--- name: "strace" section: "1" platform: "linux" package: "strace" version: "7.3-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/strace" --- # strace(1) trace system calls and signals ## NAME strace - trace system calls and signals ## SYNOPSIS **strace** \[**-ACdffhikqqrtttTvVwxxyyYzZ**\] \[**-a** *column*\] \[**-b** *execve*\] \[**-e** *expr*\]... \[**-I** *n*\] \[**-o** *file*\] \[**-O** *overhead*\] \[**-p** *pid*\]... \[**-P** *path*\]... \[**-s** *strsize*\] \[**-S** *sortby*\] \[**-U** *columns*\] \[**-X** *format*\] \[**--seccomp-bpf**\] \[**--stack-trace-frame-limit=*limit***\] \[**--syscall-limit=*limit***\] \[**--tips\[=*format*\]**\]{ **-p** *pid*\| \[**-DDD**\] \[**-E** *var\[=val\]*\]... \[**-u** *username*\] *command* \[*args*\]} **strace** **-c** \[**-dfwzZ**\] \[**-b** *execve*\] \[**-e** *expr*\]... \[**-I** *n*\] \[**-O** *overhead*\] \[**-p** *pid*\]... \[**-P** *path*\]... \[**-S** *sortby*\] \[**-U** *columns*\] \[**--seccomp-bpf**\] \[**--syscall-limit=*limit***\] \[**--tips\[=*format*\]**\]{ **-p** *pid*\| \[**-DDD**\] \[**-E** *var\[=val\]*\]... \[**-u** *username*\] *command* \[*args*\]} **strace** **--tips\[=*format*\]** ## DESCRIPTION In its simplest use case, **strace** runs the specified *command* until it exits. It intercepts and records the system calls made by a process and the signals a process receives. The name of each system call, its arguments, and its return value are printed to standard error or to the file specified with the **-o** option. **strace** is a useful diagnostic, instructional, and debugging tool. System administrators, diagnosticians, and troubleshooters will find it invaluable for solving problems with programs for which source code is not readily available, as recompilation is not required for tracing. Students, hackers, and the overly-curious will discover that a great deal can be learned about a system and its system calls by tracing even ordinary programs. Programmers will find that since system calls and signals occur at the user/kernel interface, a close examination of this boundary is very useful for bug isolation, sanity checking, and attempting to capture race conditions. Each line in the trace contains the system call name, followed by its arguments in parentheses and its return value. An example from tracing the command "cat /dev/null" is: ``` open("/dev/null", O_RDONLY) = 3 ``` Errors, typically indicated by a return value of -1, have the *errno* symbol and error string appended. ``` open("/foo/bar", O_RDONLY) = -1 ENOENT (No such file or directory) ``` Signals are printed as a signal symbol and a decoded *siginfo* structure. An excerpt from tracing and interrupting the command "sleep 666" is: ``` sigsuspend([] --- SIGINT {si_signo=SIGINT, si_code=SI_USER, si_pid=...} --- +++ killed by SIGINT +++ ``` If a system call is being executed while another is called from a different thread or process, **strace** will attempt to preserve the order of these events and mark the ongoing call as *unfinished*. When the call returns, it will be marked as *resumed*. ``` [pid 28772] select(4, [3], NULL, NULL, NULL [pid 28779] clock_gettime(CLOCK_REALTIME, {tv_sec=1130322148, tv_nsec=3977000}) = 0 [pid 28772] <... select resumed> ) = 1 (in [3]) ``` The interruption of a (restartable) system call by a signal delivery is handled differently, as the kernel terminates the system call and arranges for its immediate re-execution after the signal handler completes. ``` read(0, 0x7ffff72cf5cf, 1) = ? ERESTARTSYS (To be restarted) --- SIGALRM {si_signo=SIGALRM, si_code=SI_KERNEL} --- rt_sigreturn({mask=[]}) = 0 read(0, "", 1) = 0 ``` Arguments are printed in symbolic form with passion. This example shows the shell performing "\>\>xyzzy" output redirection: ``` open("xyzzy", O_WRONLY|O_APPEND|O_CREAT, 0666) = 3 ``` Here, the second and third arguments of [**open**(2)](/open/2.md) are decoded by breaking down the flag argument into its three bitwise-OR constituents and printing the mode value in octal, following tradition. Where traditional or native usage differs from ANSI or POSIX, the latter forms are preferred. In some cases, **strace** output has proven to be more readable than the source code itself. Structure pointers are dereferenced, and their members are displayed as appropriate. In most cases, arguments are formatted in the most C-like fashion possible. For example, the essence of the command "ls -l /dev/null" is captured as: ``` lstat("/dev/null", {st_mode=S_IFCHR|0666, st_rdev=makedev(0x1, 0x3), ...}) = 0 ``` Notice how the *struct stat* argument is dereferenced and how each member is displayed symbolically. In particular, observe how the *st\_mode* member is carefully decoded into a bitwise-OR of symbolic and numeric values. Also, note that in this example, the first argument to [**lstat**(2)](/lstat/2.md) is an input to the system call, and the second argument is an output. Since output arguments are not modified if the system call fails, arguments may not always be dereferenced. For example, retrying the "ls -l" example with a non-existent file produces the following line: ``` lstat("/foo/bar", 0xb004) = -1 ENOENT (No such file or directory) ``` In this case, the porch light is on but nobody is home. The pointer's value is displayed because the structure it points to was not populated due to the error. System calls unknown to **strace** are printed in a raw format, with the hexadecimal system call number prefixed with "syscall\_": ``` syscall_0xbad(0x1, 0x2, 0x3, 0x4, 0x5, 0x6) = -1 ENOSYS (Function not implemented) ``` Character pointers are dereferenced and printed as C strings. Non-printing characters in strings are normally represented by standard C escape codes. Only the first *strsize* (32 by default) bytes of strings are printed; longer strings have an ellipsis appended following the closing quote. Here is a line from "ls -l" where the [**getpwuid**(3)](/getpwuid/3.md) library routine is reading the password file: ``` read(3, "root::0:0:System Administrator:/"..., 1024) = 422 ``` While structures are annotated using curly braces, pointers to basic types and arrays are printed using square brackets with commas separating the elements. Here is an example from the command [**id**(1)](/id/1.md) on a system with supplementary group IDs: ``` getgroups(32, [100, 0]) = 2 ``` On the other hand, bit-sets are also shown using square brackets, but set elements are separated only by a space. Here is the shell, preparing to execute an external command: ``` sigprocmask(SIG_BLOCK, [CHLD TTOU], []) = 0 ``` Here, the second argument is a bit-set of two signals, **SIGCHLD** and **SIGTTOU**. In some cases, the bit-set is so full that it is more valuable to print the unset elements. In that case, the bit-set is prefixed by a tilde, like this: ``` sigprocmask(SIG_UNBLOCK, ~[], NULL) = 0 ``` Here, the second argument represents the full set of all signals. ## OPTIONS ### General - **-e** *expr* Modifies which events to trace or how to trace them by specifying a qualifying expression. The format of the expression is: - \[*qualifier***=**\]\[**!**\]*value*\[**,***value*\]... - where *qualifier* is one of **trace** (or **t**), **trace-fds** (or **trace-fd** or **fd** or **fds**), **abbrev** (or **a**), **verbose** (or **v**), **raw** (or **x**), **signal** (or **signals** or **s**), **read** (or **reads** or **r**), **write** (or **writes** or **w**), **fault**, **inject**, **status**, **quiet** (or **silent** or **silence** or **q**), **decode-fds** (or **decode-fd**), **decode-pids** (or **decode-pid**), or **kvm**, and *value* is a qualifier-dependent symbol or number. The default qualifier is **trace**. Using an exclamation mark negates the set of values. For example, **-e** **open** is equivalent to **-e** **trace**=**open**, which in turn means trace only the **open** system call. By contrast, **-e** **trace**=!**open** means to trace every system call except **open**. In addition, the special values **all** and **none** may be used to trace every event or no events, respectively. - Note that some shells use the exclamation mark for history expansion even inside quoted arguments. In that case, the exclamation mark must be escaped with a backslash. ### Startup - **-E** *var*=*val* **--env**=***var***=***val*** Runs the command with the environment variable *var*=*val* set for execution. - **-E** *var* **--env**=***var*** Removes *var* from the inherited environment variables before executing the command. - **-p** *pid* **--attach**=***pid*** Attaches to the process with the process ID *pid* and begin tracing. The trace may be terminated at any time by a keyboard interrupt signal (**CTRL-C**). **strace** will respond by detaching itself from the traced processes, leaving them to continue running. - Multiple **-p** options can be used to attach to several processes in addition to the command, which is optional if at least one **-p** option is given. - A single **-p** option can accept multiple process IDs separated by a comma (“,”), space (“ ”), tab, or newline. Consequently, syntaxes like **-p** "$(pidof PROG)" and **-p** "$(pgrep PROG)" are supported. - **-u** *username* **--user**=***username*** **-u** *UID:GID* **--user**=***UID:GID*** Runs command with the user ID, group ID, and supplementary groups of *username*. This option is only useful when running as root, as it enables the correct execution of setuid and/or setgid binaries. Unless this option is used, setuid and setgid programs are executed without their effective privileges. Alternative syntax where the program is started with exactly the given user and group IDs, and an empty list of supplementary groups. In this case, user and group name lookups are not performed. - **--argv0**=***name*** Sets the executed command's argv\[0\] to *name*. This is useful for tracing multi-call executables that interpret argv\[0\], such as busybox or kmod. ### Tracing - **-b** *syscall* **--detach-on**=***syscall*** Detaches from the traced process if the specified system call is reached. Currently, only **execve** keyword is supported, which includes [**execve**(2)](/execve/2.md) and [**execveat**(2)](/execveat/2.md) system calls. This option is useful for tracing a multi-threaded process with **-f** without also tracing its (potentially very complex) child processes. - **-D** **--daemonize** **--daemonize**=**grandchild** Runs the tracer process as a grandchild of the tracee, not as its parent. This reduces the visible effect of **strace** by keeping the tracee a direct child of the calling process. - **-DD** **--daemonize**=**pgroup** **--daemonize**=**pgrp** Runs tracer process as tracee's grandchild in a separate process group. In addition to reducing the visible effect of **strace**, this also prevents **strace** from being terminated by a [**kill**(2)](/kill/2.md) signal sent to the entire process group. - **-DDD** **--daemonize**=**session** Runs the tracer process as the tracee's grandchild in a separate session (known as "true daemonisation"). In addition to reduction of the visible effect of **strace**, this also prevents **strace** from being terminated upon session termination. - **-f** **--follow-forks** Traces child processes as they are created by currently traced processes as a result of the [**fork**(2)](/fork/2.md), [**vfork**(2)](/vfork/2.md) and [**clone**(2)](/clone/2.md) system calls. Note that if process *PID* is multi-threaded, using **-f** **-p** *PID* attaches to all of its threads, not just the one with *thread\_id* = *PID*. - **--output-separately** If the **--output**=***filename*** option is in effect, the trace for each process is written to a separate *filename*.*pid* file, where *pid* is the process ID. - **-ff** **--follow-forks --output-separately** Combines the effects of **--follow-forks** and **--output-separately** options. This is incompatible with **-c**, since no per-process counts are kept. - Use [**strace-log-merge**(1)](/strace-log-merge/1.md) to get a combined view of the log files. - **-I** *interruptible* **--interruptible**=***interruptible*** Controls when **strace** can be interrupted by signals (such as pressing **CTRL-C**). - **1**, **anywhere** **2**, **waiting** **3**, **never** **4**, **never\_tstp** no signals are blocked; fatal signals are blocked while decoding system call (default); fatal signals are always blocked (default if **-o** ***FILE*** ***PROG***); fatal signals and **SIGTSTP** (**CTRL-Z**) are always blocked (useful to make **strace -o** *FILE PROG* not stop on **CTRL-Z**, default if **-D**). - **--syscall-limit**=***limit*** Detaches all tracees after *limit* system calls have been captured. System calls filtered out via **--trace**, **--trace-path** or **--status** options are not considered when keeping track of the number of system calls that are captured. - **--kill-on-exit** Applies the **PTRACE\_O\_EXITKILL** ptrace option to all tracees, which sends a SIGKILL signal to a tracee if the tracer exits. This prevents tracees from being left running after the tracer exits, as they will not be detached on cleanup. **--kill-on-exit** is not compatible with **-p**/**--attach** options. ### Filtering - **-e trace**=*syscall\_set* **-e t**=*syscall\_set* **--trace**=*syscall\_set* Traces only the specified set of system calls. *syscall\_set* is defined as \[**!**\]*value*\[**,***value*\], and *value* can be one of the following: - *syscall* Traces specific system call, specified by its name (see [**syscalls**(2)](/syscalls/2.md) for a reference, but also see **NOTES**). - **?***value* A question mark preceding the qualification suppresses errors if no matching system calls are found. - *value***@64** Limits the system call specification described by *value* to the 64-bit personality. - *value***@32** Limits the system call specification described by *value* to the 32-bit personality. - *value***@x32** Limits the system call specification described by *value* to the x32 personality. - **all** Traces all system calls. - **/***regex* Traces only those system calls that match the *regex*. You can use **POSIX** Extended Regular Expression syntax (see [**regex**(7)](/regex/7.md)). - **%file** **file** Traces all system calls that take a file name as an argument. You can think of this as an abbreviation for **--trace**=**open**,**stat**,**chmod**,**unlink**,... which is useful to seeing what files the process is referencing. Furthermore, using the abbreviation will ensure that you don't accidentally forget to include a call like [**newfstatat**(2)](/newfstatat/2.md) in the list. The syntax without a preceding percent sign ("**--trace**=**file**") is deprecated. - **%process** **process** Traces system calls associated with process lifecycle (creation, exec, termination). The syntax without a preceding percent sign ("**--trace**=**process**") is deprecated. - **%net** **%network** **network** Traces all the network related system calls. The syntax without a preceding percent sign ("**--trace**=**network**") is deprecated. - **%signal** **signal** Traces all signal related system calls. The syntax without a preceding percent sign ("**--trace**=**signal**") is deprecated. - **%ipc** **ipc** Traces all IPC related system calls. The syntax without a preceding percent sign ("**--trace**=**ipc**") is deprecated. - **%desc** **desc** Traces all file descriptor related system calls. The syntax without a preceding percent sign ("**--trace**=**desc**") is deprecated. - **%memory** **memory** Traces all memory mapping related system calls. The syntax without a preceding percent sign ("**--trace**=**memory**") is deprecated. - **%creds** Traces system calls that read or modify user and group identifiers or capability sets. - **%stat** Traces stat system call variants. - **%lstat** Traces lstat system call variants. - **%fstat** Traces fstat, fstatat, and statx system call variants. - **%%stat** Traces system calls used for requesting file status (stat, lstat, fstat, fstatat, statx, and their variants). - **%statfs** Traces statfs, statfs64, statvfs, osf\_statfs, and osf\_statfs64 system calls. The same effect can be achieved with **--trace**=**/^(.\*\_)?statv?fs** regular expression. - **%fstatfs** Traces fstatfs, fstatfs64, fstatvfs, osf\_fstatfs, and osf\_fstatfs64 system calls. The same effect can be achieved with **--trace**=**/fstatv?fs** regular expression. - **%%statfs** Traces system calls related to file system statistics (statfs-like, fstatfs-like, and ustat). The same effect can be achieved with **--trace**=**/statv?fs\|fsstat\|ustat** regular expression. - **%clock** Traces system calls that read or modify system clocks. - **%pure** Traces system calls that always succeed and have no arguments. Currently, this list includes **arc\_gettls**(2), [**getdtablesize**(2)](/getdtablesize/2.md), [**getegid**(2)](/getegid/2.md), [**getegid32**(2)](/getegid32/2.md), [**geteuid**(2)](/geteuid/2.md), [**geteuid32**(2)](/geteuid32/2.md), [**getgid**(2)](/getgid/2.md), [**getgid32**(2)](/getgid32/2.md), [**getpagesize**(2)](/getpagesize/2.md), [**getpgrp**(2)](/getpgrp/2.md), [**getpid**(2)](/getpid/2.md), [**getppid**(2)](/getppid/2.md), [**get\_thread\_area**(2)](/get_thread_area/2.md) (on architectures other than x86), [**gettid**(2)](/gettid/2.md), **get\_tls**(2), [**getuid**(2)](/getuid/2.md), [**getuid32**(2)](/getuid32/2.md), **getxgid**(2), **getxpid**(2), **getxuid**(2), **kern\_features**(2), and **metag\_get\_tls**(2) system calls. - The **-c** option is useful for determining which system calls might be useful to trace. For example, **--trace**=**open,close,read,write** means to only trace those four system calls. Be careful when making inferences about the user/kernel boundary if only a subset of system calls are being monitored. The default is **--trace**=**all**. - **-e trace-fd**=*set* **-e trace-fds**=*set* **-e fd**=*set* **-e fds**=*set* **--trace-fds**=*set* Traces only the system calls that operate on the specified subset of (non-negative) file descriptors. Note that usage of this option also filters out all the system calls that do not operate on file descriptors at all. - This filter is combined with the **--trace-path** filter; a system call is traced if it matches either of them. - **-e signal**=*set* **-e signals**=*set* **-e s**=*set* **--signal**=*set* Traces only the specified subset of signals. The default is **--signal**=**all**. For example, **--signal**=!**SIGIO** (or **--signal**=!**io**) causes **SIGIO** signals not to be traced. - **-e status**=*set* **--status**=*set* Prints only system calls with the specified return status. The default is **--status**=**all**. When using the **status** qualifier, the chronological order of events may not be preserved. This is because **strace** must wait for a system call to complete before deciding whether to print it. If two system calls are executed by concurrent threads, **strace** will first print both the entry and exit of the first system call to exit, regardless of their respective entry time. The entry and exit of the second system call to exit will be printed afterwards. Here is an example when [**select**(2)](/select/2.md) is called, but a different thread calls [**clock\_gettime**(2)](/clock_gettime/2.md) before [**select**(2)](/select/2.md) finishes: ``` [pid 28779] 1130322148.939977 clock_gettime(CLOCK_REALTIME, {1130322148, 939977000}) = 0 [pid 28772] 1130322148.438139 select(4, [3], NULL, NULL, NULL) = 1 (in [3]) ``` *set* can include the following elements: - **successful** **failed** **unfinished** **unavailable** **detached** Traces system calls that returned without an error code. The **-z** option has the effect of **--status**=**successful**. Traces system calls that returned with an error code. The **-Z** option has the effect of **--status**=**failed**. Traces system calls that did not return. This might happen, for example, due to an execve call in a different thread from the same thread group. Traces system calls that returned but strace failed to fetch the error status. Traces system calls for which strace detached before the return. - **-P** *path* **--trace-path**=***path*** Traces only system calls accessing *path*. Multiple **-P** options can be used to specify several paths. This filter is combined with the **--trace-fds** filter; a system call is traced if it matches either option. - **-z** **--successful-only** Prints only system calls that returned without an error code. - **-Z** **--failed-only** Prints only system calls that returned with an error code. ### Output format - **-a** *column* **--columns**=***column*** Aligns return values in a specific column (default column 40). - **-e abbrev**=*syscall\_set* **-e a**=*syscall\_set* **--abbrev**=*syscall\_set* Abbreviates the output from printing each member of large structures. The syntax of the *syscall\_set* specification is the same as in the **--trace** option. The default is **--abbrev**=**all**. The **-v** option has the effect of **--abbrev**=**none**. - **-e verbose**=*syscall\_set* **-e v**=*syscall\_set* **--verbose**=*syscall\_set* Dereferences structures for the specified set of system calls. The syntax of the *syscall\_set* specification is the same as in the **--trace** option. The default is **--verbose**=**all**. - **-e raw**=*syscall\_set* **-e x**=*syscall\_set* **--raw**=*syscall\_set* Prints raw, undecoded arguments for the specified set of system calls. The syntax of the *syscall\_set* specification is the same as in the **--trace** option. This option has the effect of causing all arguments to be printed in hexadecimal. This option is useful if the decoding is not trusted, or if the actual numeric value of an argument is needed. See also **-X raw** option. - **-e read**=*set* **-e reads**=*set* **-e r**=*set* **--read**=*set* Performs a full hexadecimal and ASCII dump of all the data read from file descriptors listed in the specified set. For example, to see all input activity on file descriptors *3* and *5* use **--read**=*3*,*5*. Note that this is independent from the normal tracing of the [**read**(2)](/read/2.md) system call that is controlled by the option **--trace**=**read**. - **-e write**=*set* **-e writes**=*set* **-e w**=*set* **--write**=*set* Performs a full hexadecimal and ASCII dump of all the data written to file descriptors listed in the specified set. For example, to see all output activity on file descriptors *3* and *5* use **--write**=*3*,*5*. Note that this is independent from the normal tracing of the [**write**(2)](/write/2.md) system call that is controlled by the option **--trace**=**write**. - **-e quiet**=*set* **-e silent**=*set* **-e silence**=*set* **-e q**=*set* **--quiet**=*set* **--silent**=*set* **--silence**=*set* Suppresses various information messages. The default is **--quiet**=**none**. *set* can include the following elements: - **attach** **exit** **path-resolution** **personality** **thread-execve** **superseded** Suppresses messages about attaching and detaching ("**\[ Process NNNN attached \]**", "**\[ Process NNNN detached \]**"). Suppress messages about process exits ("**+++ exited with SSS +++**"). Suppress messages about resolution of paths provided via the **-P** option ("**Requested path "..." resolved into "..."**"). Suppress messages about process personality changes ("**\[ Process PID=NNNN runs in PPP mode. \]**"). Suppress messages about process being superseded by [**execve**(2)](/execve/2.md) in another thread ("**+++ superseded by execve in pid NNNN +++**"). - **-e decode-fds**=*set* **--decode-fds**=*set* Decodes various information associated with file descriptors. The default is **--decode-fds**=**none**. *set* can include the following elements: - **path** **socket** **dev** **eventfd** **pidfd** **signalfd** Prints file paths. Also enables printing of tracee's current working directory when **AT\_FDCWD** constant is used. Prints socket protocol-specific information. Prints character/block device numbers. Prints eventfd object details associated with eventfd file descriptors. Prints PIDs associated with pidfd file descriptors. Prints signal masks associated with signalfd file descriptors. - **-e decode-pids**=*set* **--decode-pids**=*set* Decodes various information associated with process IDs (and also thread IDs, process group IDs, and session IDs). The default is **--decode-pids**=**none**. *set* can include the following elements: - **comm** **pidns** Prints command names associated with thread or process IDs. Prints thread, process, process group, and session IDs in strace's PID namespace if the tracee is in a different PID namespace. - **-e kvm**=**vcpu\|vcpu+** **--kvm**=**vcpu\|vcpu+** Prints the exit reason of kvm vcpu (=**vcpu**), or prints the whole kvm\_run struct including the reason (=**vcpu+**). Requires Linux kernel version 4.16.0 or higher. - The **vcpu+** output uses prefixes **VCPU:***CPU#***\<** and **VCPU:***CPU#***\>**. Lines starting with the former indicate the state of the structure before calling KVM\_RUN, and those starting with the latter show the state after calling KVM\_RUN. - **-e namespace**=**new** **--namespace**=**new** Prints the new namespaces entered by the tracee. The following system calls are supported: [**clone**(2)](/clone/2.md), [**clone3**(2)](/clone3/2.md), [**setns**(2)](/setns/2.md), and [**unshare**(2)](/unshare/2.md). - **-i** **--instruction-pointer** Prints the instruction pointer at the time of the system call. - **-n** **--syscall-number** Prints the system call number. - **-N** **--arg-names** Prints the system call argument names. - **-k** **--stack-trace**\[=**symbol**\] Prints the execution stack trace of the traced processes after each system call. - **--stack-trace-frame-limit**=***limit*** Prints no more than this amount of stack trace frames when backtracing a system call (the default is 256). Use this option with the **--stack-trace** (or **-k**) option. - **-o** *filename* **--output**=***filename*** Writes the trace output to the file *filename* rather than to stderr. *filename*.*pid* form is used if **-ff** option is supplied. If the argument begins with '\|' or '!', the rest of the argument is treated as a command and all output is piped to it. This is convenient for piping the debugging output to a program without affecting the redirections of executed programs. Piping output to a command is not currently compatible with the **-ff** option. - **-A** **--output-append-mode** Opens the file provided in the **-o** option in append mode. - **--color**=***when*** Colorize the trace output. The default is **--color**=**auto**, which enables color only when the trace output stream is a terminal (by default stderr; see **-o**) that supports at least 8 colors. If the **NO\_COLOR** environment variable is set, color is disabled unless **--color**=**always** is specified. The *when* parameter can be one of **auto**, **always**, or **never**. Custom color schemes can be specified with the **STRACE\_COLORS** environment variable, for example:\ STRACE\_COLORS="syscall=1;33:argname=39:argval=35:call=1;34:const=1;36:comment=36:punct=0:retval=32:error=31" - The format of this value is **key**=***value*** pairs separated by **:** with semicolon-separated SGR codes as values. - The default palette uses standard ANSI colors (SGR 30–37) and bold attributes rather than bright colors (SGR 90–97), so that terminal color themes can adapt them for both dark and light backgrounds. - Recognized color keys are: **syscall**, **argname**, **argval**, **call**, **const**, **comment**, **punct**, **retval**, **error**. - **-q** **--quiet** **--quiet**=**attach**,**personality** Suppresses messages about attaching, detaching, and personality changes. This happens automatically when output is redirected to a file and the command is run directly instead of attaching. - **-qq** **--quiet**=**attach**,**personality**,**exit** Suppresses messages about attaching, detaching, personality changes, and process exit status. - **-qqq** **--quiet**=**all** Suppresses all suppressible messages (please refer to the **--quiet** option description for the full list of suppressible messages). - **-r** **--relative-timestamps**\[=***precision***\] Prints a relative timestamp upon entry to each system call. This records the time difference between the beginning of successive system calls. *precision* can be one of **s** (for seconds), **ms** (milliseconds), **us** (microseconds), or **ns** (nanoseconds), and allows setting the precision of time value being printed. Default is **us** (microseconds). Note that because the **-r** option uses the monotonic clock, its measurements may differ from the time differences reported by the **-t** option, which uses the wall clock. - **-s** *strsize* **--string-limit**=***strsize*** Specifies the maximum string size to print (the default is 32). Note that filenames are not considered strings and are always printed in full. - **--absolute-timestamps**\[=\[\[**format:**\]***format***\],\[\[**precision:**\]***precision***\]\] **--timestamps**\[=\[\[**format:**\]***format***\],\[\[**precision:**\]***precision***\]\] Prefixes each line of the trace with the wall clock time in the specified *format* with the specified *precision*. *format* can be one of the following: - **none** **time** **unix** No time stamp is printed. Can be used to override the previous setting. Wall clock time ([**strftime**(3)](/strftime/3.md) format string is **%T**). Number of seconds since the epoch ([**strftime**(3)](/strftime/3.md) format string is **%s**). - *precision* can be one of **s** (for seconds), **ms** (milliseconds), **us** (microseconds), or **ns** (nanoseconds). Default arguments for the option are **format:time**,**precision:s**. - **-t** **--absolute-timestamps** Prefixes each line of the trace with the wall clock time. - **-tt** **--absolute-timestamps**=**precision:us** Prints the wall clock time with microsecond precision. - **-ttt** **--absolute-timestamps**=**format:unix**,**precision:us** Prints the wall clock time as seconds since the epoch, with microsecond precision. - **-T** **--syscall-times**\[=***precision***\] Shows the time spent in system calls. This records the time difference between the beginning and the end of each system call. *precision* can be one of **s** (for seconds), **ms** (milliseconds), **us** (microseconds), or **ns** (nanoseconds), and allows setting the precision of time value being printed. Default is **us** (microseconds). - **-v** **--no-abbrev** Prints unabbreviated versions of environment, stat, termios, etc. calls. These structures are very common, so the default behavior is to display a reasonable subset of their members. Use this option to see all members in full detail. - **--strings-in-hex**\[=***option***\] Controls the use of hexadecimal escape sequences when printing strings. This option alters the default escaping behavior. - Normally (when neither this option nor -x is used), **strace** introduces escape sequences in two situations: to represent non-printable and non-ASCII characters (i.e., those with character codes less than 32 or greater than 127), or to disambiguate output, for example, by escaping the quotation marks that enclose a string or the angle brackets used in file descriptor paths. When a character must be escaped, **strace** prioritizes symbolic C-standard sequences if one exists: “**\\t**” (tab), “**\\n**” (newline), “**\\v**” (vertical tab), “**\\f**” (form feed), and “**\\r**” (carriage return). For all other characters that require escaping, **strace** defaults to using an octal representation of the character's byte value. This option allows you to override this default behavior and use hexadecimal escapes instead of octal ones. - *option* can be one of the following: - **none** **non-ascii-chars** **non-ascii** **all** Hexadecimal numbers are not used in the output at all. When there is a need to emit an escape sequence, octal numbers are used. Hexadecimal numbers are used instead of octal in the escape sequences. Strings that contain non-ASCII characters are printed using escape sequences with hexadecimal numbers. All strings are printed using escape sequences with hexadecimal numbers. - When the option is supplied without an argument, **all** is assumed. - **-x** **--strings-in-hex**=**non-ascii** Prints all non-ASCII strings in hexadecimal string format. - **-xx** **--strings-in-hex**\[=**all**\] Prints all strings in hexadecimal string format. - **-X** *format* **--const-print-style**=***format*** Sets the format for printing of named constants and flags. Supported *format* values are: - **raw** **abbrev** **verbose** Raw number output, without decoding. Outputs a named constant or a set of flags instead of the raw number if they are found. This is the default **strace** behaviour. Outputs both the raw value and the decoded string (as a comment). - **-y** **--decode-fds** **--decode-fds**=**path** Prints paths associated with file descriptor arguments and with the **AT\_FDCWD** constant. - **-yy** **--decode-fds**=**all** Prints all available information associated with file descriptors: protocol-specific information associated with socket file descriptors, block/character device number associated with device file descriptors, and PIDs associated with pidfd file descriptors. - **--pidns-translation** **--decode-pids**=**pidns** If strace and tracee are in different PID namespaces, print PIDs in strace's namespace, too. - **-Y** **--decode-pids**=**comm** Prints command names for PIDs. - **--always-show-pid** Shows PID prefix also for the process started by strace. Implied when -f and -o are both specified. ### Statistics - **-c** **--summary-only** Counts time, calls, and errors for each system call and report a summary on program exit, suppressing the regular output. This shows system time (CPU time spent in the kernel), which is independent of wall clock time. If **-c** is used with **-f**, only aggregate totals for all traced processes are kept. - **-C** **--summary** Like **-c**, but also prints the regular output while processes are running. - **-O** *overhead* **--summary-syscall-overhead**=***overhead*** Sets the overhead for tracing system calls to *overhead*. This is useful for overriding the default heuristic, which estimates the time spent in the measurement process itself when timing system calls with the **-c** option. The same overhead is subtracted from polymorphic time columns and from **wall-total**, **wall-min**, **wall-max**, and **wall-avg**. The accuracy of the heuristic can be gauged by timing a given program run without tracing (using [**time**(1)](/time/1.md)) and comparing the accumulated system call time to the total produced using **-c**. - The format of *overhead* specification is described in section *Time specification format description*. - **-S** *sortby* **--summary-sort-by**=***sortby*** Sorts the output of the histogram printed by the **-c** option by the specified criterion. Valid values are **time** (or **time-percent** or **time-total** or **total-time**), **min-time** (or **shortest** or **time-min**), **max-time** (or **longest** or **time-max**), **avg-time** (or **time-avg**), **wall-total** (or **total-wall**), **wall-min** (or **min-wall**), **wall-max** (or **max-wall**), **wall-avg** (or **avg-wall**), **calls** (or **count**), **errors** (or **error**), **name** (or **syscall** or **syscall-name**), and **nothing** (or **none**); default is **time**. - **-U** *columns* **--summary-columns**=***columns*** Configures the set and order of columns shown in the call summary. The *columns* argument is a comma-separated list containing one or more of the following values: - **time-percent** (or **time**) **total-time** (or **time-total**) **min-time** (or **shortest** or **time-min**) **max-time** (or **longest** or **time-max**) **avg-time** (or **time-avg**) **wall-total** (or **total-wall**) **wall-min** (or **min-wall**) **wall-max** (or **max-wall**) **wall-avg** (or **avg-wall**) **calls** (or **count**) **errors** (or **error**) **name** (or **syscall** or **syscall-name**) Percentage of cumulative time consumed by a specific system call. Total system (or wall clock, if **-w** option is provided) time consumed by a specific system call. Minimum observed call duration in seconds. Maximum observed call duration in seconds. Average call duration in microseconds per call. Total wall-clock time consumed by a specific system call. Always wall-clock, regardless of the **-w** option, allowing it to be displayed alongside **total-time** in a single run. Minimum observed wall-clock call duration in seconds. Maximum observed wall-clock call duration in seconds. Average wall-clock call duration in microseconds per call. Call count. Error count. System call name. - The default value is **time-percent**,**total-time**,**avg-time**,**calls**,**errors**,**name**. If the **name** field is not supplied explicitly, it is added as the last column. - **-w** **--summary-wall-clock** Summarizes the wall clock time for each system call, measured from its beginning to its end. The default is to summarize the system time. ### Tampering - **--inject**=*syscall\_set*\[:**error**=*errno*\|:**retval**=*value*\]\[:**signal**=*sig*\]\[:**syscall**=*syscall*\]\[:**delay\_enter**=*delay*\]\[:**delay\_exit**=*delay*\]\[:**poke\_enter**=*@argN=DATAN,@argM=DATAM...*\]\[:**poke\_exit**=*@argN=DATAN,@argM=DATAM...*\]\[:**when**=*expr*\] { Performs system call tampering for the specified set of system calls. - The syntax of the *syscall\_set* specification is the same as in the **--trace** option. - At least one of **error**, **retval**, **signal**, **delay\_enter**, **delay\_exit**, **poke\_enter**, or **poke\_exit** action options must be specified. **error** and **retval** are mutually exclusive. - If the **error**=*errno* option is specified, a fault is injected into the system call. This is achieved by replacing the system call number with -1 (representing an invalid system call) and setting the error code to the specified *errno*. This behavior of replacing the syscall number with -1 can be overridden using the **syscall**= option. The *errno* can be a symbolic name like **ENOSYS** or a numeric value in the range 1..4095. - If the **retval**=*value* option is specified, a success value is injected. The system call number is replaced as with the **error**= option, but instead of an error, the specified success *value* is returned to the caller process. - If the **signal**=*sig* option is specified with either a symbolic value like **SIGSEGV** or a numeric value within 1..**SIGRTMAX** range, that signal is delivered on entering every system call specified by the *syscall\_set*. - If the **delay\_enter**=*delay* or **delay\_exit**=*delay* options are specified, delay injection is performed: the tracee is delayed by time period specified by *delay* on entering or exiting the system call, respectively. The format of *delay* specification is described in section *Time specification format description*. - If the **poke\_enter**=*@argN=DATAN,@argM=DATAM...* or **poke\_exit**=*@argN=DATAN,@argM=DATAM...* options are specified, tracee's memory at locations, pointed to by system call arguments *argN* and *argM* (going from *arg1* to *arg7*) is overwritten by data *DATAN* and *DATAM* (specified in hexadecimal format; for example **poke\_enter**=*@arg1=0000DEAD0000BEEF*). The **poke\_enter** option modifies memory on system call enter, while **poke\_exit** does so on system call exit. - The injection actions are independent. For example, specifying only **signal**= delivers a signal without altering the system call's outcome or delaying it. Similarly, specifying only **error**= injects a system call fault without adding a signal or delay. - If the **signal**=*sig* option is specified together with **error**=*errno* or **retval**=*value*, then both injection of a fault or success and signal delivery are performed. - If the **syscall**=*syscall* option is specified, the given *syscall* is injected instead of the default -1. The specified *syscall* must have no side effects; currently, only system calls from the **%pure** set are supported. - Unless the **when**=*expr* subexpression is specified, an injection is being made into every invocation of each system call from the *syscall\_set*. - The format of the subexpression is: - *first*\[**..***last*\]\[**+**\[*step*\]\] - Number *first* stands for the first invocation number in the range, number *last* stands for the last invocation number in the range, and *step* stands for the step between two consecutive invocations. The following combinations are useful: - *first* *first***..***last* *first***+** *first***+***step* *first***..***last***+***step* Injects into invocation number *first* only for each system call in the *syscall\_set*. Injects into invocations from *first* through *last* (inclusive) for each system call in the *syscall\_set*. Injects into every invocation, starting with number *first*, for each system call in the *syscall\_set*. Injects into invocations number *first*, *first*+*step*, *first*+*step*+*step*, and so on, for each system call in the *syscall\_set*. Same as the previous, but consider only invocations with numbers up to *last* (inclusive). - For example, to fail each third and subsequent chdir system calls with **ENOENT**, use **--inject**=*chdir*:**error**=*ENOENT*:**when**=*3***+**. - The valid range for numbers *first* and *step* is 1..65535, and for number *last* is 1..65534. - An injection expression can contain at most one fault or return value specification (i.e., either **error**= or **retval**=) and at most one **signal**= specification. If an injection expression contains multiple **when**= specifications, the last one takes precedence. - Accounting of system calls that are subject to injection is done per system call and per tracee. - Specification of system call injection can be combined with other system call filtering options, for example, **-P** */dev/urandom* **--inject**=*file*:**error**=*ENOENT*. - **-e inject**=*args* This is equivalent to **--inject**=*args*. - **--fault**=*syscall\_set*\[:**error**=*errno*\]\[:**when**=*expr*\] Performs system call fault injection for the specified set of system calls. - This is a shortcut for the more general **--inject**= option, using a default *errno* of **ENOSYS**. - **-e fault**=*args* This is equivalent to **--fault**=*args*. ### Miscellaneous - **-d** **--debug** Shows some debugging output of **strace** itself on the standard error. - **-F** This option is deprecated. It is retained for backward compatibility only and may be removed in future releases. Using multiple **-F** options is equivalent to a single **-f**. This option is ignored entirely if used in conjunction with one or more **-f** options. - **-h** **--help** Prints the help summary. - **--seccomp-bpf** Attempts to use seccomp-bpf (see [**seccomp**(2)](/seccomp/2.md)) to cause the kernel to stop the tracee only for the system calls that are being traced. - This option has no effect unless **-f**/**--follow-forks** is also specified. **--seccomp-bpf** is not compatible with **--syscall-limit** and **-b**/**--detach-on** options. It is also not applicable to processes attached using **-p**/**--attach** option. - An attempt to enable system calls filtering using seccomp-bpf may fail for various reasons, e.g. there are too many system calls to filter, the seccomp API is not available, or **strace** itself is being traced. If the seccomp-bpf filter setup fails, **strace** proceeds as usual, stopping traced processes on every system call. - When **--seccomp-bpf** is activated and **-p**/**--attach** option is not used, **--kill-on-exit** option is activated as well. - Note that in cases when the tracee has another seccomp filter that returns an action value with a precedence greater than **SECCOMP\_RET\_TRACE**, **strace --seccomp-bpf** will not be notified. That is, if another seccomp filter, for example, disables the system call or kills the tracee, then **strace --seccomp-bpf** will not be aware of that system call invocation at all. - **--tips**\[=\[\[**id:**\]***id***\],\[\[**format:**\]***format***\]\] Shows strace tips, tricks, and tweaks before exit. The *id* can be a non-negative integer to print a specific tip (note: these IDs are not guaranteed to be stable). It can also be **random** (the default), in which case a random tip is printed. *format* can be one of the following: - **none** **compact** **full** No tip is printed. Can be used to override the previous setting. Prints the tip just big enough to contain all the text. Prints the tip in its full glory. - Default is **id:random,format:compact**. - **-V** **--version** Prints the version number of **strace** and the list of enabled optional features. Multiple instances of this option beyond specific threshold tend to increase der Strauss awareness. ### Time specification format description Time values are specified as a decimal floating point number (in a format accepted by [**strtod**(3)](/strtod/3.md)), optionally followed by a suffix to indicate the unit of time: **s** (seconds), **ms** (milliseconds), **us** (microseconds), or **ns** (nanoseconds). If no suffix is specified, the value defaults to microseconds. The described format is used for **-O**, **--inject**=**delay\_enter**, and **--inject**=**delay\_exit** options. ## DIAGNOSTICS When *command* exits, **strace** exits with the same exit status. If *command* is terminated by a signal, **strace** terminates itself with the same signal, so that **strace** can be used as a wrapper process transparent to the invoking parent process. Note that the parent-child relationship (signal stop notifications, the [**getppid**(2)](/getppid/2.md) value, etc) between the traced process and its parent is not preserved unless **-D** is used. When using **-p** without a *command*, the exit status of **strace** is zero unless no processes have been attached or an unexpected error occurred during tracing. ## SETUID INSTALLATION If **strace** is installed setuid to root, then the invoking user will be able to attach to and trace processes owned by any user. In addition, setuid and setgid programs will be executed and traced with the correct effective privileges. Since these capabilities should only be granted to users with full root privileges, installing **strace** as setuid to root is only appropriate when its use is restricted to such trusted users. For example, a special version of **strace** could be installed with mode 'rwsr-x---', user **root,** and group **trace**. In this configuration, only trusted users who are members of the **trace** group could execute it. If you use this feature, remember to also install a regular, non-setuid version of **strace** for ordinary users. ## MULTIPLE PERSONALITIES SUPPORT On some architectures, **strace** can decode system calls for processes that use a different Application Binary Interface (ABI) from the one **strace** uses. Specifically, in addition to decoding native ABI, **strace** can decode the following ABIs on the following architectures: | | | | --- | --- | | **Architecture** | **ABIs supported** | | x86\_64 | i386, x32 \[1\]; i386 \[2\] | | AArch64 | ARM 32-bit EABI | | PowerPC 64-bit \[3\] | PowerPC 32-bit | | s390x | s390 | | SPARC 64-bit | SPARC 32-bit | | TILE 64-bit | TILE 32-bit | - \[1\] \[2\] \[3\] When **strace** is built as an x86\_64 application When **strace** is built as an x32 application Big endian only This support is optional and depends on the ability to generate and parse structure definitions at build time. Refer to the output of the **strace -V** command to determine which ABIs are supported by your **strace** build. In this context, "non-native" refers to an ABI that differs from the one **strace** is using: - **m32-mpers** **no-m32-mpers** **mx32-mpers** **no-mx32-mpers** **strace** can trace and properly decode non-native 32-bit binaries. **strace** can trace, but cannot properly decode non-native 32-bit binaries. **strace** can trace and properly decode non-native 32-on-64-bit binaries. **strace** can trace, but cannot properly decode non-native 32-on-64-bit binaries. If the output contains neither **m32-mpers** nor **no-m32-mpers**, it means that support for decoding non-native 32-bit binaries is not applicable to the architecture. Likewise, if the output contains neither **mx32-mpers** nor **no-mx32-mpers**, it means that support for decoding non-native 32-on-64-bit binaries is not applicable to the architecture. ## NOTES Systems that use shared libraries often produce a large amount of tracing output when loading them. It is instructive to think about system call inputs and outputs as data-flow across the user/kernel boundary. Because user-space and kernel-space are separate and address-protected, it is sometimes possible to make deductive inferences about process behavior using inputs and outputs as propositions. In some cases, a system call will differ from the documented behavior or have a different name. For example, the underlying [**faccessat**(2)](/faccessat/2.md) system call does not have a *flags* argument, and the [**setrlimit**(2)](/setrlimit/2.md) library function is implemented using [**prlimit64**(2)](/prlimit64/2.md) system call on modern (2.6.38+) kernels. These discrepancies are normal characteristics of the system call interface and are handled by C library wrapper functions. Some system calls have different names in different architectures and personalities. In these cases, system call filtering and printing uses the names that match corresponding **\_\_NR\_**\* kernel macros of the tracee's architecture and personality. There are two exceptions from this general rule: [**arm\_fadvise64\_64**(2)](/arm_fadvise64_64/2.md) ARM system call and **xtensa\_fadvise64\_64**(2) Xtensa system call are filtered and printed as [**fadvise64\_64**(2)](/fadvise64_64/2.md). On the x32 ABI, some system calls are intended for 64-bit processes but can be invoked from x32 by setting the **\_\_X32\_SYSCALL\_BIT** flag. When this occurs, **strace** designates these calls with a **#64** suffix. An example is [**readv**(2)](/readv/2.md), which is syscall number 19 on x86\_64, whereas its distinct x32 counterpart is syscall number 515. On some platforms, a process attached with the **-p** option may receive a spurious **EINTR** error from a non-restartable system call. This can have an unpredictable effect on the process if it does not attempt to restart the call. Ideally, all system calls should be restarted on **strace** attach, making the attach invisible to the traced process, but a few system calls aren't. Arguably, every instance of such behavior is a kernel bug. Since **strace** executes the specified *command* directly without a shell, scripts that lack a shebang line (e.g., #!/bin/sh) will fail with an **ENOEXEC** error, even if a shell could run them correctly. It is advisable to manually supply a shell as a *command* with the script as its argument. ## BUGS Programs that use the *setuid* bit do not have effective user ID privileges while being traced. A traced process runs more slowly than a non-traced one. The performance impact can be mitigated by using the **--seccomp-bpf** option. When tracing a *command*, its descendant processes may be left running after **strace** is terminated by an interrupt signal (such as CTRL-C**).** This can be prevented by using the **--kill-on-exit** option, or by using **--seccomp-bpf** option in a way that implies **--kill-on-exit**. A traced process can use the **CLONE\_UNTRACED** flag with the **clone** system call to create a child process that is not traced by strace. This breaks a guarantee of the **--seccomp-bpf** option, as this untraced child may be left with an active seccomp filter after strace terminates. ## HISTORY The original **strace** was written by Paul Kranenburg for SunOS and was inspired by its **trace** utility. The SunOS version of **strace** was ported to Linux and enhanced by Branko Lankester, who also wrote the Linux kernel support. Even though Paul released **strace** 2.5 in 1992, Branko's work was based on Paul's **strace** 1.5 release from 1991. In 1993, Rick Sladkey took on the project. He merged **strace** 2.5 for SunOS with the second release of **strace** for Linux, added many features from SVR4's [**truss**(1)](/truss/1.md), and produced a version of **strace** that worked on both platforms. In 1994 Rick ported **strace** to SVR4 and Solaris and wrote the automatic configuration support. In 1995 he ported **strace** to Irix (and became tired of writing about himself in the third person). Beginning with 1996, **strace** was maintained by Wichert Akkerman. During his tenure, **strace** development migrated to CVS; ports to FreeBSD and many architectures on Linux (including ARM, IA-64, MIPS, PA-RISC, PowerPC, s390, SPARC) were introduced. In 2002, responsibility for **strace** maintenance was transferred to Roland McGrath. Since then, **strace** gained support for several new Linux architectures (AMD64, s390x, SuperH), bi-architecture support for some of them, and received numerous additions and improvements in system calls decoders on Linux; **strace** development migrated to **Git** during that period. Since 2009, **strace** has been actively maintained by Dmitry Levin. During this period, **strace** has gained support for the AArch64, ARC, AVR32, Blackfin, C-SKY, LoongArch, Meta, Nios II, OpenRISC 1000, RISC-V, Tile/TileGx, and Xtensa architectures. In 2012, unmaintained and apparently broken support for non-Linux operating systems was removed. Also, in 2012 **strace** gained support for path tracing and file descriptor path decoding. In 2014, support for stack trace printing was added. In 2016, system call tampering was implemented. For the additional information, please refer to the **NEWS** file and **strace** repository commit log. ## REPORTING BUGS Problems with **strace** should be reported to the **strace** mailing list ⟨mailto:strace\\-devel@lists.strace.io⟩. ## SEE ALSO [**strace-log-merge**(1)](/strace-log-merge/1.md), [**ltrace**(1)](/ltrace/1.md), [**perf-trace**(1)](/perf-trace/1.md), [**trace-cmd**(1)](/trace-cmd/1.md), [**time**(1)](/time/1.md), [**ptrace**(2)](/ptrace/2.md), [**seccomp**(2)](/seccomp/2.md), [**syscall**(2)](/syscall/2.md), [**proc**(5)](/proc/5.md), [**signal**(7)](/signal/7.md) **strace** Home Page ⟨https://strace.io/⟩ ## AUTHORS The complete list of **strace** contributors can be found in the **CREDITS** file.