--- name: "ptrace" section: "2" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/ptrace" --- # ptrace(2) process tracing and debugging ## NAME **ptrace** – process tracing and debugging ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** **#include \** *int*\ **ptrace**(*int request*, *pid\_t pid*, *caddr\_t addr*, *int data*); ## DESCRIPTION The **ptrace**() system call provides tracing and debugging facilities. It allows one process (the *tracing* process) to control another (the *traced* process). The tracing process must first attach to the traced process, and then issue a series of **ptrace**() system calls to control the execution of the process, as well as access process memory and register state. For the duration of the tracing session, the traced process will be “re-parented”, with its parent process ID (and resulting behavior) changed to the tracing process. It is permissible for a tracing process to attach to more than one other process at a time. When the tracing process has completed its work, it must detach the traced process; if a tracing process exits without first detaching all processes it has attached, those processes will be killed. Most of the time, the traced process runs normally, but when it receives a signal (see [sigaction(2)](/sigaction/2.md)), it stops. The tracing process is expected to notice this via [wait(2)](/wait/2.md) or the delivery of a SIGCHLD signal, examine the state of the stopped process, and cause it to terminate or continue as appropriate. The signal may be a normal process signal, generated as a result of traced process behavior, or use of the [kill(2)](/kill/2.md) system call; alternatively, it may be generated by the tracing facility as a result of attaching, stepping by the tracing process, or an event in the traced process. The tracing process may choose to intercept the signal, using it to observe process behavior (such as SIGTRAP), or forward the signal to the process if appropriate. The **ptrace**() system call is the mechanism by which all this happens. A traced process may report additional signal stops corresponding to events in the traced process. These additional signal stops are reported as SIGTRAP or SIGSTOP signals. The tracing process can use the PT\_LWPINFO request to determine which events are associated with a SIGTRAP or SIGSTOP signal. Note that multiple events may be associated with a single signal. For example, events indicated by the PL\_FLAG\_BORN, PL\_FLAG\_FORKED, and PL\_FLAG\_EXEC flags are also reported as a system call exit event (PL\_FLAG\_SCX). The signal stop for a new child process enabled via PTRACE\_FORK will report a SIGSTOP signal. All other additional signal stops use SIGTRAP. ## DETACH AND TERMINATION Normally, exiting tracing process should wait for all pending debugging events and then detach from all alive traced processes before exiting using PT\_DETACH request. If tracing process exits without detaching, for instance due to abnormal termination, the destiny of the traced children processes is determined by the kern.kill\_on\_debugger\_exit sysctl control. If the control is set to the default value 1, such traced processes are terminated. If set to zero, kernel implicitly detaches traced processes. Traced processes are un-stopped if needed, and then continue the execution without tracing. Kernel drops any SIGTRAP signals queued to the traced children, which could be either generated by not yet consumed debug events, or sent by other means, the later should not be done anyway. ## SELECTING THE TARGET The *pid* argument of the call specifies the target on which to perform the requested operation. For operations affecting the global process state, the process ID is typically passed there. Similarly, for operations affecting only a thread, the thread ID needs to be passed. Still, for global operations, the ID of any thread can be used as the target, and system will perform the request on the process owning that thread. If a thread operation got the process ID as *pid*, the system randomly selects a thread from among the threads owned by the process. For single-threaded processes there is no difference between specifying process or thread ID as the target. ## DISABLING PTRACE The **ptrace** subsystem provides rich facilities to manipulate other processes state. Sometimes it may be desirable to disallow it either completely, or limit its scope. The following controls are provided for this: - *security.bsd.allow\_ptrace* Setting this sysctl to zero makes **ptrace** return ENOSYS always as if the syscall is not implemented by the kernel. - *security.bsd.unprivileged\_proc\_debug* Setting this sysctl to zero disallows the use of **ptrace**() by unprivileged processes. - *security.bsd.see\_other\_uids* Setting this sysctl to zero prevents **ptrace**() requests from targeting processes with a real user identifier different from the caller's. These requests will fail with error ESRCH. - *security.bsd.see\_other\_gids* Setting this sysctl to zero disallows **ptrace**() requests from processes that have no groups in common with the target process, considering their sets of real and supplementary groups. These requests will fail with error ESRCH. - *security.bsd.see\_jail\_proc* Setting this sysctl to zero disallows **ptrace**() requests from processes belonging to a different jail than that of the target process, even if the requesting process' jail is an ancestor of the target process'. These requests will fail with error ESRCH. - security.bsd.allow\_ptrace\_in\_cap\_mode Setting this sysctl to true allows the **ptrace**() requests issued by the processes in capability mode, see pdptrace(2). - securelevel and init The [init(1)](/init/1.md) process can only be traced with **ptrace** if securelevel is zero. - [procctl(2)](/procctl/2.md) PROC\_TRACE\_CTL Process can deny attempts to trace itself with [procctl(2)](/procctl/2.md) PROC\_TRACE\_CTL request. In this case requests return EPERM error. ## TRACING EVENTS Each traced process has a tracing event mask. An event in the traced process only reports a signal stop if the corresponding flag is set in the tracing event mask. The current set of tracing event flags include: - PTRACE\_EXEC Report a stop for a successful invocation of [execve(2)](/execve/2.md). This event is indicated by the PL\_FLAG\_EXEC flag in the *pl\_flags* member of *struct ptrace\_lwpinfo*. - PTRACE\_SCE Report a stop on each system call entry. This event is indicated by the PL\_FLAG\_SCE flag in the *pl\_flags* member of *struct ptrace\_lwpinfo*. - PTRACE\_SCX Report a stop on each system call exit. This event is indicated by the PL\_FLAG\_SCX flag in the *pl\_flags* member of *struct ptrace\_lwpinfo*. - PTRACE\_SYSCALL Report stops for both system call entry and exit. - PTRACE\_FORK This event flag controls tracing for new child processes of a traced process. When this event flag is enabled, new child processes will enable tracing and stop before executing their first instruction. The new child process will include the PL\_FLAG\_CHILD flag in the *pl\_flags* member of *struct ptrace\_lwpinfo*. The traced process will report a stop that includes the PL\_FLAG\_FORKED flag. The process ID of the new child process will also be present in the *pl\_child\_pid* member of *struct ptrace\_lwpinfo*. If the new child process was created via [vfork(2)](/vfork/2.md), the traced process's stop will also include the PL\_FLAG\_VFORKED flag. Note that new child processes will be attached with the default tracing event mask; they do not inherit the event mask of the traced process. When this event flag is not enabled, new child processes will execute without tracing enabled. - PTRACE\_LWP This event flag controls tracing of LWP (kernel thread) creation and destruction. When this event is enabled, new LWPs will stop and report an event with PL\_FLAG\_BORN set before executing their first instruction, and exiting LWPs will stop and report an event with PL\_FLAG\_EXITED set before completing their termination. Note that new processes do not report an event for the creation of their initial thread, and exiting processes do not report an event for the termination of the last thread. - PTRACE\_VFORK Report a stop event when a parent process resumes after a [vfork(2)](/vfork/2.md). When a thread in the traced process creates a new child process via [vfork(2)](/vfork/2.md), the stop that reports PL\_FLAG\_FORKED and PL\_FLAG\_SCX occurs just after the child process is created, but before the thread waits for the child process to stop sharing process memory. If a debugger is not tracing the new child process, it must ensure that no breakpoints are enabled in the shared process memory before detaching from the new child process. This means that no breakpoints are enabled in the parent process either. The PTRACE\_VFORK flag enables a new stop that indicates when the new child process stops sharing the process memory of the parent process. A debugger can reinsert breakpoints in the parent process and resume it in response to this event. This event is indicated by setting the PL\_FLAG\_VFORK\_DONE flag. The default tracing event mask when attaching to a process via PT\_ATTACH, PT\_TRACE\_ME, or PTRACE\_FORK includes only PTRACE\_EXEC events. All other event flags are disabled. ## PTRACE REQUESTS The *request* argument specifies what operation is being performed; the meaning of the rest of the arguments depends on the operation, but except for one special case noted below, all **ptrace**() calls are made by the tracing process, and the *pid* argument specifies the process ID of the traced process or a corresponding thread ID. The *request* argument can be: - PT\_TRACE\_ME This request is the only one used by the traced process; it declares that the process expects to be traced by its parent. All the other arguments are ignored. (If the parent process does not expect to trace the child, it will probably be rather confused by the results; once the traced process stops, it cannot be made to continue except via **ptrace**().) When a process has used this request and calls [execve(2)](/execve/2.md) or any of the routines built on it (such as [execv(3)](/execv/3.md)), it will stop before executing the first instruction of the new image. Also, any setuid or setgid bits on the executable being executed will be ignored. If the child was created by [vfork(2)](/vfork/2.md) system call or [rfork(2)](/rfork/2.md) call with the RFMEM flag specified, the debugging events are reported to the parent only after the [execve(2)](/execve/2.md) is executed. - PT\_READ\_I, PT\_READ\_D These requests read a single *int* of data from the traced process's address space. Traditionally, **ptrace**() has allowed for machines with distinct address spaces for instruction and data, which is why there are two requests: conceptually, PT\_READ\_I reads from the instruction space and PT\_READ\_D reads from the data space. In the current FreeBSD implementation, these two requests are completely identical. The *addr* argument specifies the address (in the traced process's virtual address space) at which the read is to be done. This address does not have to meet any alignment constraints. The value read is returned as the return value from **ptrace**(). - PT\_WRITE\_I, PT\_WRITE\_D These requests parallel PT\_READ\_I and PT\_READ\_D, except that they write rather than read. The *data* argument supplies the value to be written. - PT\_IO This request allows reading and writing arbitrary amounts of data in the traced process's address space. The *addr* argument specifies a pointer to a *struct ptrace\_io\_desc*, which is defined as follows: ``` struct ptrace_io_desc { int piod_op; /* I/O operation */ void *piod_offs; /* child offset */ void *piod_addr; /* parent offset */ size_t piod_len; /* request length */ }; /* * Operations in piod_op. */ #define PIOD_READ_D 1 /* Read from D space */ #define PIOD_WRITE_D 2 /* Write to D space */ #define PIOD_READ_I 3 /* Read from I space */ #define PIOD_WRITE_I 4 /* Write to I space */ ``` The *data* argument is ignored. The actual number of bytes read or written is stored in *piod\_len* upon return. - PT\_CONTINUE The traced process continues execution. The *addr* argument is an address specifying the place where execution is to be resumed (a new value for the program counter), or (*caddr\_t*)1 to indicate that execution is to pick up where it left off. The *data* argument provides a signal number to be delivered to the traced process as it resumes execution, or 0 if no signal is to be sent. - PT\_STEP The traced process is single stepped one instruction. The *addr* argument should be passed (*caddr\_t*)1. The *data* argument provides a signal number to be delivered to the traced process as it resumes execution, or 0 if no signal is to be sent. - PT\_KILL The traced process terminates, as if PT\_CONTINUE had been used with SIGKILL given as the signal to be delivered. - PT\_ATTACH This request allows a process to gain control of an otherwise unrelated process and begin tracing it. It does not need any cooperation from the process to trace. In this case, *pid* specifies the process ID of the process to trace, and the other two arguments are ignored. This request requires that the target process must have the same real UID as the tracing process, and that it must not be executing a setuid or setgid executable. (If the tracing process is running as root, these restrictions do not apply.) The tracing process will see the newly-traced process stop and may then control it as if it had been traced all along. - PT\_DETACH This request is like PT\_CONTINUE, except that it does not allow specifying an alternate place to continue execution, and after it succeeds, the traced process is no longer traced and continues execution normally. The parent of the traced process will be sent a SIGCHLD to indicate that the process has continued from a stopped state regardless of whether the process was in a stopped state prior to the corresponding PT\_ATTACH request. A [wait(2)](/wait/2.md) for the traced process would indicate that it had been continued. - PT\_GETREGS This request reads the traced process's machine registers into the“ *struct reg*” (defined in ⟨machine/reg.h⟩) pointed to by *addr*. - PT\_SETREGS This request is the converse of PT\_GETREGS; it loads the traced process's machine registers from the“ *struct reg*” (defined in ⟨machine/reg.h⟩) pointed to by *addr*. - PT\_GETFPREGS This request reads the traced process's floating-point registers into the“ *struct fpreg*” (defined in ⟨machine/reg.h⟩) pointed to by *addr*. - PT\_SETFPREGS This request is the converse of PT\_GETFPREGS; it loads the traced process's floating-point registers from the“ *struct fpreg*” (defined in ⟨machine/reg.h⟩) pointed to by *addr*. - PT\_GETDBREGS This request reads the traced process's debug registers into the“ *struct dbreg*” (defined in ⟨machine/reg.h⟩) pointed to by *addr*. - PT\_SETDBREGS This request is the converse of PT\_GETDBREGS; it loads the traced process's debug registers from the“ *struct dbreg*” (defined in ⟨machine/reg.h⟩) pointed to by *addr*. - PT\_GETREGSET This request reads the registers from the traced process. The *data* argument specifies the register set to read, with the *addr* argument pointing at a *struct iovec* where the *iov\_base* field points to a register set specific structure to hold the registers, and the *iov\_len* field holds the length of the structure. - PT\_SETREGSET This request writes to the registers of the traced process. The *data* argument specifies the register set to write to, with the *addr* argument pointing at a *struct iovec* where the *iov\_base* field points to a register set specific structure to hold the registers, and the *iov\_len* field holds the length of the structure. If *iov\_base* is NULL the kernel will return the expected length of the register set specific structure in the *iov\_len* field and not change the target register set. - PT\_LWPINFO This request can be used to obtain information about the kernel thread, also known as light-weight process, that caused the traced process to stop. The *addr* argument specifies a pointer to a *struct ptrace\_lwpinfo*, which is defined as follows: ``` struct ptrace_lwpinfo { lwpid_t pl_lwpid; int pl_event; int pl_flags; sigset_t pl_sigmask; sigset_t pl_siglist; siginfo_t pl_siginfo; char pl_tdname[MAXCOMLEN + 1]; pid_t pl_child_pid; u_int pl_syscall_code; u_int pl_syscall_narg; }; ``` The *data* argument is to be set to the size of the structure known to the caller. This allows the structure to grow without affecting older programs. The fields in the *struct ptrace\_lwpinfo* have the following meaning: - *pl\_lwpid* LWP id of the thread - *pl\_event* Event that caused the stop. Currently defined events are: - PL\_EVENT\_NONE No reason given - PL\_EVENT\_SIGNAL Thread stopped due to the pending signal - *pl\_flags* Flags that specify additional details about observed stop. Currently defined flags are: - PL\_FLAG\_SCE The thread stopped due to system call entry, right after the kernel is entered. The debugger may examine syscall arguments that are stored in memory and registers according to the ABI of the current process, and modify them, if needed. - PL\_FLAG\_SCX The thread is stopped immediately before syscall is returning to the usermode. The debugger may examine system call return values in the ABI-defined registers and/or memory. - PL\_FLAG\_EXEC When PL\_FLAG\_SCX is set, this flag may be additionally specified to inform that the program being executed by debuggee process has been changed by successful execution of a system call from the **execve**(*2*) family. - PL\_FLAG\_SI Indicates that *pl\_siginfo* member of *struct ptrace\_lwpinfo* contains valid information. - PL\_FLAG\_FORKED Indicates that the process is returning from a call to **fork**(*2*) that created a new child process. The process identifier of the new process is available in the *pl\_child\_pid* member of *struct ptrace\_lwpinfo*. - PL\_FLAG\_CHILD The flag is set for first event reported from a new child which is automatically attached when PTRACE\_FORK is enabled. - PL\_FLAG\_BORN This flag is set for the first event reported from a new LWP when PTRACE\_LWP is enabled. It is reported along with PL\_FLAG\_SCX. - PL\_FLAG\_EXITED This flag is set for the last event reported by an exiting LWP when PTRACE\_LWP is enabled. Note that this event is not reported when the last LWP in a process exits. The termination of the last thread is reported via a normal process exit event. - PL\_FLAG\_VFORKED Indicates that the thread is returning from a call to [vfork(2)](/vfork/2.md) that created a new child process. This flag is set in addition to PL\_FLAG\_FORKED. - PL\_FLAG\_VFORK\_DONE Indicates that the thread has resumed after a child process created via [vfork(2)](/vfork/2.md) has stopped sharing its address space with the traced process. - *pl\_sigmask* The current signal mask of the LWP - *pl\_siglist* The current pending set of signals for the LWP. Note that signals that are delivered to the process would not appear on an LWP siglist until the thread is selected for delivery. - *pl\_siginfo* The siginfo that accompanies the signal pending. Only valid for PL\_EVENT\_SIGNAL stop when PL\_FLAG\_SI is set in *pl\_flags*. - *pl\_tdname* The name of the thread. - *pl\_child\_pid* The process identifier of the new child process. Only valid for a PL\_EVENT\_SIGNAL stop when PL\_FLAG\_FORKED is set in *pl\_flags*. - *pl\_syscall\_code* The ABI-specific identifier of the current system call. Note that for indirect system calls this field reports the indirected system call. Only valid when PL\_FLAG\_SCE or PL\_FLAG\_SCX is set in *pl\_flags*. - *pl\_syscall\_narg* The number of arguments passed to the current system call not counting the system call identifier. Note that for indirect system calls this field reports the arguments passed to the indirected system call. Only valid when PL\_FLAG\_SCE or PL\_FLAG\_SCX is set in *pl\_flags*. - PT\_GETNUMLWPS This request returns the number of kernel threads associated with the traced process. - PT\_GETLWPLIST This request can be used to get the current thread list. A pointer to an array of type *lwpid\_t* should be passed in *addr*, with the array size specified by *data*. The return value from **ptrace**() is the count of array entries filled in. - PT\_SETSTEP This request will turn on single stepping of the specified process. Stepping is automatically disabled when a single step trap is caught. - PT\_CLEARSTEP This request will turn off single stepping of the specified process. - PT\_SUSPEND This request will suspend the specified thread. - PT\_RESUME This request will resume the specified thread. - PT\_TO\_SCE This request will set the PTRACE\_SCE event flag to trace all future system call entries and continue the process. The *addr* and *data* arguments are used the same as for PT\_CONTINUE. - PT\_TO\_SCX This request will set the PTRACE\_SCX event flag to trace all future system call exits and continue the process. The *addr* and *data* arguments are used the same as for PT\_CONTINUE. - PT\_SYSCALL This request will set the PTRACE\_SYSCALL event flag to trace all future system call entries and exits and continue the process. The *addr* and *data* arguments are used the same as for PT\_CONTINUE. - PT\_GET\_SC\_ARGS For the thread which is stopped in either PL\_FLAG\_SCE or PL\_FLAG\_SCX state, that is, on entry or exit to a syscall, this request fetches the syscall arguments. The arguments are copied out into the buffer pointed to by the *addr* pointer, sequentially. Each syscall argument is stored as the machine word. Kernel copies out as many arguments as the syscall accepts, see the *pl\_syscall\_narg* member of the *struct* *ptrace\_lwpinfo*, but not more than the *data* bytes in total are copied. - PT\_GET\_SC\_RET Fetch the system call return values on exit from a syscall. This request is only valid for threads stopped in a syscall exit (the PL\_FLAG\_SCX state). The *addr* argument specifies a pointer to a *struct ptrace\_sc\_ret*, which is defined as follows: ``` struct ptrace_sc_ret { register_t sr_retval[2]; int sr_error; }; ``` The *data* argument is set to the size of the structure. If the system call completed successfully, *sr\_error* is set to zero and the return values of the system call are saved in *sr\_retval*. If the system call failed to execute, *sr\_error* field is set to a positive [errno(2)](/errno/2.md) value. If the system call completed in an unusual fashion, *sr\_error* is set to a negative value: - ERESTART System call will be restarted. - EJUSTRETURN System call completed sucessfully but did not set a return value (for example, [setcontext(2)](/setcontext/2.md) and [sigreturn(2)](/sigreturn/2.md)). - PT\_SET\_SC\_RET Set the current system call return values. This request is only valid for threads stopped in a syscall entry (the PL\_FLAG\_SCE state). The *addr* argument specifies a pointer to a *struct ptrace\_sc\_ret*, the *data* argument is set to the size of the structure. The current system call handler is not executed. After the thread is resumed, it returns to userspace with machine state set as though the system call handler had returned the values passed via the content of the *struct ptrace\_sc\_ret* argument. - PT\_FOLLOW\_FORK This request controls tracing for new child processes of a traced process. If *data* is non-zero, PTRACE\_FORK is set in the traced process's event tracing mask. If *data* is zero, PTRACE\_FORK is cleared from the traced process's event tracing mask. - PT\_LWP\_EVENTS This request controls tracing of LWP creation and destruction. If *data* is non-zero, PTRACE\_LWP is set in the traced process's event tracing mask. If *data* is zero, PTRACE\_LWP is cleared from the traced process's event tracing mask. - PT\_GET\_EVENT\_MASK This request reads the traced process's event tracing mask into the integer pointed to by *addr*. The size of the integer must be passed in *data*. - PT\_SET\_EVENT\_MASK This request sets the traced process's event tracing mask from the integer pointed to by *addr*. The size of the integer must be passed in *data*. - PT\_VM\_TIMESTAMP This request returns the generation number or timestamp of the memory map of the traced process as the return value from **ptrace**(). This provides a low-cost way for the tracing process to determine if the VM map changed since the last time this request was made. - PT\_VM\_ENTRY This request is used to iterate over the entries of the VM map of the traced process. The *addr* argument specifies a pointer to a *struct ptrace\_vm\_entry*, which is defined as follows: ``` struct ptrace_vm_entry { int pve_entry; int pve_timestamp; u_long pve_start; u_long pve_end; u_long pve_offset; u_int pve_prot; u_int pve_pathlen; long pve_fileid; uint32_t pve_fsid; char *pve_path; }; ``` The first entry is returned by setting *pve\_entry* to zero. Subsequent entries are returned by leaving *pve\_entry* unmodified from the value returned by previous requests. The *pve\_timestamp* field can be used to detect changes to the VM map while iterating over the entries. The tracing process can then take appropriate action, such as restarting. By setting *pve\_pathlen* to a non-zero value on entry, the pathname of the backing object is returned in the buffer pointed to by *pve\_path*, provided the entry is backed by a vnode. The *pve\_pathlen* field is updated with the actual length of the pathname (including the terminating null character). The *pve\_offset* field is the offset within the backing object at which the range starts. The range is located in the VM space at *pve\_start* and extends up to *pve\_end* (inclusive). The *data* argument is ignored. - PT\_COREDUMP This request creates a coredump for the stopped program. The *addr* argument specifies a pointer to a *struct ptrace\_coredump*, which is defined as follows: ``` struct ptrace_coredump { int pc_fd; uint32_t pc_flags; off_t pc_limit; }; ``` The fields of the structure are: - pc\_fd File descriptor to write the dump to. It must refer to a regular file, opened for writing. - pc\_flags Flags. The following flags are defined: - PC\_COMPRESS Request compression of the dump. - PC\_ALL Include non-dumpable entries into the dump. The dumper ignores MAP\_NOCORE flag of the process map entry, but device mappings are not dumped even with PC\_ALL set. - pc\_limit Maximum size of the coredump. Specify zero for no limit. The size of *struct ptrace\_coredump* must be passed in *data*. - PT\_SC\_REMOTE Request to execute a syscall in the context of the traced process, in the specified thread. The *addr* argument must point to the *struct ptrace\_sc\_remote*, which describes the requested syscall and its arguments, and receives the result. The size of *struct ptrace\_sc\_remote* must be passed in *data.* ``` struct ptrace_sc_remote { struct ptrace_sc_ret pscr_ret; u_int pscr_syscall; u_int pscr_nargs; u_long *pscr_args; }; ``` The pscr\_syscall contains the syscall number to execute, the pscr\_nargs is the number of supplied arguments, which are supplied in the pscr\_args array. Result of the execution is returned in the pscr\_ret member. Note that the request and its result do not affect the returned value from the currently executed syscall, if any. - PT\_GET\_CHILDREN Returns a report describing the children of the process specified by *pid*. Only children visible to the current thread are reported. If the *addr* argument is NULL, the instantaneous number of children is returned as the result of the request. If the *addr* argument is not NULL, it must point to the array of ``` struct ptrace_child { pid_t pid; int flags; }; ``` elements. The size of the array in bytes must be passed in *data*. The PT\_GET\_CHILDREN implementation fills the array and returns the number of elements filled. For each element, the *pid* member contains the PID of the corresponding child. The *flags* member may have the following flags set: - PTCHLD\_TRACED The child specified by the *pid* member is traced. - PTCHLD\_TRACED\_BY\_ME The child specified by the *pid* member is traced by the caller (not the process specified by the *pid* argument to the function). - PTCHLD\_EXITED The child process is exiting or is a zombie. - PTCHLD\_ORPHAN The child process is currently being debugged by a process other than its parent process. - PT\_GET\_ABI\_NAME Returns the string designating the Application Binary Interface (ABI) of the traced process. Among other details, the ABI defines the interpretation of system calls numbers and arguments. The buffer where the name is returned is pointed to by the *addr* argument, the size of the buffer is passed in the *data* argument. ## PT\_COREDUMP and PT\_SC\_REMOTE usage The process must be stopped before dumping or initiating a remote system call. A single thread in the target process is temporarily unsuspended in the kernel to perform the action. If the **ptrace** call fails before a thread is unsuspended, there is no event to [waitpid(2)](/waitpid/2.md) for. If a thread was unsuspended, it will stop again before the **ptrace** call returns, and the process must be waited upon using [waitpid(2)](/waitpid/2.md) to consume the new stop event. Since it is hard to deduce whether a thread was unsuspended before an error occurred, it is recommended to unconditionally perform [waitpid(2)](/waitpid/2.md) with WNOHANG flag after PT\_COREDUMP and PT\_SC\_REMOTE, and silently accept zero result from it. For PT\_SC\_REMOTE, the selected thread must be stopped in the safe place, which is currently defined as a syscall exit, or a return from kernel to user mode (basically, a signal handler call place). Kernel returns EBUSY status if attempt is made to execute remote syscall at unsafe stop. Note that neither kern.trap\_enotcap sysctl setting, nor the corresponding [procctl(2)](/procctl/2.md) flag PROC\_TRAPCAP\_CTL\_ENABLE are obeyed during the execution of the syscall by PT\_SC\_REMOTE. In other words, SIGTRAP signal is not sent to a process executing in capability mode, which violated a mode access restriction. Note that due to the mode of execution for the remote syscall, in particular, the setting where only one thread is allowed to run, the syscall might block on resources owned by suspended threads. This might result in the target process deadlock. In this situation, the only way out is to kill the target. ## ARM MACHINE-SPECIFIC REQUESTS - PT\_GETVFPREGS Return the thread's VFP machine state in the buffer pointed to by *addr*. The *data* argument is ignored. - PT\_SETVFPREGS Set the thread's VFP machine state from the buffer pointed to by *addr*. The *data* argument is ignored. ## x86 MACHINE-SPECIFIC REQUESTS - PT\_GETXMMREGS Copy the XMM FPU state into the buffer pointed to by the argument *addr*. The buffer has the same layout as the 32-bit save buffer for the machine instruction FXSAVE. This request is only valid for i386 programs, both on native 32-bit systems and on amd64 kernels. For 64-bit amd64 programs, the XMM state is reported as part of the FPU state returned by the PT\_GETFPREGS request. The *data* argument is ignored. - PT\_SETXMMREGS Load the XMM FPU state for the thread from the buffer pointed to by the argument *addr*. The buffer has the same layout as the 32-bit load buffer for the machine instruction FXRSTOR. As with PT\_GETXMMREGS, this request is only valid for i386 programs. The *data* argument is ignored. - PT\_GETXSTATE\_INFO Report which XSAVE FPU extensions are supported by the CPU and allowed in userspace programs. The *addr* argument must point to a variable of type *struct* *ptrace\_xstate\_info*, which contains the information on the request return. *struct* *ptrace\_xstate\_info* is defined as follows: ``` struct ptrace_xstate_info { uint64_t xsave_mask; uint32_t xsave_len; }; ``` The xsave\_mask field is a bitmask of the currently enabled extensions. The meaning of the bits is defined in the Intel and AMD processor documentation. The xsave\_len field reports the length of the XSAVE area for storing the hardware state for currently enabled extensions in the format defined by the x86 XSAVE machine instruction. The *data* argument value must be equal to the size of the *struct* *ptrace\_xstate\_info*. - PT\_GETXSTATE Return the content of the XSAVE area for the thread. The *addr* argument points to the buffer where the content is copied, and the *data* argument specifies the size of the buffer. The kernel copies out as much content as allowed by the buffer size. The buffer layout is specified by the layout of the save area for the XSAVE machine instruction. - PT\_SETXSTATE Load the XSAVE state for the thread from the buffer specified by the *addr* pointer. The buffer size is passed in the *data* argument. The buffer must be at least as large as the *struct* *savefpu* (defined in *x86/fpu.h*) to allow the complete x87 FPU and XMM state load. It must not be larger than the XSAVE state length, as reported by the xsave\_len field from the *struct* *ptrace\_xstate\_info* of the PT\_GETXSTATE\_INFO request. Layout of the buffer is identical to the layout of the load area for the XRSTOR machine instruction. - PT\_GETFSBASE Return the value of the base used when doing segmented memory addressing using the %fs segment register. The *addr* argument points to an *unsigned* *long* variable where the base value is stored. The *data* argument is ignored. - PT\_GETGSBASE Like the PT\_GETFSBASE request, but returns the base for the %gs segment register. - PT\_SETFSBASE Set the base for the %fs segment register to the value pointed to by the *addr* argument. *addr* must point to the *unsigned* *long* variable containing the new base. The *data* argument is ignored. - PT\_SETGSBASE Like the PT\_SETFSBASE request, but sets the base for the %gs segment register. ## PowerPC MACHINE-SPECIFIC REQUESTS - PT\_GETVRREGS Return the thread's ALTIVEC machine state in the buffer pointed to by *addr*. The *data* argument is ignored. - PT\_SETVRREGS Set the thread's ALTIVEC machine state from the buffer pointed to by *addr*. The *data* argument is ignored. - PT\_GETVSRREGS Return doubleword 1 of the thread's VSX registers VSR0-VSR31 in the buffer pointed to by *addr*. The *data* argument is ignored. - PT\_SETVSRREGS Set doubleword 1 of the thread's VSX registers VSR0-VSR31 from the buffer pointed to by *addr*. The *data* argument is ignored. Additionally, other machine-specific requests can exist. ## RETURN VALUES Most requests return 0 on success and -1 on error. Some requests can cause **ptrace**() to return -1 as a non-error value, among them are PT\_READ\_I and PT\_READ\_D, which return the value read from the process memory on success. To disambiguate, *errno* can be set to 0 before the call and checked afterwards. The current **ptrace**() implementation always sets *errno* to 0 before calling into the kernel, both for historic reasons and for consistency with other operating systems. It is recommended to assign zero to *errno* explicitly for forward compatibility. ## ERRORS The **ptrace**() system call may fail if: - \[ESRCH\] - No process having the specified process ID exists. - \[EINVAL\] - A process attempted to use PT\_ATTACH on itself. - The *request* argument was not one of the legal requests. - The signal number (in *data*) to PT\_CONTINUE was neither 0 nor a legal signal number. - PT\_GETREGS, PT\_SETREGS, PT\_GETFPREGS, PT\_SETFPREGS, PT\_GETDBREGS, or PT\_SETDBREGS was attempted on a process with no valid register set. (This is normally true only of system processes.) - PT\_VM\_ENTRY was given an invalid value for *pve\_entry*. This can also be caused by changes to the VM map of the process. - The size (in *data*) provided to PT\_LWPINFO was less than or equal to zero, or larger than the *ptrace\_lwpinfo* structure known to the kernel. - The size (in *data*) provided to the x86-specific PT\_GETXSTATE\_INFO request was not equal to the size of the *struct* *ptrace\_xstate\_info*. - The size (in *data*) provided to the x86-specific PT\_SETXSTATE request was less than the size of the x87 plus the XMM save area. - The size (in *data*) provided to the x86-specific PT\_SETXSTATE request was larger than returned in the xsave\_len member of the *struct* *ptrace\_xstate\_info* from the PT\_GETXSTATE\_INFO request. - The base value, provided to the amd64-specific requests PT\_SETFSBASE or PT\_SETGSBASE, pointed outside of the valid user address space. This error will not occur in 32-bit programs. - \[EBUSY\] - PT\_ATTACH was attempted on a process that was already being traced. - A request attempted to manipulate a process that was being traced by some process other than the one making the request. - A request (other than PT\_ATTACH) specified a process that was not stopped. - \[EPERM\] - A request (other than PT\_ATTACH) attempted to manipulate a process that was not being traced at all. - An attempt was made to use PT\_ATTACH on a process in violation of the requirements listed under PT\_ATTACH above. - \[ENOENT\] - PT\_VM\_ENTRY previously returned the last entry of the memory map. No more entries exist. - \[ENOMEM\] - A PT\_READ\_I, PT\_READ\_D, PT\_WRITE\_I, or PT\_WRITE\_D request attempted to access an invalid address, or a memory allocation failure occurred when accessing process memory. - \[ENAMETOOLONG\] - PT\_VM\_ENTRY cannot return the pathname of the backing object because the buffer is not big enough. *pve\_pathlen* holds the minimum buffer size required on return. - \[ECAPMODE\] The process issuing the **ptrace**() call is in capability mode. ## SEE ALSO [execve(2)](/execve/2.md), pdptrace(2), [sigaction(2)](/sigaction/2.md), [wait(2)](/wait/2.md), [execv(3)](/execv/3.md), i386\_clr\_watch(3), i386\_set\_watch(3) ## HISTORY The **ptrace**() function appeared in Version 6 AT&T UNIX.