--- name: "ptrace" section: "2" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/ptrace" --- # ptrace(2) process tracing and debugging ## NAME **ptrace** – process tracing and debugging ## SYNOPSIS **#include \** **#include \** *int*\ **ptrace**(*int request*, *pid\_t pid*, *caddr\_t addr*, *int data*); ## DESCRIPTION **ptrace**() provides tracing and debugging facilities. It allows one process (the *tracing* process) to control another (the *traced* process). 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. **ptrace**() is the mechanism by which all this happens. The *request* argument specifies what operation is being performed; the meaning of the rest of the arguments depends on the operation. The following **ptrace**() operations are made by the traced process, in which case the *pid* argument is ignored: - PT\_TRACE\_ME This request 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. - PT\_DENY\_ATTACH This request allows a process that is not currently being traced to deny future traces by its parent. All other arguments are ignored. If the process is currently being traced, it will exit with the exit status of ENOTSUP; otherwise, it sets a flag that denies future traces. An attempt by the parent to trace a process which has set this flag will result in a segmentation violation in the parent. - PT\_SIGEXC This request causes any signals from the calling process to be delivered as Mach exceptions on the task's exception port (see EXC\_SOFT\_SIGNAL). All other arguments are ignored. The calling process must be actively marked as traced via PT\_TRACE\_ME or PT\_ATTACHEXC. The following **ptrace**() operations are made by the tracing process, and the *pid* argument specifies the process ID of the traced process: - PT\_CONTINUE The traced process continues execution. *addr* 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. *data* 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 continues execution for a single step. The parameters are identical to those passed to PT\_CONTINUE. - 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 call has been replaced with PT\_ATTACHEXC. - PT\_ATTACHEXC 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 to-be-traced process. In this case, *pid* specifies the process ID of the to-be-traced process, 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. Note that this call differs from the prior call (PT\_ATTACH) in that signals from the child are delivered to the parent as Mach exceptions on the task's exception port (see EXC\_SOFT\_SIGNAL). - PT\_DETACH This request is like PT\_CONTINUE, except that *addr* and *data* are ignored, and after it succeeds, the traced process is no longer traced and continues execution normally. ## ERRORS Some requests can cause **ptrace**() to return -1 as a non-error value; to disambiguate, *errno* can be set to 0 before the call and checked afterwards. The possible errors are: - \[ESRCH\] No process having the specified process ID exists. - \[EINVAL\] - A process attempted to use PT\_ATTACHEXC on itself. - A process attempted to use PT\_SIGEXC while not actively traced. - The *request* 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, or PT\_SETFPREGS was attempted on a process with no valid register set. (This is normally true only of system processes.) - \[EBUSY\] - PT\_ATTACHEXC 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\_ATTACHEXC) specified a process that wasn't stopped. - \[EPERM\] - A request (other than PT\_ATTACHEXC) attempted to manipulate a process that wasn't being traced at all. - An attempt was made to use PT\_ATTACHEXC on a process in violation of the requirements listed under PT\_ATTACHEXC above.