ptrace(2)
NAME
ptrace − process trace
SYNTAX
#include <signal.h>
ptrace(request, pid, addr, data)
int request, pid, *addr, data;
DESCRIPTION
The ptrace system call provides a means by which a parent process may control the execution of a child process, and examine and change its core image. Its primary use is for the implementation of breakpoint debugging. There are four arguments whose interpretation depends on a request argument. Generally, pid is the process ID of the traced process, which must be a child (no more distant descendant) of the tracing process. A process being traced behaves normally until it encounters some signal whether internally generated like “illegal instruction” or externally generated like “interrupt”. See sigvec() for the list. Then the traced process enters a stopped state and its parent is notified via wait(.). When the child is in the stopped state, its core image can be examined and modified using ptrace. If desired, another ptrace request can then cause the child either to terminate or to continue, possibly ignoring the signal.
The value of the request argument determines the precise action of the call:
0 This request is the only one used by the child process; it declares that the process is to be traced by its parent. All the other arguments are ignored. Peculiar results will ensue if the parent does not expect to trace the child.
1,2 The word in the child process’s address space at addr is returned. If I and D space are separated (for example, historically on a pdp-11), request 1 indicates I space, 2 D space. The addr must be even. The child must be stopped. The input data is ignored.
3 The word of the system’s per-process data area corresponding to addr is returned. The addr must be even and less than 512. This space contains the registers and other information about the process; its layout corresponds to the user structure in the system.
4,5 The given data is written at the word in the process’s address space corresponding to addr, which must be even. No useful value is returned. If I and D space are separated, request 4 indicates I space, 5 D space. Attempts to write in pure procedure fail if another process is executing the same file.
6 The process’s system data is written, as it is read with request 3. Only a few locations can be written in this way: the general registers, the floating point status and registers, and certain bits of the processor status word.
7 The data argument is taken as a signal number and the child’s execution continues at location addr as if it had incurred that signal. Normally the signal number will be either 0 to indicate that the signal that caused the stop should be ignored, or that value fetched out of the process’s image indicating which signal caused the stop. If addr is (int *)1 then execution continues from where it stopped.
8 The traced process terminates.
9 Execution continues as in request 7; however, as soon as possible after execution of at least one instruction, execution stops again. The signal number from the stop is SIGTRAP. (On the VAX-11 the T-bit is used and just one instruction is executed.) This is part of the mechanism for implementing breakpoints.
As indicated, these calls (except for request 0) can be used only when the subject process has stopped. The wait call is used to determine when a process stops; in such a case the “termination” status returned by wait has the value 0177 to indicate stoppage rather than genuine termination.
To forestall possible fraud, ptrace inhibits the set-user-id and set-group-id facilities on subsequent execve() calls. If a traced process calls execve, it will stop before executing the first instruction of the new image showing signal SIGTRAP.
On a VAX, “word” also means a 32-bit integer, but the “even” restriction does not apply.
ENVIRONMENT
When your program is compiled using the System V environment, requests 7 and 9 return the value of the data argument on success, errno is set to ESRCH if the pid does not exist, EIO if the address is out of bounds.
RETURN VALUE
A 0 value is returned if the call succeeds. If the call fails then a −1 is returned and the global variable errno is set to indicate the error.
RESTRICTIONS
In ULTRIX, the ptrace system call will only succeed if the user owns the binary being traced or if the user is root.
DIAGNOSTICS
[EIO] The request code is invalid.
[ESRCH] The specified process does not exist.
[EIO] The given signal number is invalid.
[EIO] The specified address is out of bounds.
[EPERM] The specified process cannot be traced.