PTRACE(2)

NAME
ptrace - process tracing and debugging

SYNOPSIS
#include <sys/types.h>
#include <sys/ptrace.h>
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)), it stops. The tracing process is expected to notice this via wait(2) 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, 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. request 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) or any of the routines built on it ( such as execv(3)), 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_READ_I, PT_READ_D
    These requests read a single int of data from the traced process' 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' 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_READ_U
    This request reads an int from the traced process' user structure. The addr argument specifies the location of the int relative to the base of the user structure; it will usually be an integer value cast to caddr_t either explicitly or via the presence of a prototype for ptrace(). Unlike PT_READ_I and PT_READ_D, addr must be aligned on an int boundary. The value read is returned as the return value from ptrace().

PT_WRITE_U
    This request writes an int into the traced process' user structure. addr specifies the offset, just as for PT_READ_U, and data specifies the value to be written, just as for PT_WRITE_I and PT_WRITE_D.

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_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 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.

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.

Additionally, machine-specific requests can exist. On the i386, these are:

PT_GETREGS
    This request reads the traced process' 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' machine registers from the “struct reg” (defined in <machine/reg.h>) pointed to by addr.

PT_GETFPREGS
    This request reads the traced process' 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' floating-point registers from the “struct fpreg” (defined in <machine/reg.h>) pointed to by addr.

RETURN VALUES
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.

ERRORS
The ptrace() function may fail if:

[ESRCH]
    •   No process having the specified process ID exists.

[EINVAL]
    •   A process attempted to use PT_ATTACH on itself.
    •   The request was not one of the legal requests.
    •   The addr to PT_READ_U or PT_WRITE_U was not int-aligned.
    •   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_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 wasn't stopped.

[EPERM]
    •   A request (other than PT_ATTACH) attempted to manipulate a process that wasn't 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.

SEE ALSO
execve(2), sigaction(2), wait(2), execv(3)

HISTORY
A ptrace() function call appeared in AT&T UNIX.