man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

ptrace(3C)

Name

ptrace - allows a parent process to control the execution of a child process

Synopsis

#include <unistd.h>
#include <sys/types.h>

int ptrace(int request, pid_t pid, int addr, int data);

Description

The ptrace() function allows a parent process to control the execution of a child process. Its primary use is for the implementation of breakpoint debugging. The child process behaves normally until it encounters a signal (see signal.h(3HEAD)), at which time it enters a stopped state and its parent is notified by the wait(3C) function. When the child is in the stopped state, its parent can examine and modify its “core image” using ptrace(). Also, the parent can cause the child either to terminate or continue, with the possibility of ignoring the signal that caused it to stop.

The request argument determines the action to be taken by ptrace() and is one of the following:

0

This request must be issued by the child process if it is to be traced by its parent. It turns on the child's trace flag that stipulates that the child should be left in a stopped state on receipt of a signal rather than the state specified by func (see signal(3C)). The pid, addr, and data arguments are ignored, and a return value is not defined for this request. Peculiar results ensue if the parent does not expect to trace the child.

The remainder of the requests can only be used by the parent process. For each, pid is the process ID of the child. The child must be in a stopped state before these requests are made.

1, 2

With these requests, the word at location addr in the address space of the child is returned to the parent process. If instruction and data space are separated, request 1 returns a word from instruction space, and request 2 returns a word from data space. If instruction and data space are not separated, either request 1 or request 2 may be used with equal results. The data argument is ignored. These two requests fail if addr is not the start address of a word, in which case −1 is returned to the parent process and the parent's errno is set to EIO.

3

With this request, the word at location addr in the child's user area in the system's address space (see <sys/user.h>) is returned to the parent process. The data argument is ignored. This request fails if addr is not the start address of a word or is outside the user area, in which case −1 is returned to the parent process and the parent's errno is set to EIO.

4, 5

With these requests, the value given by the data argument is written into the address space of the child at location addr. If instruction and data space are separated, request 4 writes a word into instruction space, and request 5 writes a word into data space. If instruction and data space are not separated, either request 4 or request 5 may be used with equal results. On success, the value written into the address space of the child is returned to the parent. These two requests fail if addr is not the start address of a word. On failure −1 is returned to the parent process and the parent's errno is set to EIO.

6

With this request, a few entries in the child's user area can be written. data gives the value that is to be written and addr is the location of the entry. The few entries that can be written are the general registers and the condition codes of the Processor Status Word.

7

This request causes the child to resume execution. If the data argument is 0, all pending signals including the one that caused the child to stop are canceled before it resumes execution. If the data argument is a valid signal number, the child resumes execution as if it had incurred that signal, and any other pending signals are canceled. The addr argument must be equal to 1 for this request. On success, the value of data is returned to the parent. This request fails if data is not 0 or a valid signal number, in which case −1 is returned to the parent process and the parent's errno is set to EIO.

8

This request causes the child to terminate with the same consequences as exit(2).

9

This request sets the trace bit in the Processor Status Word of the child and then executes the same steps as listed above for request 7. The trace bit causes an interrupt on completion of one machine instruction. This effectively allows single stepping of the child.

To forestall possible fraud, ptrace() inhibits the set-user-ID facility on subsequent calls to one of the exec family of functions (see exec( 2)). If a traced process calls one of these functions, it stops before executing the first instruction of the new image showing signal SIGTRAP.

Errors

The ptrace() function will fail if:

EIO

The request argument is an illegal number.

EPERM

The calling process does not have appropriate privileges to control the calling process. See proc(4).

ESRCH

The pid argument identifies a child that does not exist or has not executed a ptrace() call with request 0.

Usage

The ptrace() function is available only with the 32-bit version of libc(3LIB). It is not available with the 64-bit version of this library.

The /proc debugging interfaces should be used instead of ptrace(), which provides quite limited debugger support and is itself implemented using the /proc interfaces. There is no actual ptrace() system call in the kernel. See proc(4) for descriptions of the /proc debugging interfaces.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe
Standard

See Also

exec(2), exit(2), libc(3LIB), signal(3C), signal.h(3HEAD), wait(3C), proc(4), attributes(5)