ptrace(3C) Standard C Library Functions ptrace(3C)NAMEptrace - 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 break‐
point 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 sepa‐
rated, 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 execu‐
tion. 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 con‐
sequences 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 inter‐
faces.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Standard │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOexec(2), exit(2), libc(3LIB), signal(3C), signal.h(3HEAD), wait(3C),
proc(4), attributes(5)SunOS 5.10 22 Mar 2004 ptrace(3C)
