ptrace(2)ptrace(2)NAMEptrace - process trace
SYNOPSIS
#include <unistd.h>
#include <sys/types.h>
int ptrace(int request, pid_t pid, int addr, int data);
DESCRIPTIONptrace allows a parent process to control the execution of a child
process. Its primary use is for the implementation of breakpoint
debugging [see dbx(1)]. The child process behaves normally until it
encounters a signal [see signal(5)], at which time it enters a stopped
state and its parent is notified via the wait(2) system call. 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(2)]. 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 a value of -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 a value of -1 is returned to
the parent process and the parent's errno is set to EIO.
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ptrace(2)ptrace(2)
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 a value of -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 a value of -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 exec(2) calls. If a traced process calls exec(2), the call
fails with errno ENOEXEC. ptrace in general fails if one or more of the
following are true:
EIO request is an illegal number.
ESRCH pid identifies a child that does not exist or has not
executed a ptrace with request 0.
SEE ALSOdbx(1), exec(2), signal(2), wait(2).
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