PTRACE(2-SVR4) RISC/os Reference Manual PTRACE(2-SVR4)
NAME
ptrace - process trace
SYNOPSIS
#include <unistd.h>
#include <sys/types.h>
int ptrace(int request, pidt pid, int addr, int data);
DESCRIPTION
ptrace 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 sig-
nal(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 B 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
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PTRACE(2-SVR4) RISC/os Reference Manual PTRACE(2-SVR4)
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.
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 argu-
ment must be equal to 1 for this request. On suc-
cess, 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.
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PTRACE(2-SVR4) RISC/os Reference Manual PTRACE(2-SVR4)
To forestall possible fraud, ptrace inhibits the set-user-ID
facility on subsequent exec(2) calls. If a traced process
calls exec(2), it stops before executing the first instruc-
tion of the new image showing signal SIGTRAP. ptrace in gen-
eral 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.
EPERM the invoking subject does not have the appropri-
ate MAC privileges.
SEE ALSO
dbx(1), exec(2), proc(4), signal(2), wait(2).
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