4.3BSD-UWisc/man/cat2/ptrace.2
PTRACE(2) UNIX Programmer's Manual PTRACE(2)
NAME
ptrace - process trace
SYNOPSIS
#include <sys/signal.h>
#include <sys/ptrace.h>
ptrace(request, pid, addr, data)
int request, pid, *addr, data;
DESCRIPTION
_P_t_r_a_c_e provides a means by which a parent process may con-
trol the execution of a child process, and examine and
change its core image. Its primary use is for the implemen-
tation of breakpoint debugging. There are four arguments
whose interpretation depends on a _r_e_q_u_e_s_t argument. Gen-
erally, _p_i_d is the process ID of the traced process, which
must be a child (no more distant descendant) of the tracing
process. A process being traced behaves normally until it
encounters some signal whether internally generated like
"illegal instruction" or externally generated like "inter-
rupt". See _s_i_g_v_e_c(2) for the list. Then the traced process
enters a stopped state and its parent is notified via
_w_a_i_t(2). When the child is in the stopped state, its core
image can be examined and modified using _p_t_r_a_c_e. If
desired, another _p_t_r_a_c_e request can then cause the child
either to terminate or to continue, possibly ignoring the
signal.
The value of the _r_e_q_u_e_s_t argument determines the precise
action of the call:
PT_TRACE_ME
This request is the only one used by the child process;
it declares that the process is to be traced by its
parent. All the other arguments are ignored. Peculiar
results will ensue if the parent does not expect to
trace the child.
PT_READ_I, PT_READ_D
The word in the child process's address space at _a_d_d_r is
returned. If I and D space are separated (e.g. histori-
cally on a pdp-11), request PT_READ_I indicates I space,
PT_READ_D D space. _A_d_d_r must be even on some machines.
The child must be stopped. The input _d_a_t_a is ignored.
PT_READ_U
The word of the system's per-process data area
corresponding to _a_d_d_r is returned. _A_d_d_r must be even on
some machines and less than 512. This space contains
the registers and other information about the process;
its layout corresponds to the _u_s_e_r structure in the
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PTRACE(2) UNIX Programmer's Manual PTRACE(2)
system.
PT_WRITE_I, PT_WRITE_D
The given _d_a_t_a is written at the word in the process's
address space corresponding to _a_d_d_r, which must be even
on some machines. No useful value is returned. If I
and D space are separated, request PT_WRITE_I indicates
I space, PT_WRITE_D D space. Attempts to write in pure
procedure fail if another process is executing the same
file.
PT_WRITE_U
The process's system data is written, as it is read with
request PT_READ_U. Only a few locations can be written
in this way: the general registers, the floating point
status and registers, and certain bits of the processor
status word.
PT_CONTINUE
The _d_a_t_a argument is taken as a signal number and the
child's execution continues at location _a_d_d_r as if it
had incurred that signal. Normally the signal number
will be either 0 to indicate that the signal that caused
the stop should be ignored, or that value fetched out of
the process's image indicating which signal caused the
stop. If _a_d_d_r is (int *)1 then execution continues from
where it stopped.
PT_KILL
The traced process terminates.
PT_STEP
Execution continues as in request PT_CONTINUE; however,
as soon as possible after execution of at least one
instruction, execution stops again. The signal number
from the stop is SIGTRAP. (On the VAX-11 the T-bit is
used and just one instruction is executed.) This is part
of the mechanism for implementing breakpoints.
As indicated, these calls (except for request PT_TRACE_ME)
can be used only when the subject process has stopped. The
_w_a_i_t call is used to determine when a process stops; in such
a case the "termination" status returned by _w_a_i_t has the
value 0177 to indicate stoppage rather than genuine termina-
tion.
To forestall possible fraud, _p_t_r_a_c_e inhibits the set-user-id
and set-group-id facilities on subsequent _e_x_e_c_v_e(2) calls.
If a traced process calls _e_x_e_c_v_e, it will stop before exe-
cuting the first instruction of the new image showing signal
SIGTRAP.
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PTRACE(2) UNIX Programmer's Manual PTRACE(2)
On a VAX-11, "word" also means a 32-bit integer, but the
"even" restriction does not apply.
RETURN VALUE
A 0 value is returned if the call succeeds. If the call
fails then a -1 is returned and the global variable _e_r_r_n_o is
set to indicate the error.
ERRORS
[EIO] The request code is invalid.
[ESRCH] The specified process does not exist.
[EIO] The given signal number is invalid.
[EIO] The specified address is out of bounds.
[EPERM] The specified process cannot be traced.
SEE ALSO
wait(2), sigvec(2), adb(1)
BUGS
_P_t_r_a_c_e is unique and arcane; it should be replaced with a
special file that can be opened and read and written. The
control functions could then be implemented with _i_o_c_t_l(2)
calls on this file. This would be simpler to understand and
have much higher performance.
The request PT_TRACE_ME call should be able to specify sig-
nals that are to be treated normally and not cause a stop.
In this way, for example, programs with simulated floating
point (which use "illegal instruction" signals at a very
high rate) could be efficiently debugged.
The error indication, -1, is a legitimate function value;
_e_r_r_n_o, (see _i_n_t_r_o(2)), can be used to disambiguate.
It should be possible to stop a process on occurrence of a
system call; in this way a completely controlled environment
could be provided.
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