ptrace(2) System Calls Manual ptrace(2)
Name
ptrace - process trace
Syntax
#include <signal.h>
#include <sys/ptrace.h>
ptrace(request, pid, addr, data)
int request, pid, *addr, data;
Description
The system call provides a means by which a process can control the execution of another process, and examine and change its core image.
Its primary use is for the implementation of breakpoint debugging. There are four arguments whose interpretation depends on a request
argument. Generally, pid is the process ID of the traced process. A process being traced behaves normally until it encounters some sig-
nal, whether internally generated, like "illegal instruction," or externally generated, like "interrupt." For more information, see
Upon encountering a signal, the traced process enters a stopped state and its tracing process is notified by means of If the traced process
stops with a SIGTRAP, the process might have been stopped for a number of reasons. Two status words addressable as registers in the traced
process's uarea qualify SIGTRAPs: TRAPCAUSE, which contains the cause of the trap, and TRAPINFO, which contains extra information concern-
ing the trap.
When the traced process is in the stopped state, its core image can be examined and modified using If desired, another request can then
cause the traced process either to terminate or to continue, possibly ignoring the signal.
The value of the request argument determines the precise action of the call:
0 This request is the only one that can be used by a child process. The child process can declare that it is to be traced by its parent.
All other arguments are ignored. Unexpected results occur if the parent process does not expect to trace the child process.
1,2 The word in the traced process's address space at addr is returned. If I and D space are separated (for example, historically on a
PDP-11), request 1 indicates I space, request 2 indicates D space. The addr must be 4-byte aligned. The traced process must be
stopped. The input data is ignored.
3 The word of the system's per-process data area corresponding to addr is returned. The addr is a constant defined in ptrace.h. This
space contains the registers and other information about the process; the constants correspond to fields in the user structure in the
system.
4,5 The given data is written at the word in the process's address space corresponding to addr, which must be 4-byte aligned. The old
value at the address is returned. If I and D space are separated, request 4 indicates I space, request 5 indicates D space. Attempts
to write in pure procedure fail if another process is executing the same file.
6 The process's system data is written, as it is read with request 3. Only a few locations can be written in this way: the general reg-
isters, the floating point status and registers, and certain bits of the processor status word. The old value at the address is
returned.
7 The data argument is taken as a signal number and the traced process's execution continues at location addr as if it had incurred that
signal. Normally, the signal number is 0 to indicate that the signal causing the stop should be ignored. The signal number might be
the value fetched out of the process's image, which identifies the signal that caused the stop. If addr is (int *)1, execution contin-
ues from where it stopped.
8 The traced process terminates.
9 Execution continues as in request 7; however, as soon as possible after execution of at least one instruction, execution stops again.
The signal number from the stop is SIGTRAP. TRAPCAUSE contains CAUSESINGLE. This is part of the mechanism for implementing break-
points.
20 This is the same as zero, except it is executed by the tracing process and the pid field is nonzero. The process with that pid stops
and becomes a traced process. On a signal, the traced process returns control to the tracing process, rather than the parent. The
tracing process must have the same uid as the traced process.
21,22
Returns MAXREG general or MAXFREG floating registers, respectively. Their values are copied to the locations starting at the address
in the tracing process specified by the addr argument.
24,25
Same as 20 and 21, but writes the registers instead of reading them.
26 Specifies a watchpoint in the data or stack segment of the traced process. If any byte address starting at the addr argument and con-
tinuing for the number of bytes specified by the data argument is accessed in an instruction, the traced process stops execution with a
SIGTRAP. TRAPCAUSE contains CAUSEWATCH, and TRAPINFO contains the address causing the trap. This ptrace returns a watchpoint identi-
fier (wid). MAXWIDS specifies the maximum number of watchpoints for each process.
27 The data argument specifies a wid to delete.
28 Turns off the tracing for the traced process with the specified pid.
29 Returns an open file descriptor for the file attached to pid. This request is useful in accessing the symbol table of a process cre-
ated with the call.
As indicated, these calls (except for request 0 and 20) can be used only when the subject process has stopped. The wait call is used to
determine when a process stops; in such a case, the "termination" status returned by wait has the value 0177. This value indicates that the
process has stopped, rather than terminated. If multiple processes are being traced, can be called multiple times, and it returns the sta-
tus for the next stopped or terminated child or traced process.
To forestall possible fraud, inhibits the set-user-id and set-group-id facilities on subsequent calls. If a traced process calls it stops
before executing the first instruction of the new image showing signal SIGTRAP. In this case, TRAPCAUSE contains CAUSEEXEC and TRAPINFO
does not contain anything interesting. If a traced process calls again, the same thing occurs.
If a traced process forks, both parent and child are traced. Breakpoints from the parent are not copied into the child. At the time of
the fork, the child is stopped with a SIGTRAP. The tracing process can then terminate the trace if desired. TRAPCAUSE contains CAUSEFORK
and TRAPINFO contains the pid of its parent.
Restrictions
On an ULTRIX system, the system call succeeds only if the user owns the binary being traced or if the user is root.
The request 0 call should be able to specify signals that are to be treated normally and not cause a stop. In this way, for example, pro-
grams with simulated floating point, which use "illegal instruction" signals at a very high rate, can be efficiently debugged.
The error indication, -1, is a legitimate function value; when an error occurs, the errno variable is set to explain the condition that
caused the error.
It should be possible to stop a process on occurrence of a system call; in this way, a completely controlled environment could be provided.
Return Values
A 0 value is returned if the call succeeds. If the call fails, a -1 is returned, and the global variable errno is set to indicate the
error.
Diagnostics
The call fails under the following conditions:
[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
dbx(1), wait(2), sigvec(2)
RISC ptrace(2)