PTRACE(2) BSD System Calls Manual PTRACE(2)
ptrace -- process tracing and debugging
ptrace(int request, pid_t pid, caddr_t addr, int data);
ptrace() provides tracing and debugging facilities. It allows one process (the tracing
process) to control another (the traced process). Most of the time, the traced process runs
normally, but when it receives a signal (see sigaction(2)), it stops. The tracing process
is expected to notice this via wait(2) or the delivery of a SIGCHLD signal, examine the
state of the stopped process, and cause it to terminate or continue as appropriate.
ptrace() is the mechanism by which all this happens.
The request argument specifies what operation is being performed; the meaning of the rest of
the arguments depends on the operation, but except for one special case noted below, all
ptrace() calls are made by the tracing process, and the pid argument specifies the process
ID of the traced process. request can be:
PT_TRACE_ME This request is the only one used by the traced process; it declares that the
process expects to be traced by its parent. All the other arguments are
ignored. (If the parent process does not expect to trace the child, it will
probably be rather confused by the results; once the traced process stops, it
cannot be made to continue except via ptrace().) When a process has used this
request and calls execve(2) or any of the routines built on it (such as
execv(3)), it will stop before executing the first instruction of the new
image. Also, any setuid or setgid bits on the executable being executed will
These requests read a single int of data from the traced process' address
space. Traditionally, ptrace() has allowed for machines with distinct address
spaces for instruction and data, which is why there are two requests: concep-
tually, PT_READ_I reads from the instruction space and PT_READ_D reads from
the data space. In the current OpenBSD implementation, these two requests are
completely identical. The addr argument specifies the address (in the traced
process' virtual address space) at which the read is to be done. This address
does not have to meet any alignment constraints. The value read is returned
as the return value from ptrace().
These requests parallel PT_READ_I and PT_READ_D, except that they write rather
than read. The data argument supplies the value to be written.
PT_CONTINUE The traced process continues execution. addr is an address specifying the
place where execution is to be resumed (a new value for the program counter),
or (caddr_t)1 to indicate that execution is to pick up where it left off.
data provides a signal number to be delivered to the traced process as it
resumes execution, or 0 if no signal is to be sent.
PT_KILL The traced process terminates, as if PT_CONTINUE had been used with SIGKILL
given as the signal to be delivered.
PT_ATTACH This request allows a process to gain control of an otherwise unrelated
process and begin tracing it. It does not need any cooperation from the to-
be-traced process. In this case, pid specifies the process ID of the to-be-
traced process, and the other two arguments are ignored. This request
requires that the target process must have the same real UID as the tracing
process, and that it must not be executing a setuid or setgid executable. (If
the tracing process is running as root, these restrictions do not apply.) The
tracing process will see the newly-traced process stop and may then control it
as if it had been traced all along.
PT_DETACH This request is like PT_CONTINUE, except that it does not allow specifying an
alternate place to continue execution, and after it succeeds, the traced
process is no longer traced and continues execution normally.
Additionally, machine-specific requests can exist. On the SPARC, these are:
PT_GETREGS This request reads the traced process' machine registers into the ``struct
reg'' (defined in <machine/reg.h>) pointed to by addr.
PT_SETREGS This request is the converse of PT_GETREGS; it loads the traced process'
machine registers from the ``struct reg'' (defined in <machine/reg.h>) pointed
to by addr.
PT_GETFPREGS This request reads the traced process' floating-point registers into the
``struct fpreg'' (defined in <machine/reg.h>) pointed to by addr.
PT_SETFPREGS This request is the converse of PT_GETFPREGS; it loads the traced process'
floating-point registers from the ``struct fpreg'' (defined in
<machine/reg.h>) pointed to by addr.
Some requests can cause ptrace() to return -1 as a non-error value; to disambiguate, errno
can be set to 0 before the call and checked afterwards. The possible errors are:
No process having the specified process ID exists.
o A process attempted to use PT_ATTACH on itself.
o The request was not one of the legal requests.
o The signal number (in data) to PT_CONTINUE was neither 0 nor a legal signal num-
o PT_GETREGS, PT_SETREGS, PT_GETFPREGS, or PT_SETFPREGS was attempted on a process
with no valid register set. (This is normally true only of system processes.)
o PT_ATTACH was attempted on a process that was already being traced.
o A request attempted to manipulate a process that was being traced by some process
other than the one making the request.
o A request (other than PT_ATTACH) specified a process that wasn't stopped.
o A request (other than PT_ATTACH) attempted to manipulate a process that wasn't
being traced at all.
o An attempt was made to use PT_ATTACH on a process in violation of the requirements
listed under PT_ATTACH above.
On the SPARC, the PC is set to the provided PC value for PT_CONTINUE and similar calls, but
the NPC is set willy-nilly to 4 greater than the PC value. Using PT_GETREGS and PT_SETREGS
to modify the PC, passing (caddr_t)1 to ptrace(), should be able to sidestep this.
Single-stepping is not available.
BSD November 7, 1994 BSD