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 con-
fused 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 be ignored.
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: conceptually, 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
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 after-
wards. 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 number.
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.
November 7, 1994 BSD