PTRACE(2) BSD System Calls Manual PTRACE(2)NAMEptrace — process tracing and debugging
ptrace(int request, pid_t pid, caddr_t addr, int data);
DESCRIPTIONptrace() 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 par‐
ent. All the other arguments are ignored. (If the parent
process does not expect to trace the child, it will proba‐
bly 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 instruc‐
tion 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 align‐
ment 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 sup‐
plies 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 indi‐
cate 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
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 other‐
wise 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
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 reg‐
isters 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.
· A process attempted to use PT_ATTACH on itself.
· The request was not one of the legal requests.
· The signal number (in data) to PT_CONTINUE was neither 0 nor a
legal signal number.
· 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.)
· PT_ATTACH was attempted on a process that was already being
· A request attempted to manipulate a process that was being
traced by some process other than the one making the request.
· A request (other than PT_ATTACH) specified a process that
· A request (other than PT_ATTACH) attempted to manipulate a
process that wasn't being traced at all.
· 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