PTRACE.2
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- PTRACE(2) Minix Programmer's Manual PTRACE(2)
- NAME
- ptrace - process trace
- SYNOPSIS
- #include <sys/types.h>
- #include <sys/signal.h>
- #include <sys/ptrace.h>
- int ptrace(int request, pid_t pid, long addr, long data)
- DESCRIPTION
- Note: This manual page has no relation to Minix. Someone who knows
- ptrace() has to check, or rewrite, this page. (kjb)
- Ptrace provides a means by which a parent process may control the
- execution of a child 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, 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 "interrupt". See sigaction(2) for the list. Then the traced
- process enters a stopped state and its parent is notified via wait(2).
- When the child is in the stopped state, its core image can be examined
- and modified using ptrace. If desired, another ptrace request can then
- cause the child either to terminate or to continue, possibly ignoring the
- signal.
- The value of the request 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 addr is returned.
- If I and D space are separated (e.g. historically on a pdp-11),
- request PT_READ_I indicates I space, PT_READ_D D space. Addr must be
- even on some machines. The child must be stopped. The input data is
- ignored.
- PT_READ_U
- The word of the system's per-process data area corresponding to addr
- is returned. Addr 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 user structure in the system.
- 4BSD May 23, 1986 1
-
- PTRACE(2) Minix Programmer's Manual PTRACE(2)
- PT_WRITE_I, PT_WRITE_D
- The given data is written at the word in the process's address space
- corresponding to addr, 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 data argument is taken as a signal number and the child's
- execution continues at location addr 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 addr 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 wait call is used to
- determine when a process stops; in such a case the "termination" status
- returned by wait has the value 0177 to indicate stoppage rather than
- genuine termination.
- To forestall possible fraud, ptrace inhibits the set-user-id and set-
- group-id facilities on subsequent execve(2) calls. If a traced process
- calls execve, it will stop before executing the first instruction of the
- new image showing signal SIGTRAP.
- On a VAX-11, "word" also means a 32-bit integer, but the "even"
- restriction does not apply.
- 4BSD May 23, 1986 2
-
- PTRACE(2) Minix Programmer's Manual PTRACE(2)
- RETURN VALUE
- A 0 value is returned if the call succeeds. If the call fails then a -1
- is returned and the global variable errno 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), sigaction(2), mdb(1).
- BUGS
- Ptrace 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 ioctl(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 signals 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; errno, (see
- intro(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.
- 4BSD May 23, 1986 3
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