ptrace -- process tracing and debugging
Standard C Library (libc, -lc)
#include <sys/types.h>
#include <sys/ptrace.h>
int
ptrace(int request, pid_t pid, caddr_t addr, int data);
The ptrace() system call 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. The ptrace() system call 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. The request argument 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 be ignored.
PT_READ_I, PT_READ_D
These requests read a single int of data from the traced
process's 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
FreeBSD implementation, these two requests are completely
identical. The addr argument specifies the address (in the
traced process's 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().
PT_WRITE_I, PT_WRITE_D
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_IO This request allows reading and writing arbitrary amounts
of data in the traced process's address space. The addr
argument specifies a pointer to a struct ptrace_io_desc,
which is defined as follows:
struct ptrace_io_desc {
int piod_op; /* I/O operation */
void *piod_offs; /* child offset */
void *piod_addr; /* parent offset */
size_t piod_len; /* request length */
};
/*
* Operations in piod_op.
*/
#define PIOD_READ_D 1 /* Read from D space */
#define PIOD_WRITE_D 2 /* Write to D space */
#define PIOD_READ_I 3 /* Read from I space */
#define PIOD_WRITE_I 4 /* Write to I space */
The data argument is ignored. The actual number of bytes
read or written is stored in piod_len upon return.
PT_CONTINUE The traced process continues execution. The addr argument
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. The data argument 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_STEP The traced process is single stepped one instruction. The
addr argument should be passed (caddr_t)1. The data argument
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.
PT_GETREGS This request reads the traced process's 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's machine registers from the ``struct reg''
(defined in <machine/reg.h>) pointed to by addr.
PT_GETFPREGS This request reads the traced process's 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's floating-point registers from the ``struct
fpreg'' (defined in <machine/reg.h>) pointed to by addr.
PT_GETDBREGS This request reads the traced process's debug registers
into the ``struct dbreg'' (defined in <machine/reg.h>)
pointed to by addr.
PT_SETDBREGS This request is the converse of PT_GETDBREGS; it loads the
traced process's debug registers from the ``struct dbreg''
(defined in <machine/reg.h>) pointed to by addr.
Additionally, machine-specific requests can exist.
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 ptrace() system call may fail if:
[ESRCH]
+o No process having the specified process ID exists.
[EINVAL]
+o A process attempted to use PT_ATTACH on itself.
+o The request argument 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,
PT_SETFPREGS, PT_GETDBREGS, or PT_SETDBREGS was
attempted on a process with no valid register set.
(This is normally true only of system processes.)
[EBUSY]
+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.
[EPERM]
+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.
execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch(3),
i386_set_watch(3)
The ptrace() function appeared in Version 7 AT&T UNIX.
FreeBSD 5.2.1 August 11, 2003 FreeBSD 5.2.1 [ Back ] |
