DragonFly On-Line Manual Pages
PTRACE(2) DragonFly System Calls Manual PTRACE(2)
NAME
ptrace -- process tracing and debugging
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/ptrace.h>
int
ptrace(int request, pid_t pid, caddr_t addr, int data);
DESCRIPTION
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 be ignored.
PT_READ_I, PT_READ_D
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
DragonFly 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().
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. 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_STEP The traced process is single stepped one instruction. addr
should be passed (caddr_t)1. data is not used.
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 x86_64, 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.
PT_GETDBREGS This request reads the traced process' 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' debug registers from the ``struct dbreg''
(defined in <machine/reg.h>) pointed to by addr.
RETURN VALUES
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.
ERRORS
The ptrace() function 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 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 The pid represents a system process.
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.
SEE ALSO
execve(2), sigaction(2), wait(2), execv(3)
HISTORY
A ptrace() function call appeared in Version 7 AT&T UNIX.
DragonFly 4.1 June 22, 2015 DragonFly 4.1