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==================
Process management
==================
Historic note: The process code comes from a time before posix_spawn. It also
relies on the POSIX functions ``fork``, ``execv``, ``execvp`` and ``pipe``, so
it may not be available everywhere. Where this is the case, the functions will
return ``-ENOSYS``.
Process metadata structure
==========================
.. code-block:: c
#include <libHX/proc.h>
struct HXproc {
const struct HXproc_ops *p_ops;
void *p_data;
unsigned int p_flags;
/* Following members should only be read */
int p_stdin, p_stdout, p_stderr;
int p_pid;
char p_status;
bool p_exited, p_terminated;
};
When creating a new process with the intent of running it asynchronously (using
``HXproc_run_async``), the first three fields must be filled in by the user.
``p_ops``
A table of callbacks, generally used for setting and/or
restoring signals before/after execution. This member may be
``NULL``.
``p_data``
The user may provide a pointer of his choosing. It will be
passed the callback functions when invoked.
``p_flags``
Process creation flags, see below.
After the subprocess has been started, ``HXproc_run_async`` will have filled in
some fields:
``p_stdin``
If ``HXPROC_STDIN`` was specified in ``p_flags``, ``p_stdin`` will be
assigned the write side file descriptor of the subprocess's to-be
stdin. The subprocess will get the read side file descriptor in this
member. This is so that the correct fd is used in when
``p_ops->p_postfork`` is called.
``p_stdout``
If ``HXPROC_STDOUT`` is specified in ``p_flags``, ``p_stdout`` will be
assigned the read side file descriptor of the subprocess's to-be
stdout. The subprocess will get the write side file descriptor value
from this member.
``p_stderr``
If ``HXPROC_STDERR`` is specified in ``p_flags``, ``p_stderr`` will be
assigned the read side file descriptor of the subprocess's to-be stderr, and
the subprocess will get the write side fd.
``p_pid``
The process ID of the spawned process.
Upon calling ``HXproc_wait``, further fields will have been filled when the
function returns:
``p_exited``
Whether the process exited normally (cf. signalled/terminated).
``p_terminated``
Whether the process was terminated (signalled).
``p_status``
The exit status of the process or the termination signal.
Flags
-----
Possible values for the ``p_flags`` member are:
``HXPROC_STDIN``
The subprocess's stdin file descriptor shall be connected to the master
program, that is, not inherit the stdin of the master. Cannot be used
with ``HXproc_run_sync`` (because there would be no one to provide data
in a sync operation).
``HXPROC_STDOUT``
Connect the stdout file descriptor of the subprocess with the master.
Cannot be used with ``HXproc_run_sync``.
``HXPROC_STDERR``
Connect the stderr file descriptor of the subprocess with the master.
Cannot be used with ``HXproc_run_sync``.
``HXPROC_NULL_STDIN``
The subprocess's stdin file descriptor shall be connected to
``/dev/null``. ``HXPROC_STDIN`` and ``HXPROC_NULL_STDIN`` are mutually
exclusive.
``HXPROC_NULL_STDOUT``
Connect the stdout file descriptor of the subprocess to ``/dev/null``,
thereby essentially discarding its output. ``HXPROC_STDOUT`` and
``HXPROC_NULL_STDOUT`` are mutually exclusive.
``HXPROC_NULL_STDERR``
Connect the stderr file descriptor of the subprocess to ``/dev/null``,
thereby essentially discarding its output. ``HXPROC_STDERR`` and
``HXPROC_NULL_STDERR`` are mutually exclusive.
``HXPROC_VERBOSE``
Have the subprocess print an error message to stderr if exec'ing
returned an error.
``HXPROC_A0``
``argv[0]`` refers to program file, while ``argv[1]`` to the program
invocation name, with ``argv[2]`` being the first argument, etc.
Without this flag, ``argv[0]`` will be both the program file and
program invocation name, and arguments begin at ``argv[1]``.
``HXPROC_EXECV``
Normally, ``execvp`` will be used which scans ``$PATH for the program.
Use this flag to use ``execv`` instead, which will not do such thing.
Callbacks
=========
.. code-block:: c
#include <libHX/proc.h>
struct HXproc_ops {
void (*p_prefork)(void *);
void (*p_postfork)(void *);
void (*p_complete)(void *);
};
``struct HXproc_ops`` provides a way to run user-specified functions just
before the fork, after, and when the process has been waited for. They can be
used to set and/or restore signals as needed, for example. The function
pointers can be ``NULL``. The ``p_data`` member is passed as an argument.
``p_prefork``
Run immediately before calling ``fork``. This is useful for taking any
action regarding signals, like setting ``SIGCHLD`` to ``SIG_DFL``, or
``SIGPIPE`` to ``SIG_IGN``, for example.
``p_postfork``
Run in the subprocess (and only there) after forking. Useful
to do a ``setuid`` or other change in privilege level.
``p_complete``
Run in ``HXproc_wait`` when the process has been waited for.
Useful to restore the signal handler(s).
Process control
===============
.. code-block:: c
#include <libHX/proc.h>
int HXproc_run_async(const char *const *argv, struct HXproc *proc);
int HXproc_run_sync(const char *const *argv, unsigned int flags);
int HXproc_wait(struct HXproc *proc);
``HXproc_run_async``
Start a subprocess according to the parameters in proc. Returns a
negative errno code if something went wrong, or positive non-zero on
success.
``HXproc_run_sync``
Start a subprocess synchronously, similar to calling ``system``, but
with the luxury of being able to specify arguments as separate strings
(via argv) rather than one big command line that is run through the
shell. ``flags`` is a value composed of the HXproc flags mentioned
above. ``HXPROC_STDIN``, ``HXPROC_STDOUT`` and ``HXPROC_STDERR`` are ignored
because there would be no one in a synchronous execution that
could supply data to these file descriptors or read from them.[#f1]_
.. [#f1] Even for threads, please just use the async model.
``HXproc_wait``
Wait for a subprocess to terminate, if it has not already. It will also
retrieve the exit status of the process and store it in the ``struct
HXproc``.
The return value will be positive non-zero on success, or negative on
error. Underlying system function's errors are returned, plus:
``EINVAL``
Flags were not accepted.
User identity control
=====================
.. code-block: c
#include <libHX/proc.h>
int HXproc_switch_user(const char *user, const char *group);
``HXproc_switch_user`` is a wrapper for changing process identity to an
unprivileged user. This utilizes ``setuid``, and possibly ``setgid`` plus
``initgroups``.
``user`` can either be a username or a numeric UID in string form, the latter
of which will be parsed with strtoul in automatic base. If ``user`` is ``NULL``
or the empty string, no change of process user identity occurs.
``group`` can likewise be a name or GID. When ``group`` is ``NULL``, the
process group(s) will change to the the user's group(s) — both primary and
secondary — provided a user was specified (see above). When ``gruop`` is the
empty string, no change of process group identity occurs.
Process information
===================
.. code-block:: c
#include <libHX/proc.h>
int HXproc_top_fd(void);
``HXproc_top_fd``
This function gives a heuristic for the highest fd in the process.
The returned number may be higher than the highest live fd actually.
On error, negative errno is returned.
|
