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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
.TH "PTHREAD_ATFORK" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" pthread_atfork
.SH NAME
pthread_atfork \- register fork handlers
.SH SYNOPSIS
.LP
\fB#include <pthread.h>
.br
.sp
int pthread_atfork(void (*\fP\fIprepare\fP\fB)(void), void (*\fP\fIparent\fP\fB)(void),
.br
\ \ \ \ \ \ void (*\fP\fIchild\fP\fB)(void)); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIpthread_atfork\fP() function shall declare fork handlers to
be called before and after \fIfork\fP(), in the context of the thread
that called \fIfork\fP(). The \fIprepare\fP fork handler shall be
called before \fIfork\fP() processing commences. The \fIparent\fP
fork handle shall be called after \fIfork\fP() processing completes
in the parent process. The \fIchild\fP fork handler shall be
called after \fIfork\fP() processing completes in the child process.
If no handling is
desired at one or more of these three points, the corresponding fork
handler address(es) may be set to NULL.
.LP
The order of calls to \fIpthread_atfork\fP() is significant. The \fIparent\fP
and \fIchild\fP fork handlers shall be called
in the order in which they were established by calls to \fIpthread_atfork\fP().
The \fIprepare\fP fork handlers shall be called
in the opposite order.
.SH RETURN VALUE
.LP
Upon successful completion, \fIpthread_atfork\fP() shall return a
value of zero; otherwise, an error number shall be returned
to indicate the error.
.SH ERRORS
.LP
The \fIpthread_atfork\fP() function shall fail if:
.TP 7
.B ENOMEM
Insufficient table space exists to record the fork handler addresses.
.sp
.LP
The \fIpthread_atfork\fP() function shall not return an error code
of [EINTR].
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
There are at least two serious problems with the semantics of \fIfork\fP()
in a
multi-threaded program. One problem has to do with state (for example,
memory) covered by mutexes. Consider the case where one
thread has a mutex locked and the state covered by that mutex is inconsistent
while another thread calls \fIfork\fP(). In the child, the mutex is
in the locked state (locked by a nonexistent thread and thus
can never be unlocked). Having the child simply reinitialize the mutex
is unsatisfactory since this approach does not resolve the
question about how to correct or otherwise deal with the inconsistent
state in the child.
.LP
It is suggested that programs that use \fIfork\fP() call an \fIexec\fP
function very soon afterwards in the child process, thus resetting
all states. In the
meantime, only a short list of async-signal-safe library routines
are promised to be available.
.LP
Unfortunately, this solution does not address the needs of multi-threaded
libraries. Application programs may not be aware that
a multi-threaded library is in use, and they feel free to call any
number of library routines between the \fIfork\fP() and \fIexec\fP
calls, just as they always have.
Indeed, they may be extant single-threaded programs and cannot, therefore,
be expected to obey new restrictions imposed by the
threads library.
.LP
On the other hand, the multi-threaded library needs a way to protect
its internal state during \fIfork\fP() in case it is re-entered later
in the child process. The problem arises especially in
multi-threaded I/O libraries, which are almost sure to be invoked
between the \fIfork\fP()
and \fIexec\fP calls to effect I/O redirection. The solution may require
locking mutex
variables during \fIfork\fP(), or it may entail simply resetting the
state in the child after
the \fIfork\fP() processing completes.
.LP
The \fIpthread_atfork\fP() function provides multi-threaded libraries
with a means to protect themselves from innocent
application programs that call \fIfork\fP(), and it provides multi-threaded
application
programs with a standard mechanism for protecting themselves from
\fIfork\fP() calls in a
library routine or the application itself.
.LP
The expected usage is that the \fIprepare\fP handler acquires all
mutex locks and the other two fork handlers release them.
.LP
For example, an application can supply a \fIprepare\fP routine that
acquires the necessary mutexes the library maintains and
supply \fIchild\fP and \fIparent\fP routines that release those mutexes,
thus ensuring that the child gets a consistent snapshot
of the state of the library (and that no mutexes are left stranded).
Alternatively, some libraries might be able to supply just a
\fIchild\fP routine that reinitializes the mutexes in the library
and all associated states to some known value (for example, what
it was when the image was originally executed).
.LP
When \fIfork\fP() is called, only the calling thread is duplicated
in the child process.
Synchronization variables remain in the same state in the child as
they were in the parent at the time \fIfork\fP() was called. Thus,
for example, mutex locks may be held by threads that no longer exist
in the child process, and any associated states may be inconsistent.
The parent process may avoid this by explicit code that
acquires and releases locks critical to the child via \fIpthread_atfork\fP().
In addition, any critical threads need to be
recreated and reinitialized to the proper state in the child (also
via \fIpthread_atfork\fP()).
.LP
A higher-level package may acquire locks on its own data structures
before invoking lower-level packages. Under this scenario,
the order specified for fork handler calls allows a simple rule of
initialization for avoiding package deadlock: a package
initializes all packages on which it depends before it calls the \fIpthread_atfork\fP()
function for itself.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIatexit\fP() , \fIfork\fP() , the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<sys/types.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
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