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fork(2) [osx man page]

FORK(2) 						      BSD System Calls Manual							   FORK(2)

fork -- create a new process SYNOPSIS
#include <unistd.h> pid_t fork(void); DESCRIPTION
Fork() causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process) except for the following: o The child process has a unique process ID. o The child process has a different parent process ID (i.e., the process ID of the parent process). o The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that, for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the child process can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish standard input and output for newly created processes as well as to set up pipes. o The child processes resource utilizations are set to 0; see setrlimit(2). RETURN VALUES
Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indicate the error. ERRORS
Fork() will fail and no child process will be created if: [EAGAIN] The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration- dependent. [EAGAIN] The system-imposed limit MAXUPRC (<sys/param.h>) on the total number of processes under execution by a single user would be exceeded. [ENOMEM] There is insufficient swap space for the new process. LEGACY SYNOPSIS
#include <sys/types.h> #include <unistd.h> The include file <sys/types.h> is necessary. SEE ALSO
execve(2), sigaction(2), wait(2), compat(5) HISTORY
A fork() function call appeared in Version 6 AT&T UNIX. CAVEATS
There are limits to what you can do in the child process. To be totally safe you should restrict yourself to only executing async-signal safe operations until such time as one of the exec functions is called. All APIs, including global data symbols, in any framework or library should be assumed to be unsafe after a fork() unless explicitly documented to be safe or async-signal safe. If you need to use these frame- works in the child process, you must exec. In this situation it is reasonable to exec yourself. 4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution

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PTHREAD_ATFORK(3)					   BSD Library Functions Manual 					 PTHREAD_ATFORK(3)

pthread_atfork -- register handlers to be called when process forks LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <pthread.h> int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void)); DESCRIPTION
The pthread_atfork() function registers the provided handler functions to be called when the fork(2) function is called. Each of the three handlers is called at a different place in the fork(2) sequence. The prepare handler is called in the parent process before the fork hap- pens, the parent handler is called in the parent process after the fork has happened, and the child handler is called in the child process after the fork has happened. The parent and child handlers are called in the order in which they were registered, while the prepare handlers are called in reverse of the order in which they were registered. Any of the handlers given may be NULL. The intended use of pthread_atfork() is to provide a consistent state to a child process from a multithreaded parent process where locks may be acquired and released asynchronously with respect to the fork(2) call. Each subsystem with locks that are used in a child process should register handlers with pthread_atfork() that acquires those locks in the prepare handler and releases them in the parent handler. RETURN VALUES
The pthread_atfork() function returns 0 on success and an error number on failure. ERRORS
The following error code may be returned: [ENOMEM] Insufficient memory exists to register the fork handlers. SEE ALSO
The pthread_atfork() function conforms to IEEE Std 1003.1c-1995 (``POSIX.1''). HISTORY
The pthread_atfork() function first appeared in NetBSD 2.0. CAVEATS
After calling fork(2) from a multithreaded process, it is only safe to call async-signal-safe functions until calling one of the exec(3) functions. The pthread_*() functions are not async-signal-safe, so it is not safe to use such functions in the child handler. BUGS
There is no way to unregister a handler registered with pthread_atfork(). BSD
February 12, 2003 BSD

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