NAME

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:

• The child process has a unique process ID.
• The child process has a different parent process ID (i.e., the process ID of the parent process).
• 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.
• 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 frameworks in the child process, you must exec. In this situation it is reasonable to exec yourself.