FORK(2) System Calls Manual FORK(2)

forkcreate a new process

#include <unistd.h>


() 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:

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.

fork() will fail and no child process will be created if:

The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration-dependent.
The system-imposed limit MAXUPRC (⟨sys/param.h⟩) on the total number of processes under execution by a single user would be exceeded.
There is insufficient swap space for the new process.

#include <sys/types.h> #include <unistd.h>

The include file <sys/types.h> is necessary.

execve(2), sigaction(2), wait(2), compat(5)

A fork() function call appeared in Version 6 AT&T UNIX.

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.

June 4, 1993 BSD 4