bsd man page for fork

FORK(2) 							System Calls Manual							   FORK(2)

NAME
       fork - create a new process

SYNOPSIS
       pid = fork()
       int pid;

DESCRIPTION
       Fork causes creation of a new process.  The new process (child process) is an exact copy of the calling 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 VALUE
       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 one or more of the following are true:

       [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.

SEE ALSO
       execve(2), wait(2)

3rd Berkeley Distribution					   May 22, 1986 							   FORK(2)