2.9BSD/usr/man/man2/fork.2

.TH FORK 2 
.UC
.SH NAME
fork  \-  spawn new process
.SH SYNOPSIS
.B fork()
.SH DESCRIPTION
.I Fork
is one of two ways new processes are created.
The new process's core image is a copy of
that of the caller of
.IR fork .
The only distinction
is the fact
that the value returned in the old (parent) process
contains the process ID
of the new (child) process,
while the value returned in the child is 0.
Process ID's range from 1 to 30,000.
This process ID is used by
.IR wait (2).
.PP
Files open before the fork
are shared, and have a common read-write pointer.
In particular,
this is the way that standard input and output
files are passed and also how
pipes are set up.
.SH RETURN VALUE
Upon successful completion,
.I 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
.I errno
is set to indicate the error.
.SH ERRORS
.I Fork
will fail and no child process will be created if:
.TP 20
[EAGAIN]
The system-imposed limit on the total number of processes under
execution would be exceeded.
.TP 20
[EAGAIN]
The system-imposed liimit on the total number of processes under
execution by a single user would be exceeded.
.SH "SEE ALSO"
execve(2), vfork(2), exec(3)
.SH ASSEMBLER
(fork = 2.)
.br
.B sys fork
.br
(new process return)
.br
(old process return, new process ID in r0)
.PP
The return locations in the old and new process
differ by one word.
The C-bit is set in the old process if a new
process could not be created.