V4/man/man2/fork.2

.th FORK II 8/5/73
.sh NAME
fork  \*-  spawn new process
.sh SYNOPSIS
(fork = 2.)
.br
.ft B
sys fork
.br
.ft R
(new process return)
.br
(old process return)
.s3
.ft B
fork( )
.ft R
.sh DESCRIPTION
.it Fork
is the only way new processes are created.
The new process's core image is a copy of
that of the caller of
.it fork.
The only distinction
is the return location and the fact
that r0 in the old (parent) process contains the process ID
of the new (child) process.
This process ID is used by
.it wait.
.s3
From C, the returned value is 0 in the child process,
non-zero in the parent process; however,
a return of \*-1 indicates inability
to create a new process.
.sh "SEE ALSO"
wait(II), exec(II)
.sh DIAGNOSTICS
The error bit
(c-bit) is set in the old process if a new process
could not be created because of lack of process space.
From C, a return of \*-1 (not just negative)
indicates an error.