fork.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)fork.2	8.1	9/11/90
.TH fork 2
.SH Name
fork \- create a new process
.SH Syntax
.nf
.B #include <sys/types.h>
.B #include <unistd.h>
.PP
.B pid = fork()
.B pid_t pid;
.fi
.SH Description
.NXR "fork system call"
.NXR "process" "creating"
The
.PN fork
system call causes creation of a new process.
The new process (child process) is an exact copy of the
calling process except for the following:
.IP \(bu 5
The child process has a unique process ID.
.IP \(bu 5
The child process has a different parent process ID (that is,
the process ID of the parent process).
.IP \(bu 5
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 a 
.MS lseek 2
on a descriptor in the child process can affect a subsequent
.I read
or
.I 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.
.IP \(bu 5
The child processes resource utilizations are set to 0.
For further information, see 
.MS setrlimit 2 .
.SH Return Values
Upon successful completion, 
.PN 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 \fIerrno\fP is set to indicate the error.
.SH Diagnostics
The
.PN fork
system call
fails and no child process are created under the following conditions:
.TP 15
[EAGAIN]
The system-imposed limit {PROC_MAX} on the total
number of processes under execution would be exceeded.
.TP 15
[EAGAIN]
The system-imposed limit {CHILD_MAX} on the total number of
processes under execution by a single user would be exceeded.
.TP 15
[ENOMEM]
There is insufficient swap space for the new process.
.SH See Also
execve(2), wait(2)