fork(2)
create a new process
LIBRARY
Lb libc
SYNOPSIS
In unistd.h
Ft pid_t
Fn fork void
DESCRIPTION
The
Fn fork
system call 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:
-
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(2)
or
write(2)
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 process' resource utilizations
are set to 0; see
setrlimit(2).
-
All interval timers are cleared; see
setitimer(2).
-
The child process has only one thread,
corresponding to the calling thread in the parent process.
If the process has more than one thread,
locks and other resources held by the other threads are not released
and therefore only async-signal-safe functions
(see
sigaction(2))
are guaranteed to work in the child process until a call to
execve(2)
or a similar function.
RETURN VALUES
Upon successful completion,
Fn 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
The
Fn fork
system call will fail and no child process will be created if:
- Bq Er EAGAIN
-
The system-imposed limit on the total
number of processes under execution would be exceeded.
The limit is given by the
sysctl(3)
MIB variable
KERN_MAXPROC
(The limit is actually ten less than this
except for the super user).
- Bq Er EAGAIN
-
The user is not the super user, and
the system-imposed limit
on the total number of
processes under execution by a single user would be exceeded.
The limit is given by the
sysctl(3)
MIB variable
KERN_MAXPROCPERUID
- Bq Er EAGAIN
-
The user is not the super user, and
the soft resource limit corresponding to the
Fa resource
argument
RLIMIT_NPROC
would be exceeded (see
getrlimit(2)).
- Bq Er ENOMEM
-
There is insufficient swap space for the new process.
HISTORY
The
Fn fork
function appeared in
AT&T System
v6 .
