diff --git a/lib/libc/sys/fork.2 b/lib/libc/sys/fork.2 index bf934d166b03..d841b0bc38e1 100644 --- a/lib/libc/sys/fork.2 +++ b/lib/libc/sys/fork.2 @@ -1,254 +1,254 @@ .\" Copyright (c) 1980, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)fork.2 8.1 (Berkeley) 6/4/93 .\" $FreeBSD$ .\" .Dd August 2, 2021 .Dt FORK 2 .Os .Sh NAME .Nm fork .Nd create a new process .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In unistd.h .Ft pid_t .Fn fork void .Ft pid_t .Fn _Fork void .Sh DESCRIPTION The .Fn fork function 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: .Bl -bullet -offset indent .It The child process has a unique process ID. .It The child process has a different parent process ID (i.e., the process ID of the parent process). .It The child process has its own copy of the parent's descriptors, except for descriptors returned by .Xr kqueue 2 , which are not inherited from the parent process. 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 .Xr lseek 2 on a descriptor in the child process can affect a subsequent .Xr read 2 or .Xr 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. .It The child process' resource utilizations are set to 0; see .Xr setrlimit 2 . .It All interval timers are cleared; see .Xr setitimer 2 . .It The robust mutexes list (see .Xr pthread_mutexattr_setrobust 3 ) is cleared for the child. .It The atfork handlers established with the .Xr pthread_atfork 3 function are called as appropriate before fork in the parent process, and after the child is created, in parent and child. .It 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 .Xr sigaction 2 ) are guaranteed to work in the child process until a call to .Xr execve 2 or a similar function. The .Fx implementation of .Fn fork provides a usable .Xr malloc 3 , and .Xr rtld 1 services in the child process. .El .Pp The .Fn fork function is not async-signal safe and creates a cancellation point in the parent process. It cannot be safely used from signal handlers, and the atfork handlers established by .Xr pthread_atfork 3 do not need to be async-signal safe either. .Pp The .Fn _Fork function creates a new process, similarly to .Fn fork , but it is async-signal safe. .Fn _Fork does not call atfork handlers, and does not create a cancellation point. It can be used safely from signal handlers, but then no userspace services ( .Xr malloc 3 or .Xr rtld 1 ) are available in the child if forked from multi-threaded parent. In particular, if using dynamic linking, all dynamic symbols used by the child after .Fn _Fork must be pre-resolved. Note: resolving can be done globally by specifying the .Ev LD_BIND_NOW environment variable to the dynamic linker, or per-binary by passing the .Fl z Ar now option to the static linker .Xr ld 1 , or by using each symbol before the .Fn _Fork call to force the binding. .Sh RETURN VALUES Upon successful completion, .Fn fork and .Fn _Fork return a value -of 0 to the child process and returns the process ID of the child +of 0 to the child process and return 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 .Va errno is set to indicate the error. .Sh EXAMPLES The following example shows a common pattern of how .Fn fork is used in practice. .Bd -literal -offset indent #include #include #include #include int main(void) { pid_t pid; switch (pid = fork()) { case -1: err(1, "Failed to fork"); case 0: printf("Hello from child process!\en"); exit(0); default: break; } printf("Hello from parent process (child's PID: %d)!\en", pid); return (0); } .Ed .Pp The output of such a program is along the lines of: .Bd -literal -offset indent Hello from parent process (child's PID: 27804)! Hello from child process! .Ed .Sh ERRORS The .Fn fork system call will fail and no child process will be created if: .Bl -tag -width Er .It Bq Er EAGAIN The system-imposed limit on the total number of processes under execution would be exceeded. The limit is given by the .Xr sysctl 3 MIB variable .Dv KERN_MAXPROC . (The limit is actually ten less than this except for the super user). .It 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 .Xr sysctl 3 MIB variable .Dv KERN_MAXPROCPERUID . .It Bq Er EAGAIN The user is not the super user, and the soft resource limit corresponding to the .Fa resource argument .Dv RLIMIT_NPROC would be exceeded (see .Xr getrlimit 2 ) . .It Bq Er ENOMEM There is insufficient swap space for the new process. .El .Sh SEE ALSO .Xr execve 2 , .Xr rfork 2 , .Xr setitimer 2 , .Xr setrlimit 2 , .Xr sigaction 2 , .Xr vfork 2 , .Xr wait 2 , .Xr pthread_atfork 3 .Sh HISTORY The .Fn fork function appeared in .At v1 . .Pp The .Fn _Fork function was defined by Austin Group together with the removal of a requirement that the .Fn fork implementation must be async-signal safe. The .Fn _Fork function appeared in .Fx 14.0 .