DragonFly On-Line Manual Pages

FORK(2)                  DragonFly System Calls Manual                 FORK(2)

NAME
     fork -- create a new process

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <unistd.h>

     pid_t
     fork(void);

DESCRIPTION
     Fork() 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:

           o   The child process has a unique process ID.

           o   The child process has a different parent process ID (i.e., the
               process ID of the parent process).

           o   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.

           o   The child process' resource utilizations are set to 0; see
               setrlimit(2).

           o   All interval timers are cleared; see setitimer(2).

RETURN VALUES
     Upon successful completion, 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.

MULTI-THREADING CONSIDERATIONS
     fork() can create severe issues for multi-threaded programs due to the
     fact that the memory state of the child process will record the
     asynchronous state of the threads that are running in the parent.  Fork()
     will only be synchronous for the specific thread making the call.  In
     particular, locks used internally by pthread(3) and rtld(1) can be caught
     in a bad state.  To deal with these issues, the pthreads library goes to
     great lengths to synchronize internal locks when a fork() call is issued.
     The threaded program itself as well as third party libraries used by the
     program might or might not properly handle these issues when it comes to
     their own internal state.

     If at all possible, programs should use vfork(2) instead of fork() when
     forking for the purposes of issuing an exec of some sort.  Attempting to
     fork a threaded program without issuing an exec is not recommended.
     Attempting to bypass pthreads and implement threading manually is also
     not recommended as it is doubtful that homegrown implementations could
     properly deal with rtld races.

ERRORS
     Fork() will fail and no child process will be created if:

     [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).

     [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.

     [EAGAIN]           The user is not the super user, and the soft resource
                        limit corresponding to the resource parameter
                        RLIMIT_NPROC would be exceeded (see getrlimit(2)).

     [ENOMEM]           There is insufficient swap space for the new process.

SEE ALSO
     execve(2), rfork(2), setitimer(2), setrlimit(2), vfork(2), wait(2)

HISTORY
     A fork() function call appeared in Version 6 AT&T UNIX.

DragonFly 4.9                    June 4, 1993                    DragonFly 4.9