fork


SYNOPSIS
       #include <unistd.h>

       pid_t fork(void);

DESCRIPTION
       fork()  creates  a new process by duplicating the calling process.  The
       new process, referred to as the child, is an  exact  duplicate  of  the
       calling  process,  referred  to as the parent, except for the following
       points:

       *  The child has its own unique process ID, and this PID does not match
          the ID of any existing process group (setpgid(2)).

       *  The  child's  parent  process ID is the same as the parent's process
          ID.

       *  The child does not inherit  its  parent's  memory  locks  (mlock(2),
          mlockall(2)).

       *  Process  resource  utilizations (getrusage(2)) and CPU time counters
          (times(2)) are reset to zero in the child.

       *  The child's set of pending  signals  is  initially  empty  (sigpend-
          ing(2)).

       *  The  child  does  not  inherit semaphore adjustments from its parent
          (semop(2)).

       *  The child does not inherit record locks from its parent (fcntl(2)).

       *  The child does not inherit timers  from  its  parent  (setitimer(2),
          alarm(2), timer_create(2)).

       *  The  child  does not inherit outstanding asynchronous I/O operations
          from its parent (aio_read(3), aio_write(3)), nor does it inherit any
          asynchronous I/O contexts from its parent (seeio_setup(2)).

       The  process  attributes  in  the  preceding  list are all specified in
       POSIX.1-2001.  The parent and child also differ  with  respect  to  the
       following Linux-specific process attributes:

       *  The  child does not inherit directory change notifications (dnotify)
          from its parent (see the description of F_NOTIFY in fcntl(2)).

       *  The prctl(2) PR_SET_PDEATHSIG setting is reset  so  that  the  child
          does not receive a signal when its parent terminates.

       *  Memory mappings that have been marked with the madvise(2) MADV_DONT-
          FORK flag are not inherited across a fork().

       *  The  termination  signal  of  the  child  is  always  SIGCHLD   (see
          clone(2)).
          in the parent.  This means that the two descriptors share open  file
          status  flags, current file offset, and signal-driven I/O attributes
          (see the description of F_SETOWN and F_SETSIG in fcntl(2)).

       *  The child inherits copies of the parent's set of open message  queue
          descriptors  (see  mq_overview(7)).   Each  descriptor  in the child
          refers to the same open message queue description as the correspond-
          ing  descriptor  in the parent.  This means that the two descriptors
          share the same flags (mq_flags).

       *  The child inherits copies of the  parent's  set  of  open  directory
          streams  (see opendir(3)).  POSIX.1-2001 says that the corresponding
          directory streams in the parent and child may  share  the  directory
          stream positioning; on Linux/glibc they do not.

RETURN VALUE
       On success, the PID of the child process is returned in the parent, and
       0 is returned in the child.  On failure, -1 is returned in the  parent,
       no child process is created, and errno is set appropriately.

ERRORS
       EAGAIN fork()  cannot  allocate  sufficient memory to copy the parent's
              page tables and allocate a task structure for the child.

       EAGAIN It was not possible to create a new process because the caller's
              RLIMIT_NPROC  resource  limit  was  encountered.  To exceed this
              limit, the process must have either  the  CAP_SYS_ADMIN  or  the
              CAP_SYS_RESOURCE capability.

       ENOMEM fork()  failed  to  allocate  the  necessary  kernel  structures
              because memory is tight.

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

NOTES
       Under Linux, fork() is implemented using copy-on-write  pages,  so  the
       only  penalty  that it incurs is the time and memory required to dupli-
       cate the parent's page tables, and to create a  unique  task  structure
       for the child.

       Since  version  2.3.3,  rather than invoking the kernel's fork() system
       call, the glibc fork() wrapper that is provided as  part  of  the  NPTL
       threading  implementation  invokes clone(2) with flags that provide the
       same effect as the traditional system call.  The glibc wrapper  invokes
       any fork handlers that have been established using pthread_atfork(3).

EXAMPLE
       See pipe(2) and wait(2).

SEE ALSO
       clone(2),  execve(2), setrlimit(2), unshare(2), vfork(2), wait(2), dae-
       mon(3), capabilities(7), credentials(7)
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