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NAME

       fork - create a child process

SYNOPSIS

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

       pid_t fork(void);

DESCRIPTION

       fork()  creates  a  new  process  by  duplicating the calling process.  The new process is
       referred to as the child process.  The calling  process  is  referred  to  as  the  parent
       process.

       The  child  process  and the parent process run in separate memory spaces.  At the time of
       fork() both memory spaces have the same content.  Memory writes, file mappings  (mmap(2)),
       and unmappings (munmap(2)) performed by one of the processes do not affect the other.

       The  child  process  is  an exact duplicate of the parent process 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)) or session.

       *  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 (sigpending(2)).

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

       *  The child does not inherit process-associated record locks from its parent  (fcntl(2)).
          (On  the  other hand, it does inherit fcntl(2) open file description locks and flock(2)
          locks from its parent.)

       *  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 (see io_setup(2)).

       The process attributes in the preceding list are all specified in POSIX.1.  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.

       *  The default timer slack value is set to the parent's current timer  slack  value.   See
          the description of PR_SET_TIMERSLACK in prctl(2).

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

       *  Memory in address ranges that have been marked with the madvise(2) MADV_WIPEONFORK flag
          is  zeroed  in the child after a fork().  (The MADV_WIPEONFORK setting remains in place
          for those address ranges in the child.)

       *  The termination signal of the child is always SIGCHLD (see clone(2)).

       *  The port access permission bits set by ioperm(2) are not inherited by  the  child;  the
          child must turn on any bits that it requires using ioperm(2).

       Note the following further points:

       *  The  child  process  is  created  with a single thread—the one that called fork().  The
          entire virtual address space of the parent is replicated in the  child,  including  the
          states  of  mutexes,  condition  variables,  and  other  pthreads  objects;  the use of
          pthread_atfork(3) may be helpful for dealing with problems that this can cause.

       *  After a fork() in a multithreaded program, the child can safely call only async-signal-
          safe functions (see signal-safety(7)) until such time as it calls execve(2).

       *  The  child  inherits  copies  of  the parent's set of open file descriptors.  Each file
          descriptor in the child refers to the same open file description (see open(2))  as  the
          corresponding  file descriptor in the parent.  This means that the two file descriptors
          share open file status flags, 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 file descriptor in the child refers to  the  same  open  message
          queue  description as the corresponding file descriptor in the parent.  This means that
          the two file 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  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 A system-imposed limit on the number of  threads  was  encountered.   There  are  a
              number of limits that may trigger this error:

              *  the  RLIMIT_NPROC  soft  resource limit (set via setrlimit(2)), which limits the
                 number of processes and threads for a real user ID, was reached;

              *  the  kernel's  system-wide  limit  on  the  number  of  processes  and  threads,
                 /proc/sys/kernel/threads-max, was reached (see proc(5));

              *  the maximum number of PIDs, /proc/sys/kernel/pid_max, was reached (see proc(5));
                 or

              *  the  PID  limit  (pids.max)  imposed  by  the  cgroup  "process  number"  (PIDs)
                 controller was reached.

       EAGAIN The  caller  is  operating  under the SCHED_DEADLINE scheduling policy and does not
              have the reset-on-fork flag set.  See sched(7).

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

       ENOMEM An attempt was made to create a child process  in  a  PID  namespace  whose  "init"
              process has terminated.  See pid_namespaces(7).

       ENOSYS fork()  is  not supported on this platform (for example, hardware without a Memory-
              Management Unit).

       ERESTARTNOINTR (since Linux 2.6.17)
              System call was interrupted by a signal and will be restarted.  (This can  be  seen
              only during a trace.)

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

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 duplicate  the  parent's  page  tables,  and  to
       create a unique task structure for the child.

   C library/kernel differences
       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.  (A call
       to fork() is equivalent to a call to clone(2) specifying  flags  as  just  SIGCHLD.)   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), exit(2),  setrlimit(2),  unshare(2),  vfork(2),  wait(2),  daemon(3),
       pthread_atfork(3), capabilities(7), credentials(7)

COLOPHON

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       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.