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FORK(2) Linux Programmer's Manual FORK(2)


     fork - create a child process


     #include <sys/types.h>
     #include <unistd.h>
     pid_t fork(void);


     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


  • The child does not inherit its parent's memory locks (mlock(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-


  • The child does not inherit semaphore adjustments from its parent


  • 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 follow-
     ing 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_DONT-

FORK 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


  • 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

     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 descrip-

tors. 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 correspond-
        ing  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.


     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.


     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

  • 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 num-

ber" (PIDs) controller was reached.

     EAGAIN The caller is operating under the SCHED_DEADLINE scheduling pol-
            icy 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.)


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


     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.
 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).


     See pipe(2) and wait(2).


     clone(2),   execve(2),  exit(2),  setrlimit(2),  unshare(2),  vfork(2),
     wait(2), daemon(3), pthread_atfork(3), capabilities(7), credentials(7)


     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at

Linux 2017-09-15 FORK(2)

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