Provided by: manpages-dev_4.04-2_all bug


       fork - create a child process


       #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)).

       *  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

       *  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
       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().

       *  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(2)  in  a multithreaded program, the child can safely
          call only async-signal-safe functions  (see  signal(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 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

       *  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
          corresponding 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  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.



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

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

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

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


       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
       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).


       See pipe(2) and wait(2).


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


       This  page  is  part of release 4.04 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