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 de-
          scriptors 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  di-
          rectory  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  be-
              cause 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)

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