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    Command:

    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 (see io_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.
    
           *  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-
              variables,  and other pthreads objects; the use of pthread_atfork(3)
              may be helpful for dealing with problems that this can cause.
    
           *  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 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.
    
           ENOSYS fork()  is not supported on this platform (for example, hardware
                  without a Memory-Management Unit).
    
    
    

    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
    
    
    

    Linux 2013-03-12 FORK(2)

    
    
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