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



NAME
       fork - create a child process

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 (sigpending(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 (seeio_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.

       *  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 clone(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.

       *  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  descrip-
          tion 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 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.1-2001 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.

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 duplicate 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 threading implementation invokes clone(2) with flags that provide the same  effect  as  the  traditional
       system call.  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), setrlimit(2), unshare(2), vfork(2), wait(2), daemon(3), capabilities(7), credentials(7)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-04-27                                                    FORK(2)