NAME
fork - create a child process SYNOPSIS
#include
pidt 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), timercreate(2)). * The child does not inherit outstanding asynchronous I/O operations from its parent (aioread(3), aiowrite(3)), nor does it inherit any asynchronous I/O contexts from its parent (see iosetup(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 FNOTIFY in fcntl(2)). * The prctl(2) PRSETPDEATHSIG 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 PRSETTIMERSLACK in prctl(2). * Memory mappings that have been marked with the madvise(2) MADVDONT‐ FORK flag are not inherited across a fork(). * The termination signal of the child is always SIGCHLD (see clone(2)). * 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 ioperm(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 pthreadatfork(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 FSETOWN and FSETSIG in fcntl(2)). * The child inherits copies of the parent's set of open message queue descriptors (see mqoverview(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 (mqflags). * 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 RLIMITNPROC resource limit was encountered. To exceed this limit, the process must have either the CAPSYSADMIN or the CAPSYSRESOURCE 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). 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.) 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 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 pthreadatfork(3). EXAMPLE See pipe(2) and wait(2). SEE ALSO clone(2), execve(2), exit(2), setrlimit(2), unshare(2), vfork(2), wait(2), daemon(3), capabilities(7), credentials(7) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
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Linux 2013-03-12 FORK(2)