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fork — create a new process
Standard C Library (libc, -lc)
#include <unistd.h> pid_t fork(void);
The fork() system call causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process) except for the following: • The child process has a unique process ID. • The child process has a different parent process ID (i.e., the process ID of the parent process). • The child process has its own copy of the parent's descriptors, except for descriptors returned by kqueue(2), which are not inherited from the parent process. These descriptors reference the same underlying objects, so that, for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the child process can affect a subsequent read(2) or write(2) by the parent. This descriptor copying is also used by the shell to establish standard input and output for newly created processes as well as to set up pipes. • The child process' resource utilizations are set to 0; see setrlimit(2). • All interval timers are cleared; see setitimer(2). • The child process has only one thread, corresponding to the calling thread in the parent process. If the process has more than one thread, locks and other resources held by the other threads are not released and therefore only async- signal-safe functions (see sigaction(2)) are guaranteed to work in the child process until a call to execve(2) or a similar function.
Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indicate the error.
The fork() system call will fail and no child process will be created if: [EAGAIN] The system-imposed limit on the total number of processes under execution would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROC. (The limit is actually ten less than this except for the super user). [EAGAIN] The user is not the super user, and the system-imposed limit on the total number of processes under execution by a single user would be exceeded. The limit is given by the sysctl(3) MIB variable KERN_MAXPROCPERUID. [EAGAIN] The user is not the super user, and the soft resource limit corresponding to the resource argument RLIMIT_NPROC would be exceeded (see getrlimit(2)). [ENOMEM] There is insufficient swap space for the new process.
The fork() function appeared in Version 1 AT&T UNIX.