fork(2) System Calls Manual fork(2)
NAME
fork, vfork - Creates a new process
SYNOPSIS
#include <unistd.h>
pid_t fork (void);
pid_t vfork (void);
Application developers may want to specify an #include statement for <sys/types.h> before the one for <unistd.h> if programs are being
developed for multiple platforms. The additional #include statement is not required on Tru64 UNIX systems or by ISO or X/Open standards,
but may be required on other vendors' systems that conform to these standards.
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
fork(): XSH5.0
vfork(): XSH5.0
Refer to the standards(5) reference page for more information about industry standards and associated tags.
DESCRIPTION
The fork() and vfork() functions create a new process (child process) that is identical to the calling process (parent process).
The child process inherits the following attributes from the parent process: Environment Close-on-exec flags Signal handling settings Set
user ID mode bit Set group ID mode bit Trusted state Profiling on/off status Nice value All attached shared libraries Process group ID tty
group ID Current directory Root directory File mode creation mask File size limit Attached shared memory segments Attached mapped file seg-
ments All mapped regions with the same protection and sharing mode as in the parent process Its own copy of the parent's open directory
streams
The child process differs from the parent process in the following ways: The child process has a unique process ID that does not match any
active process group ID. The parent process ID of the child process matches the process ID of the parent. The child process has its own
copy of the parent process's file descriptors. Each of the child's file descriptors refers to the same open file description with the cor-
responding file descriptor of the parent process. The child process has its own copy of the parent's open directory streams. Each open
directory stream in the child process may share directory stream positioning with the corresponding directory stream of the parent. All
semadj values are cleared. Process locks, text locks, and data locks are not inherited by the child process. The child process' values of
tms_utime, tms_stime, tms_cutime, and tms_cstime are set to 0 (zero). Any pending alarms are cleared in the child process. [Tru64
UNIX] Any interval timers enabled by the parent process are reset in the child process. Any signals pending for the parent process are
cleared for the child process.
NOTES
If a multithreaded process forks a child process, the new process contains a replica of the calling thread and its entire address space,
possibly including the states of mutexes and other resources. Consequently, to avoid errors, the child process should only execute opera-
tions it knows will not cause deadlock.
The fork() and vfork() functions have essentially the same implementation at the level of the operating system kernel but may differ in how
they are supported through different libraries. Some libraries, such as libpthread and libc, support fork handler routines that can
acquire and release resources that are critical to the child process. Fork handlers therefore allow an application to manage potential
deadlock situations that might occur between the parent and child processes. Fork handlers do not work correctly if the application calls
vfork() to create the child process. Therefore, applications using libpthread and libc should call fork() to create a child process.
For more information about fork handler routines, see pthread_atfork(3). For a complete list of system calls that are reentrant with
respect to signals, see signal(4).
RETURN VALUES
Upon successful completion, the fork() and vfork() functions return a value of 0 (zero) to the child process and return the process ID of
the child process to the parent process. Otherwise, a value of -1 is returned to the parent, no child process is created, and errno is set
to indicate the error.
ERRORS
The fork() and vfork() functions set errno to the specified values for the following conditions: The limit on the total number of processes
executing for a single user would be exceeded. This limit can be exceeded by a process with superuser privilege. There is not enough
space left for this process.
RELATED INFORMATION
Functions: exec(2), exit(2), getpriority(2), getrusage(2), plock(2), ptrace(2), semop(2), shmat(2), sigaction(2), sigvec(2), umask(2),
wait(2)
Routines: nice(3), pthread_atfork(3), raise(3), times(3), ulimit(3)
Files: signal(4)
Standards: standards(5) delim off
fork(2)