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vfork(2) [freebsd man page]

VFORK(2)						      BSD System Calls Manual							  VFORK(2)

NAME
vfork -- create a new process without copying the address space LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> pid_t vfork(void); DESCRIPTION
The vfork() system call can be used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in a paged environment. It is useful when the purpose of fork(2) would have been to create a new system context for an execve(2). The vfork() system call differs from fork(2) in that the child borrows the parent's memory and thread of control until a call to execve(2) or an exit (either by a call to _exit(2) or abnormally). The parent process is suspended while the child is using its resources. The vfork() system call returns 0 in the child's context and (later) the pid of the child in the parent's context. The vfork() system call can normally be used just like fork(2). It does not work, however, to return while running in the child's context from the procedure that called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Be careful, also, to call _exit(2) rather than exit(3) if you cannot execve(2), since exit(3) will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. (Even with fork(2) it is wrong to call exit(3) since buffered data would then be flushed twice.) RETURN VALUES
Same as for fork(2). SEE ALSO
_exit(2), execve(2), fork(2), rfork(2), sigaction(2), wait(2), exit(3) HISTORY
The vfork() system call appeared in 2.9BSD. BUGS
To avoid a possible deadlock situation, processes that are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals; rather, output or ioctl(2) calls are allowed and input attempts result in an end-of-file indication. BSD
November 13, 2009 BSD

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VFORK(P)						     POSIX Programmer's Manual							  VFORK(P)

NAME
vfork - create a new process; share virtual memory SYNOPSIS
#include <unistd.h> pid_t vfork(void); DESCRIPTION
The vfork() function shall be equivalent to fork(), except that the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec family of functions. RETURN VALUE
Upon successful completion, vfork() shall return 0 to the child process and return the process ID of the child process to the parent process. Otherwise, -1 shall be returned to the parent, no child process shall be created, and errno shall be set to indicate the error. ERRORS
The vfork() function shall fail if: EAGAIN The system-wide limit on the total number of processes under execution would be exceeded, or the system-imposed limit on the total number of processes under execution by a single user would be exceeded. ENOMEM There is insufficient swap space for the new process. The following sections are informative. EXAMPLES
None. APPLICATION USAGE
Conforming applications are recommended not to depend on vfork(), but to use fork() instead. The vfork() function may be withdrawn in a future version. On some implementations, vfork() is equivalent to fork(). The vfork() function differs from fork() only in that the child process can share code and data with the calling process (parent process). This speeds cloning activity significantly at a risk to the integrity of the parent process if vfork() is misused. The use of vfork() for any purpose except as a prelude to an immediate call to a function from the exec family, or to _exit(), is not advised. The vfork() function can be used to create new processes without fully copying the address space of the old process. If a forked process is simply going to call exec, the data space copied from the parent to the child by fork() is not used. This is particularly inefficient in a paged environment, making vfork() particularly useful. Depending upon the size of the parent's data space, vfork() can give a significant performance improvement over fork(). The vfork() function can normally be used just like fork(). It does not work, however, to return while running in the child's context from the caller of vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Care should be taken, also, to call _exit() rather than exit() if exec cannot be used, since exit() flushes and closes standard I/O channels, thereby damaging the parent process' standard I/O data structures. (Even with fork(), it is wrong to call exit(), since buffered data would then be flushed twice.) If signal handlers are invoked in the child process after vfork(), they must follow the same rules as other code in the child process. RATIONALE
None. FUTURE DIRECTIONS
This function may be withdrawn in a future version. SEE ALSO
exec() , exit() , fork() , wait() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h> COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol- ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html . IEEE
/The Open Group 2003 VFORK(P)

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