| Linux / Unix Command: vfork |
NAME
vfork - create a child process and block parentSYNOPSIS
#include <sys/types.h>#include <unistd.h>
STANDARD DESCRIPTION
(From XPG4 / SUSv2 / POSIX draft.) The vfork() function has the same effect as fork(), except that the behaviour 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.ERRORS
- EAGAIN
- Too many processes - try again.
- ENOMEM
- There is insufficient swap space for the new process.
LINUX DESCRIPTION
vfork, just like fork(2), creates a child process of the calling process. For details and return value and errors, see fork(2).vfork() is a special case of clone(2). It is used to create new processes without copying the page tables of the parent process. It may be useful in performance sensitive applications where a child will be created which then immediately issues an execve().
vfork() differs from fork in that the parent is suspended until the child makes a call to execve(2) or _exit(2). The child shares all memory with its parent, including the stack, until execve() is issued by the child. The child must not return from the current function or call exit(), but may call _exit().
Signal handlers are inherited, but not shared. Signals to the parent arrive after the child releases the parent.
HISTORIC DESCRIPTION
Under Linux, fork() is implemented using copy-on-write pages, so the only penalty incurred by fork() is the time and memory required to duplicate the parent's page tables, and to create a unique task structure for the child. However, in the bad old days a fork() would require making a complete copy of the caller's data space, often needlessly, since usually immediately afterwards an exec() is done. Thus, for greater efficiency, BSD introduced the vfork system call, that did not fully copy the address space of the parent process, but borrowed the parent's memory and thread of control until a call to execve() or an exit occurred. The parent process was suspended while the child was using its resources. The use of vfork was tricky - for example, not modifying data in the parent process depended on knowing which variables are held in a register.SEE ALSO
clone(2), execve(2), fork(2), wait(2)
Important: Use the man command (% man) to see how a command is used on your particular computer.
