Standard description¶

(From POSIX.1) The vfork() function has the same effect as fork(2), 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(2) or one of the exec(3) family of functions.

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 is created which then immediately issues an execve(2).

vfork() differs from fork(2) in that the calling thread is suspended until the child terminates (either normally, by calling _exit(2), or abnormally, after delivery of a fatal signal), or it makes a call to execve(2). Until that point, the child shares all memory with its parent, including the stack. The child must not return from the current function or call exit(3) (which would have the effect of calling exit handlers established by the parent process and flushing the parent's stdio(3) buffers), but may call _exit(2).

As with fork(2), the child process created by vfork() inherits copies of various of the caller's process attributes (e.g., file descriptors, signal dispositions, and current working directory); the vfork() call differs only in the treatment of the virtual address space, as described above.

Signals sent to the parent arrive after the child releases the parent's memory (i.e., after the child terminates or calls execve(2)).

Historic description¶

Under Linux, fork(2) is implemented using copy-on-write pages, so the only penalty incurred by fork(2) 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(2) would require making a complete copy of the caller's data space, often needlessly, since usually immediately afterward an exec(3) is done. Thus, for greater efficiency, BSD introduced the vfork() system call, which 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(2) 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 were held in a register.

Caveats¶

The child process should take care not to modify the memory in unintended ways, since such changes will be seen by the parent process once the child terminates or executes another program. In this regard, signal handlers can be especially problematic: if a signal handler that is invoked in the child of vfork() changes memory, those changes may result in an inconsistent process state from the perspective of the parent process (e.g., memory changes would be visible in the parent, but changes to the state of open file descriptors would not be visible).

When vfork() is called in a multithreaded process, only the calling thread is suspended until the child terminates or executes a new program. This means that the child is sharing an address space with other running code. This can be dangerous if another thread in the parent process changes credentials (using setuid(2) or similar), since there are now two processes with different privilege levels running in the same address space. As an example of the dangers, suppose that a multithreaded program running as root creates a child using vfork(). After the vfork(), a thread in the parent process drops the process to an unprivileged user in order to run some untrusted code (e.g., perhaps via plug-in opened with dlopen(3)). In this case, attacks are possible where the parent process uses mmap(2) to map in code that will be executed by the privileged child process.

Linux notes¶

Fork handlers established using pthread_atfork(3) are not called when a multithreaded program employing the NPTL threading library calls vfork(). Fork handlers are called in this case in a program using the LinuxThreads threading library. (See pthreads(7) for a description of Linux threading libraries.)

A call to vfork() is equivalent to calling clone(2) with flags specified as:

CLONE_VM | CLONE_VFORK | SIGCHLD

History¶

The vfork() system call appeared in 3.0BSD. In 4.4BSD it was made synonymous to fork(2) but NetBSD introduced it again; see http://www.netbsd.org/Documentation/kernel/vfork.html. In Linux, it has been equivalent to fork(2) until 2.2.0-pre6 or so. Since 2.2.0-pre9 (on i386, somewhat later on other architectures) it is an independent system call. Support was added in glibc 2.0.112.