8.4. vfork Function

The function vfork has the same calling sequence and same return values as fork. But the semantics of the two functions differ.

The vfork function originated with 2.9BSD. Some consider the function a blemish, but all the platforms covered in this book support it. In fact, the BSD developers removed it from the 4.4BSD release, but all the open source BSD distributions that derive from 4.4BSD added support for it back into their own releases. The vfork function is marked as an obsolete interface in Version 3 of the Single UNIX Specification.

The vfork function is intended to create a new process when the purpose of the new process is to exec a new program (step 2 at the end of the previous section). The bare-bones shell in the program from Figure 1.7 is also an example of this type of program. The vfork function creates the new process, just like fork, without copying the address space of the parent into the child, as the child won't reference that address space; the child simply calls exec (or exit) right after the vfork. Instead, while the child is running and until it calls either exec or exit, the child runs in the address space of the parent. This optimization provides an efficiency gain on some paged virtual-memory implementations of the UNIX System. (As we mentioned in the previous section, implementations use copy-on-write to improve the efficiency of a fork followed by an exec, but no copying is still faster than some copying.)

Another difference between the two functions is that vfork guarantees that the child runs first, until the child calls exec or exit. When the child calls either of these functions, the parent resumes. (This can lead to deadlock if the child depends on further actions of the parent before calling either of these two functions.)

Example

The program in Figure 8.3 is a modified version of the program from Figure 8.1. We've replaced the call to fork with vfork and removed the write to standard output. Also, we don't need to have the parent call sleep, as we're guaranteed that it is put to sleep by the kernel until the child calls either exec or exit.

Running this program gives us

$ ./a.out
before vfork
pid = 29039, glob = 7, var = 89

Here, the incrementing of the variables done by the child changes the values in the parent. Because the child runs in the address space of the parent, this doesn't surprise us. This behavior, however, differs from fork.

Note in Figure 8.3 that we call _exit instead of exit. As we described in Section 7.3, _exit does not perform any flushing of standard I/O buffers. If we call exit instead, the results are indeterminate. Depending on the implementation of the standard I/O library, we might see no difference in the output, or we might find that the output from the parent's printf has disappeared.

If the child calls exit, the implementation flushes the standard I/O streams. If this is the only action taken by the library, then we will see no difference with the output generated if the child called _exit. If the implementation also closes the standard I/O streams, however, the memory representing the FILE object for the standard output will be cleared out. Because the child is borrowing the parent's address space, when the parent resumes and calls printf, no output will appear and printf will return -1. Note that the parent's STDOUT_FILENO is still valid, as the child gets a copy of the parent's file descriptor array (refer back to Figure 8.2).

Most modern implementations of exit will not bother to close the streams. Because the process is about to exit, the kernel will close all the file descriptors open in the process. Closing them in the library simply adds overhead without any benefit.

Figure 8.3. Example of vfork function

#include "apue.h"

int     glob = 6;       /* external variable in initialized data */

int
main(void)
{
    int     var;        /* automatic variable on the stack */
    pid_t   pid;

    var = 88;
    printf("before vfork\n");   /* we don't flush stdio */
    if ((pid = vfork()) < 0) {
        err_sys("vfork error");
    } else if (pid == 0) {      /* child */
        glob++;                 /* modify parent's variables */
        var++;
        _exit(0);               /* child terminates */
    }
    /*
     * Parent continues here.
     */
    printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var);
    exit(0);
}

Section 5.6 of McKusick et al. [1996] contains additional information on the implementation issues of fork and vfork. Exercises 8.1 and 8.2 continue the discussion of vfork.