Advanced Programming in the UNIX® Environment

ptg10805159

Section 8.4 vforkFunction 235

#include "apue.h"

int globvar=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 */

globvar++; /* modify parent’s variables */

var++;

_exit(0); /* child terminates */

}

/* parent continues here */

printf("pid = %ld, glob = %d, var = %d\n", (long)getpid(), globvar,

var);

exit(0);

}

Figure 8.3Example ofvforkfunction

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 the behavior offork.

Note in Figure8.3 that we call _exitinstead of exit.As we described in

Section 7.3,_exitdoes not perform any flushing of standardI/O buffers. If we call

exitinstead, the results areindeterminate. Depending on the implementation of the

standardI/O library, we might see no difference in the output, or we might find that the

output from the firstprintfin the parent has disappeared.

If the child callsexit,the implementation flushes the standardI/O streams. If this

is the only action taken by the library,then we will see no difference from the output

generated if the child called_exit.Ifthe implementation also closes the standardI/O

streams, however,the memory representing theFILEobject for the standardoutput

will be cleared out. Because the child is borrowing the parent’s address space, when the

parent resumes and callsprintf, no output will appear andprintfwill return−1.

Note that the parent’sSTDOUT_FILENOis still valid, as the child gets a copy of the

parent’s file descriptor array (refer back to Figure8.2).