Advanced Programming in the UNIX® Environment

ptg10805159

Appendix C Chapter 8 Solutions 921

This assumes that the standardI/O streamstdoutis closed when the child calls

exit,not the file descriptorSTDOUT_FILENO.Some versions of the standard

I/O library close the file descriptor associated with standardoutput, which would

cause thewriteto standardoutput to also fail. In this case,dupstandardoutput

to another descriptor,and use this new descriptor for thewrite.

8.2 Consider FigureC.8.

#include "apue.h"

static void f1(void), f2(void);

int

main(void)

{

f1();

f2();

_exit(0);

}

static void

f1(void)

{

pid_t pid;

if ((pid = vfork()) < 0)

err_sys("vfork error");

/* child and parent both return */

}

static void

f2(void)

{

char buf[1000]; /* automatic variables */

int i;

for (i = 0; i < sizeof(buf); i++)

buf[i] = 0;

}

Figure C.8 Incorrect use ofvfork

Whenvforkis called, the parent’s stack pointer points to the stack frame for the

f1function that callsvfork.FigureC.9 shows this. vforkcauses the child to

execute first, and the child returns fromf1.The child then callsf2,and its stack

frame overwrites the previous stack frame forf1.The child then zeros out the

automatic variablebuf,setting 1,000 bytes of the stack frame to 0. The child

returns fromf2and then calls_exit,but the contents of the stack beneath the

stack frame formainhave been changed. The parent then resumes after the call