Advanced Programming in the UNIX® Environment

ptg10805159

Section 8.13 systemFunction 267

#include "apue.h"

#include <sys/wait.h>

int

main(void)

{

int status;

if ((status = system("date")) < 0)

err_sys("system() error");

pr_exit(status);

if ((status = system("nosuchcommand")) < 0)

err_sys("system() error");

pr_exit(status);

if ((status = system("who; exit 44")) < 0)

err_sys("system() error");

pr_exit(status);

exit(0);

}

Figure 8.23 Calling thesystemfunction

Earlier systems, including SVR3.2 and 4.3BSD, didn’t have thewaitpidfunction

available. Instead, the parent waited for the child, using a statement such as

while ((lastpid = wait(&status)) != pid && lastpid != -1)

;

Aproblem occurs if the process that callssystemhas spawned its own children before

callingsystem.Because thewhilestatement above keeps looping until the child that

was generated bysystemterminates, if any children of the process terminate beforethe

process identified bypid,then the process ID and termination status of these other

children arediscarded by thewhilestatement. Indeed, this inability towaitfor a

specific child is one of the reasons given in the POSIX.1 Rationale for including the

waitpidfunction. We’ll see in Section 15.3 that the same problem occurs with the

popenandpclosefunctions if the system doesn’t provide awaitpidfunction.

Set-User-ID Programs

What happens if we callsystemfrom a set-user-ID program? Doing so creates a

security hole and should never be attempted. Figure8.24 shows a simple program that

just callssystemfor its command-line argument.