Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.18 systemFunction 369

Recall from Section 9.6 that typing the interrupt character causes the interrupt signal to

be sent to all the processes in the foreground process group. Figure10.27 shows the

arrangement of the processes when the editor is running.

login shell forkexec a.out forkexec /bin/sh forkexec /bin/ed

background process group foreground process group

Figure 10.27Foreground and background process groups for Figure10.26

In this example,SIGINTis sent to all three foreground processes. (The shell ignores

it.) As we can see from the output, both thea.outprocess and the editor catch the

signal. But when we’rerunning another program with the system function, we

shouldn’t have both the parent and the child catching the two terminal-generated

signals: interrupt and quit. Instead, these two signals should be sent to the program

that is running: the child. Since the command that is executed bysystemcan be an

interactive command (as is theedprogram in this example) and since the caller of

systemgives up control while the program executes, waiting for it to finish, the caller

ofsystemshould not be receiving these two terminal-generated signals. For this

reason, POSIX.1 specifies that thesystemfunction should ignorethese two signals

while waiting for the command to complete.

Example

Figure10.28 shows an implementation of thesystemfunction with the required signal

handling.

#include <sys/wait.h>

#include <errno.h>

#include <signal.h>

#include <unistd.h>

int

system(const char *cmdstring) /* with appropriate signal handling */

{

pid_t pid;

int status;

struct sigaction ignore, saveintr, savequit;

sigset_t chldmask, savemask;

if (cmdstring == NULL)

return(1); /* always a command processor with UNIX */

ignore.sa_handler = SIG_IGN; /* ignore SIGINT and SIGQUIT */

sigemptyset(&ignore.sa_mask);

ignore.sa_flags = 0;