Advanced Programming in the UNIX® Environment

ptg10805159

542 Interprocess Communication Chapter 15

parent cmdstring(child)

fp stdin

Figure 15.10Result offp = popen(cmdstring,"w")

One way to remember the final argument topopenis to remember that, likefopen,the

returned file pointer is readable iftypeis"r"or writable iftypeis"w".

Thepclosefunction closes the standardI/O stream, waits for the command to

terminate, and returns the termination status of the shell. (Wedescribed the

termination status in Section 8.6. Thesystemfunction, described in Section 8.13, also

returns the termination status.) If the shell cannot be executed, the termination status

returned bypcloseis as if the shell had executedexit(127).

Thecmdstringis executed by the Bourne shell, as in

sh -ccmdstring

This means that the shell expands any of its special characters incmdstring.This allows

us to say,for example,

fp = popen("ls *.c", "r");

or

fp = popen("cmd 2>&1", "r");

Example

Let’s redo the program from Figure15.6, usingpopen.This is shown in Figure15.11.

#include "apue.h"

#include <sys/wait.h>

#define PAGER "${PAGER:-more}" /* environment variable, or default */

int

main(int argc, char *argv[])

{

char line[MAXLINE];

FILE *fpin, *fpout;

if (argc != 2)

err_quit("usage: a.out <pathname>");

if ((fpin = fopen(argv[1], "r")) == NULL)

err_sys("can’t open %s", argv[1]);

if ((fpout = popen(PAGER, "w")) == NULL)

err_sys("popen error");

/* copy argv[1] to pager */