The Linux Programming Interface

Pipes and FIFOs 905

t fp = popen(popenCmd, "r");
if (fp == NULL) {
printf("popen() failed\n");
continue;
}

/* Read resulting list of pathnames until EOF */

fileCnt = 0;

while (fgets(pathname, PATH_MAX, fp) != NULL) {

printf("%s", pathname);

fileCnt++;

}

/* Close pipe, fetch and display termination status */

status = pclose(fp);

printf(" %d matching file%s\n", fileCnt, (fileCnt != 1)? "s" : "");

printf(" pclose() status == %#x\n", (unsigned int) status);

if (status != -1)

printWaitStatus("\t", status);

}

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––– pipes/popen_glob.c

The following shell session demonstrates the use of the program in Listing 44-5. In

this example, we first provide a pattern that matches two filenames, and then a pat-

tern that matches no filename:

$ ./popen_glob

pattern: popen_glob* Matches two filenames

popen_glob

popen_glob.c

2 matching files

pclose() status = 0

child exited, status=0

pattern: x* Matches no filename

0 matching files

pclose() status = 0x100 ls(1) exits with status 1

child exited, status=1

pattern: ^D$ Type Control-D to terminate

The construction of the command qr for globbing in Listing 44-5 requires some

explanation. Actual globbing of a pattern is performed by the shell. The ls command

is merely being used to list the matching filenames, one per line. We could have

tried using the echo command instead, but this would have had the undesirable

result that if a pattern matched no filenames, then the shell would leave the pattern

unchanged, and echo would simply display the pattern. By contrast, if ls is given the

name of a file that doesn’t exist, it prints an error message on stderr (which we dis-

pose of by redirecting stderr to /dev/null), prints nothing on stdout, and exits with a

status of 1.