118 lines
2.6 KiB
C
118 lines
2.6 KiB
C
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/* This file is part of the sample code and exercises
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* used by the class "Advanced Programming in the UNIX
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* Environment" taught by Jan Schaumann
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* <jschauma@netmeister.org> at Stevens Institute of
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* Technology.
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*
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* This file is in the public domain.
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*
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* You don't have to, but if you feel like
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* acknowledging where you got this code, you may
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* reference me by name, email address, or point
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* people to the course website:
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* https://stevens.netmeister.org/631/
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*/
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/* This program illustrates a more common use of a
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* pipe: one process generates data, the other exec's
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* another command with stdin connected to the pipe's
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* read end. This is equivalent to
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*
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* cat file | ${PAGER:-/usr/bin/more}
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*
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*/
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#include <sys/wait.h>
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#include <err.h>
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#include <errno.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#define DEF_PAGER "/usr/bin/more"
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#define LINE_LENGTH 128 /* arbitrary! */
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int
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main(int argc, char **argv)
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{
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int n, fd[2];
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pid_t pid;
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char line[LINE_LENGTH], *pager, *argv0;
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FILE *fp;
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if (argc != 2) {
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(void)fprintf(stderr, "Usage: %s <pathname>\n", argv[0]);
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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if ((fp = fopen(argv[1], "r")) == NULL) {
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(void)fprintf(stderr, "unable to open %s: %s\n",
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argv[1], strerror(errno));
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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if (pipe(fd) < 0) {
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err(EXIT_FAILURE, "pipe");
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/* NOTREACHED */
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}
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if ((pid = fork()) < 0) {
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err(EXIT_FAILURE, "fork");
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/* NOTREACHED */
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}
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if (pid > 0) {
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(void)close(fd[0]); /* close read end */
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while (fgets(line, LINE_LENGTH, fp) != NULL) {
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n = strlen(line);
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if (write(fd[1], line, n) != n) {
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err(EXIT_FAILURE, "write to pipe");
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/* NOTREACHED */
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}
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}
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if (ferror(fp)) {
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err(EXIT_FAILURE, "fgets");
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/* NOTREACHED */
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}
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/* We're done writing, so close write end of pipe. */
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(void)close(fd[1]);
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if (waitpid(pid, NULL, 0) < 0) {
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err(EXIT_FAILURE, "waitpid");
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/* NOTREACHED */
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}
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exit(EXIT_SUCCESS);
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/* NOTREACHED */
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} else {
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(void)close(fd[1]); /* close write end */
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if (fd[0] != STDIN_FILENO) {
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if (dup2(fd[0], STDIN_FILENO) != STDIN_FILENO) {
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err(EXIT_FAILURE, "dup2 to stdin");
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/* NOTREACHED */
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}
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}
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/* get arguments for execl() */
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if ((pager = getenv("PAGER")) == NULL)
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pager = DEF_PAGER;
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if ((argv0 = strrchr(pager, '/')) != NULL)
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argv0++;
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else
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argv0 = pager;
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/* We could of course lie about our name: */
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argv0 = "tar";
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execlp(pager, argv0, (char *) 0);
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(void)fprintf(stderr, "execl error for %s: %s\n",
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pager, strerror(errno));
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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/* NOTREACHED */
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}
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