86 lines
2.3 KiB
C
86 lines
2.3 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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/*
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* A program to show the value of some file
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* descriptors.
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*/
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#include <errno.h>
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#include <fcntl.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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int
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main() {
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int fd1, fd2, fd3;
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FILE *f;
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printf("STDIN_FILENO: %d\n", STDIN_FILENO);
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printf("stdin: %d\n", fileno(stdin));
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printf("STDOUT_FILENO: %d\n", STDOUT_FILENO);
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printf("stdout: %d\n", fileno(stdout));
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printf("STDERR_FILENO: %d\n", STDERR_FILENO);
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printf("stderr: %d\n", fileno(stderr));
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printf("\nOpening /dev/zero...\n");
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if ((fd1 = open("/dev/zero", O_RDONLY)) < 0) {
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fprintf(stderr, "Unable to open /dev/zero: %s\n", strerror(errno));
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} else {
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printf("fd1: %d\n", fd1);
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}
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printf("\nOpening /dev/zero a second time...\n");
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if ((fd2 = open("/dev/zero", O_RDONLY)) < 0) {
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fprintf(stderr, "Unable to open /dev/zero: %s\n", strerror(errno));
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exit(EXIT_FAILURE);
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}
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printf("fd2: %d\n", fd2);
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printf("\nNow closing fd1, but keeping fd2 open..\n");
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(void)close(fd1);
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printf("\nOpening /dev/zero a third time...\n");
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if ((fd3 = open("/dev/zero", O_RDONLY)) < 0) {
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fprintf(stderr, "Unable to open /dev/zero: %s\n", strerror(errno));
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exit(EXIT_FAILURE);
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}
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printf("fd3: %d\n", fd3);
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printf("\nNow closing fd2 and fd3.\n");
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(void)close(fd2);
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(void)close(fd3);
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printf("Now opening /dev/zero as a stream.\n");
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if ((f = fopen("/dev/zero", "r")) == NULL) {
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fprintf(stderr, "Unable to open /dev/zero: %s\n", strerror(errno));
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exit(EXIT_FAILURE);
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}
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printf("f: %d\n", fileno(f));
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(void)fclose(f);
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(void)close(STDERR_FILENO);
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printf("\nOpening /dev/stderr after closing STDERR...\n");
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if ((fd3 = open("/dev/stderr", O_RDONLY)) < 0) {
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fprintf(stdout, "Unable to open /dev/stderr: %s\n", strerror(errno));
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exit(EXIT_FAILURE);
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}
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printf("stderr is now: %d\n", fd3);
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return EXIT_SUCCESS;
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}
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