171 lines
3.6 KiB
C
171 lines
3.6 KiB
C
/* 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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* This trivial program illustrates how to open files. We present the
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* following use cases:
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*
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* - create a non-existent file
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* - "create" an existing file
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* - fail to create an existing file
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* - open an existing file
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* - fail to open a non-existing file
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* - truncate an existing file
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*/
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#include <sys/stat.h>
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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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#ifndef SLEEP
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#define SLEEP 10
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#endif
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void
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createFile() {
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int fd;
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printf("Checking if './newfile' exists...\n");
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system("ls -l ./newfile");
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printf("Trying to create './newfile' with O_RDONLY | O_CREAT...\n");
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if ((fd = open("./newfile", O_RDONLY | O_CREAT,
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S_IRUSR | S_IWUSR)) == -1) {
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fprintf(stderr, "Unable to create './newfile': %s\n",
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strerror(errno));
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exit(EXIT_FAILURE);
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}
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printf("'./newfile' created. File descriptor is: %d\n", fd);
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/* We are leaking a file descriptor here to illustrate that they
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* are increasing as we open more files. Normally, we would call
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* close(2) whenever we're done using the decriptor. */
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}
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void
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failExclFileCreation() {
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int fd;
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printf("Checking if './newfile' exists...\n");
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system("ls -l ./newfile");
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printf("Trying to create './newfile' with O_RDONLY | O_CREAT | O_EXCL...\n");
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if ((fd = open("./newfile", O_RDONLY | O_CREAT | O_EXCL,
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S_IRUSR | S_IWUSR)) == -1) {
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fprintf(stderr, "Unable to create './newfile': %s\n",
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strerror(errno));
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}
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/* We expect this to fail! */
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if (close(fd) == -1) {
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fprintf(stderr, "Closing failed: %s\n", strerror(errno));
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}
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}
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void
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failOpenNonexistingFile() {
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int fd;
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printf("Trying to open (non-existant) './nosuchfile' with O_RDONLY...\n");
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if ((fd = open("./nosuchfile", O_RDONLY)) == -1) {
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fprintf(stderr, "Unable to open './nosuchfile': %s\n",
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strerror(errno));
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}
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/* We know this is going to fail, but no need to complain. */
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(void)close(fd);
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}
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void
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openFile() {
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int fd;
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printf("Trying to open './openex.c' with O_RDONLY...\n");
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if ((fd = open("./openex.c", O_RDONLY)) == -1) {
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fprintf(stderr, "Unable to open './openex.c': %s\n",
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strerror(errno));
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exit(EXIT_FAILURE);
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}
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printf("'./openex.c' opened. File descriptor is: %d\n", fd);
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if (close(fd) == 0) {
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printf("'./openex.c' closed again\n");
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}
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}
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void
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truncateFile() {
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int fd;
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system("cp openex.c newfile");
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printf("Copied 'openex.c' to 'newfile'.\n");
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system("ls -l newfile");
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printf("Trying to open './newfile' with O_RDONLY | O_TRUNC...\n");
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if ((fd = open("./newfile", O_RDONLY | O_TRUNC)) == -1) {
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fprintf(stderr, "Unable to open './newfile': %s\n",
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strerror(errno));
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exit(EXIT_FAILURE);
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}
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printf("'./newfile' opened. File descriptor is: %d\n", fd);
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printf("'./newfile' truncated -- see 'ls -l newfile'\n");
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system("ls -l newfile");
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(void)close(fd);
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}
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int
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main() {
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createFile();
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system("ls -l newfile");
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printf("\n");
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sleep(SLEEP);
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createFile();
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system("ls -l newfile");
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printf("\n");
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sleep(SLEEP);
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failExclFileCreation();
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printf("\n");
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sleep(SLEEP);
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openFile();
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printf("\n");
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sleep(SLEEP);
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failOpenNonexistingFile();
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printf("\n");
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sleep(SLEEP);
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truncateFile();
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return 0;
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}
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