83 lines
1.9 KiB
C
83 lines
1.9 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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/* A simple example to illustrate the use of shared
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* memory for IPC.
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*
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* Note that multiple invocations as a writer
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* overwrite the same location (unlike message
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* queues), and that shared memory, as all SysV IPC
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* structures, remain available after the program
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* exits and need to be cleaned up explicitly.
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*
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* Use ipcs(1) to show the use of the shared memory.
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*/
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#include <sys/ipc.h>
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#include <sys/shm.h>
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#include <sys/types.h>
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#include <err.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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/* Code examples often use a char as the ftok(3) id,
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* but really any int will do. */
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#define SHM_ID 42
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#define SHM_SIZE 1024
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int
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main(int argc, char **argv) {
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key_t key;
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int shmid;
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char *data;
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if (argc > 2) {
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(void)fprintf(stderr, "usage: shmdemo [data_to_write]\n");
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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if ((key = ftok("./shmdemo.c", SHM_ID)) == -1) {
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err(EXIT_FAILURE, "ftok");
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/* NOTREACHED */
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}
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if ((shmid = shmget(key, SHM_SIZE, 0666 | IPC_CREAT)) == -1) {
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err(EXIT_FAILURE, "shmget");
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/* NOTREACHED */
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}
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data = shmat(shmid, (void *)0, 0);
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if (data == (char *)(-1)) {
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err(EXIT_FAILURE, "shmat");
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/* NOTREACHED */
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}
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if (argc == 2) {
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(void)printf("%d: writing to segment: \"%s\"\n", getpid(), argv[1]);
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(void)strncpy(data, argv[1], SHM_SIZE);
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} else
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(void)printf("%d: segment contains: \"%s\"\n", getpid(), data);
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if (shmdt(data) == -1) {
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err(EXIT_FAILURE, "shmdt");
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/* NOTREACHED */
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}
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return EXIT_SUCCESS;
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}
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