mirror of https://github.com/mkerrisk/man-pages
shmop.2: EXAMPLE: add a pair of example programs
Add example programs demonstrating usage of shmget(2), shmat(2), semget(2), semctl(2), and semop(2). Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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man2/shmop.2
204
man2/shmop.2
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@ -1,4 +1,5 @@
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.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\" and Copyright 2020 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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@ -36,8 +37,6 @@
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.\" Changed wording and placement of sentence regarding attachment
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.\" of segments marked for destruction
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.\"
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.\" FIXME . Add an example program to this page.
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.\"
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.TH SHMOP 2 2019-08-02 "Linux" "Linux Programmer's Manual"
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.SH NAME
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shmat, shmdt \- System V shared memory operations
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@ -292,6 +291,207 @@ is the same as the system page size.)
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The implementation places no intrinsic per-process limit on the
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number of shared memory segments
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.RB ( SHMSEG ).
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.SH EXAMPLE
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.PP
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The two programs shown below exchange a string using a shared memory segment.
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Further details about the programs are given below.
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First, we show a shell session demonstrating their use.
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.PP
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In one terminal window, we run the "reader" program,
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which creates a System V shared memory segment and a System V semaphore set.
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The program prints out the IDs of the created objects,
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and then waits for the semaphore to change value.
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.PP
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.in +4n
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.EX
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$ \fB./svshm_string_read \fP
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shmid = 1114194; semid = 15
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.EE
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.in
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.PP
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In another terminal window, we run the "writer" program.
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The "writer" program takes three command-line arguments:
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the IDs of the shared memory segment and semaphore set created
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by the "reader", and a string.
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It attaches the existing shared memory segment,
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copies the string to the shared memory, and modifies the semaphore value.
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.PP
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.in +4n
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.EX
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$ \fB./svshm_string_write 1114194 15 \(aqHello, world\(aq\fP
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.EE
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.in
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.PP
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Returning to the terminal where the "reader" is running,
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we see that the program has ceased waiting on the semaphore
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and has printed the string that was copied into the
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shared memory segment by the writer:
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.PP
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.in +4n
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.EX
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Hello, world
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.EE
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.in
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.PP
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.\"
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.SS Program source: svshm_string.h
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The following header file is included by the "reader" and "writer" programs.
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.PP
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.in +4n
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.EX
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#include <sys/types.h>
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#include <sys/ipc.h>
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#include <sys/shm.h>
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#include <sys/sem.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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#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
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} while (0)
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union semun { /* Used in calls to semctl() */
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int val;
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struct semid_ds * buf;
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unsigned short * array;
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#if defined(__linux__)
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struct seminfo * __buf;
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#endif
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};
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#define MEM_SIZE 4096
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.EE
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.in
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.\"
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.SS Program source: svshm_string_read.c
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The "reader" program creates a shared memory segment and a semaphore set
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containing one semaphore.
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It then attaches the shared memory object into its address space
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and initializes the semaphore value to 1.
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Finally, the program waits for the semaphore value to become 0,
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and afterwards prints the string that has been copied into the
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shared memory segment by the "writer".
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.PP
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.in +4n
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.EX
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#include "svshm_string.h"
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int
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main(int argc, char *argv[])
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{
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int semid, shmid;
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union semun arg, dummy;
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struct sembuf sop;
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char *addr;
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/* Create shared memory and semaphore set containing one
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semaphore */
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shmid = shmget(IPC_PRIVATE, MEM_SIZE, IPC_CREAT | 0600);
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if (shmid == \-1)
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errExit("shmget");
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semid = semget(IPC_PRIVATE, 1, IPC_CREAT | 0600);
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if (shmid == \-1)
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errExit("shmget");
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/* Attach shared memory into our address space */
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addr = shmat(shmid, NULL, SHM_RDONLY);
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if (addr == (void *) \-1)
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errExit("shmat");
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/* Initialize semaphore 0 in set with value 1 */
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arg.val = 1;
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if (semctl(semid, 0, SETVAL, arg) == \-1)
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errExit("semctl");
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printf("shmid = %d; semid = %d\en", shmid, semid);
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/* Wait for semaphore value to become 0 */
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sop.sem_num = 0;
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sop.sem_op = 0;
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sop.sem_flg = 0;
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if (semop(semid, &sop, 1) == \-1)
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errExit("semop");
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/* Print the string from shared memory */
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printf("%s\en", addr);
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/* Remove shared memory and semaphore set */
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if (shmctl(shmid, IPC_RMID, NULL) == \-1)
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errExit("shmctl");
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if (semctl(semid, 0, IPC_RMID, dummy) == \-1)
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errExit("semctl");
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exit(EXIT_SUCCESS);
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}
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.EE
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.in
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.\"
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.SS Program source: svshm_string_write.c
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The writer program takes three command-line arguments:
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the IDs of the shared memory segment and semaphore set
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that have already been created by the "reader", and a string.
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It attaches the shared memory segment into its address space,
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and then decrements the semaphore value to 0 in order to inform the
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"reader" that it can now examine the contents of the shared memory.
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.PP
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.in +4n
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.EX
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#include "svshm_string.h"
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int
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main(int argc, char *argv[])
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{
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int semid, shmid;
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struct sembuf sop;
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char *addr;
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size_t len;
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if (argc != 4) {
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fprintf(stderr, "Usage: %s shmid semid string\en", argv[0]);
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exit(EXIT_FAILURE);
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}
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len = strlen(argv[3]) + 1; /* +1 to include trailing \(aq\e0\(aq */
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if (len > MEM_SIZE) {
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fprintf(stderr, "String is too big!\en");
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exit(EXIT_FAILURE);
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}
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/* Get object IDs from command\-line */
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shmid = atoi(argv[1]);
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semid = atoi(argv[2]);
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/* Attach shared memory into our address space and copy string
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(including trailing null byte) into memory. */
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addr = shmat(shmid, NULL, 0);
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if (addr == (void *) \-1)
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errExit("shmat");
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memcpy(addr, argv[3], len);
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/* Decrement semaphore t0 0 */
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sop.sem_num = 0;
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sop.sem_op = \-1;
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sop.sem_flg = 0;
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if (semop(semid, &sop, 1) == \-1)
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errExit("semop");
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exit(EXIT_SUCCESS);
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}
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.EE
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.in
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.SH SEE ALSO
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.BR brk (2),
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.BR mmap (2),
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