This is svshm/svshm_xfr.h (Listing 48-1, page 1002), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/*  svshm_xfr.h

   Header file used by the svshm_xfr_reader.c and svshm_xfr_writer.c programs.
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include "binary_sems.h"        /* Declares our binary semaphore functions */
#include "tlpi_hdr.h"

/* Hard-coded keys for IPC objects */

#define SHM_KEY 0x1234          /* Key for shared memory segment */
#define SEM_KEY 0x5678          /* Key for semaphore set */

                                /* Permissions for our IPC objects */

/* Two semaphores are used to ensure exclusive, alternating access
   to the shared memory segment */

#define WRITE_SEM 0             /* Writer has access to shared memory */
#define READ_SEM 1              /* Reader has access to shared memory */

#ifndef BUF_SIZE                /* Allow "cc -D" to override definition */
#define BUF_SIZE 1024           /* Size of transfer buffer */

struct shmseg {                 /* Defines structure of shared memory segment */
    int cnt;                    /* Number of bytes used in 'buf' */
    char buf[BUF_SIZE];         /* Data being transferred */


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