svshm/svshm_xfr.h

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

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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 */
 
 #define OBJ_PERMS (S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP)
                                 /* 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 */
 #endif
 
 struct shmseg {                 /* Defines structure of shared memory segment */
     int cnt;                    /* Number of bytes used in 'buf' */
     char buf[BUF_SIZE];         /* Data being transferred */
 };