svshm/svshm_xfr_writer.c

This is svshm/svshm_xfr_writer.c (Listing 48-2, page 1003), 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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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/*  svshm_xfr_writer.c
+
+   Read buffers of data data from standard input into a System V shared memory
+   segment from which it is copied by svshm_xfr_reader.c
+
+   We use a pair of binary semaphores to ensure that the writer and reader have
+   exclusive, alternating access to the shared memory. (I.e., the writer writes
+   a block of text, then the reader reads, then the writer writes etc). This
+   ensures that each block of data is processed in turn by the writer and
+   reader.
+
+   This program needs to be started before the reader process as it creates the
+   shared memory and semaphores used by both processes.
+
+   Together, these two programs can be used to transfer a stream of data through
+   shared memory as follows:
+
+        $ svshm_xfr_writer < infile &
+        $ svshm_xfr_reader > out_file
+*/
 #include "semun.h"              /* Definition of semun union */
 #include "svshm_xfr.h"
 
 int
 main(int argc, char *argv[])
 {
     int semid, shmid, bytes, xfrs;
     struct shmseg *shmp;
     union semun dummy;
 
+    /* Create set containing two semaphores; initialize so that
+       writer has first access to shared memory. */
+
     semid = semget(SEM_KEY, 2, IPC_CREAT | OBJ_PERMS);
     if (semid == -1)
         errExit("semget");
 
     if (initSemAvailable(semid, WRITE_SEM) == -1)
         errExit("initSemAvailable");
     if (initSemInUse(semid, READ_SEM) == -1)
         errExit("initSemInUse");
 
+    /* Create shared memory; attach at address chosen by system */
+
     shmid = shmget(SHM_KEY, sizeof(struct shmseg), IPC_CREAT | OBJ_PERMS);
     if (shmid == -1)
         errExit("shmget");
 
     shmp = shmat(shmid, NULL, 0);
     if (shmp == (void *) -1)
         errExit("shmat");
 
     /* Transfer blocks of data from stdin to shared memory */
 
     for (xfrs = 0, bytes = 0; ; xfrs++, bytes += shmp->cnt) {
         if (reserveSem(semid, WRITE_SEM) == -1)         /* Wait for our turn */
             errExit("reserveSem");
 
         shmp->cnt = read(STDIN_FILENO, shmp->buf, BUF_SIZE);
         if (shmp->cnt == -1)
             errExit("read");
 
         if (releaseSem(semid, READ_SEM) == -1)          /* Give reader a turn */
             errExit("releaseSem");
 
         /* Have we reached EOF? We test this after giving the reader
            a turn so that it can see the 0 value in shmp->cnt. */
 
         if (shmp->cnt == 0)
             break;
     }
 
     /* Wait until reader has let us have one more turn. We then know
        reader has finished, and so we can delete the IPC objects. */
 
     if (reserveSem(semid, WRITE_SEM) == -1)
         errExit("reserveSem");
 
     if (semctl(semid, 0, IPC_RMID, dummy) == -1)
         errExit("semctl");
     if (shmdt(shmp) == -1)
         errExit("shmdt");
     if (shmctl(shmid, IPC_RMID, 0) == -1)
         errExit("shmctl");
 
     fprintf(stderr, "Sent %d bytes (%d xfrs)\n", bytes, xfrs);
     exit(EXIT_SUCCESS);
 }

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

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