This is mmap/anon_mmap.c (Listing 49-3, page 1036), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, 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
+/* anon_mmap.c
+   Demonstrate how to share a region of mapped memory between a parent and
+   child process without having to create a mapped file, either through the
+   creation of an anonymous memory mapping or through the mapping of /dev/zero.
 #ifdef USE_MAP_ANON
 #define _BSD_SOURCE             /* Get MAP_ANONYMOUS definition */
 #include <sys/wait.h>
 #include <sys/mman.h>
 #include <fcntl.h>
 #include "tlpi_hdr.h"
 main(int argc, char *argv[])
     int *addr;                  /* Pointer to shared memory region */
+    /* Parent creates mapped region prior to calling fork() */
 #ifdef USE_MAP_ANON             /* Use MAP_ANONYMOUS */
     addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE,
                 MAP_SHARED | MAP_ANONYMOUS, -1, 0);
     if (addr == MAP_FAILED)
 #else                           /* Map /dev/zero */
     int fd;
     fd = open("/dev/zero", O_RDWR);
     if (fd == -1)
     addr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
     if (addr == MAP_FAILED)
     if (close(fd) == -1)        /* No longer needed */
     *addr = 1;                  /* Initialize integer in mapped region */
     switch (fork()) {           /* Parent and child share mapping */
     case -1:
     case 0:                     /* Child: increment shared integer and exit */
         printf("Child started, value = %d\n", *addr);
         if (munmap(addr, sizeof(int)) == -1)
     default:                    /* Parent: wait for child to terminate */
         if (wait(NULL) == -1)
         printf("In parent, value = %d\n", *addr);
         if (munmap(addr, sizeof(int)) == -1)

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