The Linux Programming Interface

Memory Mappings 1029

addr = mmap(NULL, MEM_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

if (addr == MAP_FAILED)

errExit("mmap");

if (close(fd) == -1) /* No longer need 'fd' */

errExit("close");

printf("Current string=%.*s\n", MEM_SIZE, addr);

/* Secure practice: output at most MEM_SIZE bytes */

if (argc > 2) { /* Update contents of region */

if (strlen(argv[2]) >= MEM_SIZE)

cmdLineErr("'new-value' too large\n");

memset(addr, 0, MEM_SIZE); /* Zero out region */

strncpy(addr, argv[2], MEM_SIZE - 1);

if (msync(addr, MEM_SIZE, MS_SYNC) == -1)

errExit("msync");

printf("Copied \"%s\" to shared memory\n", argv[2]);

}

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– mmap/t_mmap.c

49.4.3 Boundary Cases

In many cases, the size of a mapping is a multiple of the system page size, and the

mapping falls entirely within the bounds of the mapped file. However, this is not

necessarily so, and we now look at what happens when these conditions don’t hold.

Figure 49-3 portrays the case where the mapping falls entirely within the

bounds of the mapped file, but the size of the region is not a multiple of the system

page size (which we assume is 4096 bytes for the purposes of this discussion).

Figure 49-3: Memory mapping whose length is not a multiple of the system page size

Memory

region

references

yield SIGSEGV

Mapped file

(9500 bytes)

remainder

of page

file offset: 0

byte offset: 0 6000

mmap(0, 6000, prot, MAP_SHARED, fd, 0);

accessible, mapped to file

9499

5999 81918192

requested size of mapping

actual mapped region of file

81918192

unmapped