The Linux Programming Interface

1028 Chapter 49

We then use our program to map the file and copy a string into the mapped

region:

$ ./t_mmap s.txt hello

Current string=

Copied "hello" to shared memory

The program displayed nothing for the current string because the initial value of

the mapped files began with a null byte (i.e., zero-length string).

Next, we use our program to again map the file and copy a new string into the

mapped region:

$ ./t_mmap s.txt goodbye

Current string=hello

Copied "goodbye" to shared memory

Finally, we dump the contents of the file, 8 characters per line, to verify its contents:

$ od -c -w8 s.txt

0000000 g o o d b y e nul

0000010 nul nul nul nul nul nul nul nul

*

0002000

Our trivial program doesn’t use any mechanism to synchronize access by multiple

processes to the mapped file. However, real-world applications typically need to

synchronize access to shared mappings. This can be done using a variety of tech-

niques, including semaphores (Chapters 47 and 53) and file locking (Chapter 55).

We explain the msync() system call used in Listing 49-2 in Section 49.5.

Listing 49-2: Using mmap() to create a shared file mapping

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

#include <sys/mman.h>

#include <fcntl.h>

#include "tlpi_hdr.h"

#define MEM_SIZE 10

int

main(int argc, char *argv[])

{

char *addr;

int fd;

if (argc < 2 || strcmp(argv[1], "--help") == 0)

usageErr("%s file [new-value]\n", argv[0]);

fd = open(argv[1], O_RDWR);

if (fd == -1)

errExit("open");