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linux_man_pages_library

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the posix shared memory object implementation on linux makes use
of a dedicated
.br tmpfs (5)
filesystem that is normally mounted under
.ir /dev/shm .
.sh examples
the programs below employ posix shared memory and posix unnamed semaphores
to exchange a piece of data.
the "bounce" program (which must be run first) raises the case
of a string that is placed into the shared memory by the "send" program.
once the data has been modified, the "send" program then prints
the contents of the modified shared memory.
an example execution of the two programs is the following:
.pp
.in +4n
.ex
$ \fb./pshm_ucase_bounce /myshm &\fp
[1] 270171
$ \fb./pshm_ucase_send /myshm hello\fp
hello
.ee
.in
.pp
further detail about these programs is provided below.
.\"
.ss program source: pshm_ucase.h
the following header file is included by both programs below.
its primary purpose is to define a structure that will be imposed
on the memory object that is shared between the two programs.
.pp
.in +4n
.ex
#include <sys/mman.h>
#include <fcntl.h>
#include <semaphore.h>
#include <sys/stat.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define errexit(msg) do { perror(msg); exit(exit_failure); \e
} while (0)
#define buf_size 1024 /* maximum size for exchanged string */
/* define a structure that will be imposed on the shared
memory object */
struct shmbuf {
sem_t sem1; /* posix unnamed semaphore */
sem_t sem2; /* posix unnamed semaphore */
size_t cnt; /* number of bytes used in \(aqbuf\(aq */
char buf[buf_size]; /* data being transferred */
};
.ee
.in
.\"
.ss program source: pshm_ucase_bounce.c
the "bounce" program creates a new shared memory object with the name
given in its command-line argument and sizes the object to
match the size of the
.i shmbuf
structure defined in the header file.
it then maps the object into the process's address space,
and initializes two posix semaphores inside the object to 0.
.pp
after the "send" program has posted the first of the semaphores,
the "bounce" program upper cases the data that has been placed
in the memory by the "send" program and then posts the second semaphore
to tell the "send" program that it may now access the shared memory.
.pp
.in +4n
.ex
/* pshm_ucase_bounce.c
licensed under gnu general public license v2 or later.
*/
#include <ctype.h>
#include "pshm_ucase.h"
int
main(int argc, char *argv[])
{
if (argc != 2) {
fprintf(stderr, "usage: %s /shm\-path\en", argv[0]);
exit(exit_failure);
}
char *shmpath = argv[1];
/* create shared memory object and set its size to the size
of our structure. */
int fd = shm_open(shmpath, o_creat | o_excl | o_rdwr,
s_irusr | s_iwusr);
if (fd == \-1)
errexit("shm_open");
if (ftruncate(fd, sizeof(struct shmbuf)) == \-1)
errexit("ftruncate");