| Updated: April 19, 2023 |
Open a shared memory object based on a pathname
#include <fcntl.h>
#include <sys/mman.h>
int shm_open( const char * name,
int oflag,
mode_t mode );
The check for the existence of the shared memory object, and the creation of the object if it doesn't exist, are atomic with respect to other processes executing shm_open(), naming the same shared memory object with O_EXCL and O_CREAT set.
libc
Use the -l c option to qcc to link against this library. This library is usually included automatically.
The shm_open() function returns a file descriptor that's associated with the shared memory object specified by name. This file descriptor can be used by other functions to refer to the shared memory object (e.g., mmap()). The FD_CLOEXEC flag in fcntl() is set for this file descriptor.
If you use the file descriptor to create a handle to the object (with shm_create_handle()) for sharing with a recipient process, the handle can support an access mode that's as privileged as or less privileged than what's indicated in oflag for shm_open(). For example, if oflag is set to O_RDWR, a process can create a handle to allow O_RDWR, O_RDONLY, or O_WRONLY access to the object. But if oflag is set to O_RDONLY, the handle can support O_RDONLY access but not O_RDWR or O_WRONLY access.
| name | Pathname space entry |
|---|---|
| SHM_ANON | — |
| entry | /dev/shmem/CWD/entry |
| /entry | /dev/shmem/entry |
where CWD is the current working directory for the program at the point that it calls shm_open().
The state of the shared memory object, including all data associated with it, persists until the object is unlinked and all other references are gone.
A nonnegative integer, which is the lowest numbered unused file descriptor, or -1 if an error occurred (errno is set).
The shared memory object exists and the permissions specified by oflag are denied, or O_TRUNC is specified and write permission is denied.
This example sets up a shared memory object, but doesn't really do anything with it:
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <limits.h>
#include <sys/mman.h>
int main( int argc, char** argv )
{
int fd;
unsigned* addr;
/*
* In case the unlink code isn't executed at the end
*/
if( argc != 1 ) {
shm_unlink( "/bolts" );
return EXIT_SUCCESS;
}
/* Create a new memory object */
fd = shm_open( "/bolts", O_RDWR | O_CREAT, 0777 );
if( fd == -1 ) {
fprintf( stderr, "Open failed:%s\n",
strerror( errno ) );
return EXIT_FAILURE;
}
/* Set the memory object's size */
if( ftruncate( fd, sizeof( *addr ) ) == -1 ) {
fprintf( stderr, "ftruncate: %s\n",
strerror( errno ) );
return EXIT_FAILURE;
}
/* Map the memory object */
addr = mmap( 0, sizeof( *addr ),
PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0 );
if( addr == MAP_FAILED ) {
fprintf( stderr, "mmap failed: %s\n",
strerror( errno ) );
return EXIT_FAILURE;
}
printf( "Map addr is 0x%08x\n", addr );
/* Write to shared memory */
*addr = 1;
/*
* The memory object remains in
* the system after the close
*/
close( fd );
/*
* To remove a memory object
* you must unlink it like a file.
*
* This may be done by another process.
*/
shm_unlink( "/bolts" );
return EXIT_SUCCESS;
}
This example uses a shared memory object to share data with a forked process:
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
main(int argc, char * argv[])
{
int fd;
unsigned *addr;
/*
* In case the unlink code isn't executed at the end
*/
if (argc != 1) {
shm_unlink("/bolts");
return EXIT_SUCCESS;
}
/* Create a new memory object */
fd = shm_open("/bolts", O_RDWR | O_CREAT, 0777);
if (fd == -1) {
fprintf(stderr, "Open failed : %s\n",
strerror(errno));
return EXIT_FAILURE;
}
/* Set the memory object's size */
if (ftruncate(fd, sizeof(*addr)) == -1) {
fprintf(stderr, "ftruncate : %s\n", strerror(errno));
return EXIT_FAILURE;
}
/* Map the memory object */
addr = mmap(0, sizeof(*addr), PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
if (addr == MAP_FAILED) {
fprintf(stderr, "mmap failed:%s\n", strerror(errno));
return EXIT_FAILURE;
}
printf("Map addr is %6.6X\n", addr);
printf("Press break to stop.\n");
sleep(3); /* So you can read above message */
/*
* We unlink so object goes away on last close.
*/
shm_unlink("/bolts");
*addr = '0';
if (fork())
for (;;)
if (*addr == '0')
putc(*addr = '1', stderr);
else
sched_yield();
else
for (;;)
if (*addr == '1')
putc(*addr = '0', stderr);
else
sched_yield();
return EXIT_SUCCESS;
}