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shmgetThis article will demonstrate various ways to allocate shared memory in C using the function.

shmgetAllocating shared memory in C using

Shared memory is one of the ways of inter-process communication, which allows two or more processes to exchange data and communicate quickly in user space. Shared memory means that multiple processes share the same area in memory, and they can modify/access this memory as needed.

In the following examples, we will demonstrate an interface called System V shared memory, which is provided by four functions: shmget, shmat, shmdtand shmctl.

• shmgetUsed to create a new shared memory segment or retrieve the identifier of an already created memory segment.
• shmatCall is used to attach the given shared memory segment to the memory space of the calling process.
• shmdtCan be used to detach a given memory segment.
• shmctlUsed to modify and reallocate segments with multiple options.

The next example code implements basic use of the shmgetand shmatfunctions for a process that creates a new shared segment and then writes some text to it. shmgetThe functions require three parameters, the first of which is the key of the memory segment. The key value can be the macro if a new segment needs to be created, or IPC_PRIVATEthe key of an existing segment if the identifier of the memory needs to be obtained by calling . shmgetThe second parameter of specifies the size of the segment, and the third parameter is the permission flags, which can be ORed to contain multiple values.

Once the memory segment is created, the segment identifier is obtained and can be passed to shmatthe __addr__ function to attach the given memory. The user can pass the specific address of a given memory segment as the second argument to __addr__ shmat. However, it is common to let the kernel choose the address and specify __addr__ NULLto indicate this.

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <wait.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/shm.h>

enum {SEGMENT_SIZE = 0x6400};

const char *data = "Hello there!";

int main(int argc, char *argv[]) {

    int status;
    int segment_id;

    segment_id = shmget (IPC_PRIVATE, SEGMENT_SIZE,
                        IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
    char *sh_mem = (char *) shmat(segment_id, NULL, 0);

    printf("Segment ID %d\n", segment_id);
    printf("Attached at %p\n", sh_mem);
    memmove(sh_mem, data, strlen(data)+1);
    printf("%s\n", sh_mem);

    shmdt(sh_mem);
    shmctl(segment_id, IPC_RMID, 0);
    exit(EXIT_SUCCESS);
}

Output:

Segment ID 1540195
Attached at 0x7f6ba1437000
Hello there!

Once shmatthe function returns a valid pointer, we can treat it as any memory address and operate on it as needed. Finally, call the shmdtand shmctlfunctions to detach the given segment and reallocate the memory. Assume that shmctlno call is made to the allocated memory segments. In this case, they will remain in the system's memory after the program terminates and may hit the limit on the total number of shared memory segments in the whole system.

On the other hand, the following sample code demonstrates how two processes can interact using shared memory. The code is the same as the previous example, except that after printing Hello there!the string, the main process is forked and a child process is created that stores a different string into the same address. At the same time, the parent process is paused, waiting for the child process to terminate and exit with a success code; the newly stored string is printed to the console. If multiple processes need to modify and access a shared memory segment at the same time, the programmer needs to employ some synchronization tools, such as semaphores.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <wait.h>
#include <pthread.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/shm.h>

enum {SEGMENT_SIZE = 0x6400};

const char *data = "Hello there!";

int main(int argc, char *argv[]) {

    int status;
    int segment_id;

    segment_id = shmget (IPC_PRIVATE, SEGMENT_SIZE,
                        IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);
    char *sh_mem = (char *) shmat(segment_id, 0, 0);

    printf("Segment ID %d\n", segment_id);
    printf("Attached at %p\n", sh_mem);
    memmove(sh_mem, data, strlen(data)+1);
    printf("%s\n", sh_mem);

    pid_t child_pid = fork();
    if (child_pid == -1)
        perror("fork");

    if (child_pid == 0) {
        strcpy(sh_mem, "NEW DATA Stored by Child Process\0");

        printf("child pid - %d\n", getpid());
        exit(EXIT_SUCCESS);
    } else {
        pid_t ret = waitpid(child_pid, &status, WUNTRACED | WCONTINUED);
        if (ret == -1)
            perror("waitpid");

        if (WIFEXITED(status))
            printf("Child exited, status - %d\n", WEXITSTATUS(status));

        if (WEXITSTATUS(status) == 0)
            printf("%s\n", sh_mem);
    }

    shmdt(sh_mem);
    shmctl(segment_id, IPC_RMID, 0);
    exit(EXIT_SUCCESS);
}

Output:

Segment ID 1540195
Attached at 0x7fd825a25000
Hello there!
child pid - 27291
Child exited, status - 0
NEW DATA Stored by Child Process