Shared memory fork with struct and malloc

Question

I have a problem with the shared memory between parent and child processes generated by a fork. I know how to use shared memory with primitive type as described here.

While I do not know how to share struct that contains a pointer that can be allocated via malloc.

For instance, suppose that I have the following code took from the previous link with some modifications.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h> 
#include <sys/file.h>
#include <sys/mman.h>
#include <sys/wait.h>

typedef struct Data {
    char * name;
} Data;

void error_and_die(const char *msg) {
    perror(msg);
    exit(EXIT_FAILURE);
}

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

    const char *memname = "sample";
    const size_t region_size = sysconf(_SC_PAGE_SIZE);

    int fd = shm_open(memname, O_CREAT | O_TRUNC | O_RDWR, 0666);
    if (fd == -1)
        error_and_die("shm_open");

    r = ftruncate(fd, region_size);
    if (r != 0)
        error_and_die("ftruncate");

    Data * data = (Data *) malloc(sizeof(data));
    data->name=(char *) malloc(100*sizeof(char));   

    void *ptr = mmap(0, region_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (ptr == MAP_FAILED)
        error_and_die("mmap");
    close(fd);

    ptr=(Data *) data; 

    pid_t pid = fork();

    if (pid == 0) {
        //u_long *d = (u_long *) ptr;
        //*d = 0xdbeebee;
        data->name="bob";
        printf("CHILD child wrote %s\n", (*(Data *)ptr).name);
        exit(0);
    }
    else {
        int status;
        waitpid(pid, &status, 0);
        //printf("child wrote %#lx\n", *(u_long *) ptr);
        printf("PARENT child wrote %s\n", (*(Data *)ptr).name);
    }

    r = munmap(ptr, region_size);
    if (r != 0)
        error_and_die("munmap");

    r = shm_unlink(memname);
    if (r != 0)
        error_and_die("shm_unlink");

    return 0;
}

I would like to change the data inside the struct between parent and child processes in order to allow IPC. How can I do?

Thank you

Answer 1

If you're asking "how to do it with malloc()", it is not possible, unless you write your own malloc() implementation that allocated from a buffer in a shared memory region. That is a possible approach, but goes beyond this answer. Otherwise the memory allocated by malloc() is not shared between parent and child; each one gets it's own copy.

If you're asking "how to change the example so that it works" - simply use the memory returned by the mmap() call.

To simplify, let's declare name as an array in the Data structure:

typedef struct Data {
    char name[100];
} Data;

Then the main code becomes:

void *ptr = mmap(0, region_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (ptr == MAP_FAILED)
    error_and_die("mmap");
close(fd);

Data * data = ptr;

pid_t pid = fork();

if (pid == 0) {
    strcpy(data->name, "bob");
    printf("CHILD child wrote %s\n", (*(Data *)ptr).name);
    exit(0);
}
else {
    int status;
    waitpid(pid, &status, 0);
    printf("PARENT child wrote %s\n", (*(Data *)ptr).name);
}

Output:

CHILD child wrote bob

PARENT child wrote bob

Answer 2

I solved by myself by following this guide and by using shmget, shmat, smhdt and shmctl instead of shm_open, mmap, munmap and shm_unlink.

In this way, I can manage dynamic shared memory as with malloc with the advantage that the memory is visible between the processes.

I report here the code as future hint.

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

typedef struct Person {
    char * name;
    int * numbers;
} Person;

typedef struct Data {
    Person * persons;
} Data;

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

    Data * data;
    Person * person;

    key_t mykey1=5500;
    key_t mykey2=5501;
    key_t mykey3=5502;
    key_t mykey4=5503;
    key_t mykey5=5504;
    int mem_id;

    mem_id=shmget(mykey1,sizeof(Person),IPC_CREAT|0666);
    if (mem_id<0) {
        perror("error shmget");
    }
    person=(Person*)shmat(mem_id,(void*)0,0);
    if(person == (Person*)(-1)) {
        perror("error shmat");
    }

    mem_id=shmget(mykey2,(100*sizeof(char)),IPC_CREAT|0666);
    if (mem_id<0) {
        perror("error shmget 2");
    }

    person->name=(char *)shmat(mem_id,(void*)0,0);
    if (person->name == (char *)(-1)) {
        perror("error shmat 2");
    }

    mem_id=shmget(mykey3,(10*sizeof(int)),IPC_CREAT|0666);
    if (mem_id<0) {
        perror("error shmget 3");
    }

    person->numbers=(int *)shmat(mem_id,(void*)0,0);
    if (person->numbers == (int *)(-1)) {
        perror("error shmat 3");
    }

    mem_id=shmget(mykey4,sizeof(Data),IPC_CREAT|0666);
    if (mem_id<0) {
        perror("error shmget 4");
    }
    data=(Data*)shmat(mem_id,(void*)0,0);
    if(data == (Data*)(-1)) {
        perror("error shmat 4");
    }

    mem_id=shmget(mykey5,(10*sizeof(int)),IPC_CREAT|0666);
    if (mem_id<0) {
        perror("error shmget 5");
    }

    data->persons=(Person *)shmat(mem_id,(void*)0,0);
    if (data->persons == (Person *)(-1)) {
        perror("error shmat 5");
    }    

    pid_t pid = fork(); 
    if (pid == 0) {
        person->name="bob";
        for(int i=0; i<10; i++) {
            person->numbers[i]=i;       
        }
        data->persons[0]=*person;
        data->persons[1].name="alice";
        printf("CHILD child wrote %s\n", person->name);
        exit(0);
    }
    else {
        int status;
        waitpid(pid, &status, 0);
        printf("PARENT child wrote %s\n", person->name);
        for(int i=0; i<10; i++) {
            printf("%d\n",person->numbers[i]);      
        }

        printf("PARENT child wrote %s\n", data->persons[0].name);
        for(int i=0; i<10; i++) {
            printf("%d\n",data->persons[0].numbers[i]);     
        }
        printf("PARENT child wrote %s\n", data->persons[1].name);
    }
    shmdt(person);
    shmdt(data);
    shmctl(mem_id,IPC_RMID,0); 

    return 0;
}