CODEWITHSUNDEEP
cpointersmmap
Larry
Larry

Reputation: 1765

mmap and double pointer

Context:

Taking advantage of my holidays to fiddle with some pointers :)

The code below is an intellectual challenge to myself more than anything else. it helps me working on pointers and so on.

And I fail.

I didn't enforce the coherence with error management, I confess.

Debian64.

The problem :

I make my way with mmap and I litteraly plundge with a double pointer assignation. here is the code :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>

static int mmap_create(const char ** restrict map, const char * restrict path, const unsigned int * restrict size)
{

    int fd;
    int result;

    fd = open(path, O_RDWR | O_CREAT,(mode_t)0777);
    if (fd == -1)
    {
        printf("fail3\n");
        close(fd);
        return -1;
    }

    result = lseek(fd, *size-1, SEEK_SET);
    if (result == -1)
    {
        printf("fail4\n");
        close(fd);
        return -1;
    }

    result = write(fd, "", 1);
    if (result != 1)
    {
        printf("fail0\n");
        close(fd);
        return -1;
    }

    /* Here is my problem since map is a pointer to pointer */
    map = mmap(0, *size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

    if (map == MAP_FAILED)
    { 
        printf("fail\n");
        close(fd);
        return -1;
    }

    printf("pointing to %p\n",map);

    return 0;
}

static void second_function(const char * restrict path, const char ** restrict handle)
{   
    printf("pointing to %p\n",handle);

    /* CREATE MMAP */
    unsigned int value = 100;
    mmap_create(handle,path,&value);


}

static void write_to(char ** map)
{
    printf("pointing to %p\n",map);
}

int main(int argc, char * argv[])
{ 
    const char path[] = "/my/path/";
    char ** handle_a;

    printf("pointing to %p\n",handle_a);

    second_function(path,handle_a);

    printf("pointing to %p\n",handle_a);

    write_to(handle_a);

    /*munmap*/

    return 0;
}

Question:

How could I do to be able to retrieve the right address of the mapped file up to the write_to function ?

The first two are nil (normal) the third is assigned but the last two ones are nil. Not good.

I think it all goes wrong in the mmap call since it gives a pointer but I have a pointer to pointer. Thereafter, the addresses are not the same anymore. And then, I am lost..

Any "pointer" please?

Thanks

Upvotes: 0

Views: 1767

Answers (1)

Soren
Soren

Reputation: 14708

handle_a has no memory allocated to store the pointer

change

char ** handle_a;

to

char * handle_a;

and then use as

second_function(path,&handle_a);

and assign it like;

*map = mmap(0, *size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

Upvotes: 1

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