CODEWITHSUNDEEP
cpointersstructmallocdynamic-memory-allocation
Pete
Pete

Reputation: 367

Malloc'ing pointer-to-pointer member of struct

I'm trying to use a structure to hold a pointer to a data block that I change sometimes when a file is updated, the idea being to free the old data block, malloc a new one of the right size, and assign the pointer to pointer in the structure to the pointer returned by malloc, and this is how I thought I should do it. But it seg faults. In fact in the larger program that I pared down to make this test program, its not seg faulting but writing to stdout does nothing after the malloc (anywhere in the program after that). I guess I am writing over the stdout FD, the cause being that I am using the pointers incorrectly when I set the pointer to the malloc()ed return value.

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

struct mystruct {
    int offset;
    int ** d;
};

int filecheck (struct mystruct *g, int * count) {
    int v, size = 0;
    FILE *f = fopen("/home/pi/schedule/default", "rb");
    if (f == NULL) { 
        return 0; 
    } 
    fseek(f, 0, SEEK_END);
    size = ftell(f);
    fseek(f, 0, SEEK_SET);
    int schedsize = sizeof(int);
    int elementcount = size / schedsize;
//  free(*(g->d));
//  seg fault next line
    if ((*(g->d) = malloc(size))==NULL) return 0; 
    if (elementcount != fread(*(g->d), schedsize, elementcount, f)) { 
        free(*(g->d));
        return 0;
    } 
    fclose(f);
    *count = elementcount;  
    return 1;
}

void setp (struct mystruct *g) {
//  if uncommented, seg fault here
//  *(g->d) = NULL;
}

int main (){
    struct mystruct g;
    setp(&g);
    int i, count = 0;
    if (filecheck(&g, &count)==0) {
        printf("Returned 0\n");
        return 0;
    }
    while (1) {
        printf("%d\n", (*(g.d))[i]);
        sleep(1);
    }
    return 0;

}

For convenience I want to set mystruct.d to NULL initially, in setp(), but even if thats comented out the code still fails, so I know its completely wrong. Maybe I don't need to be using a pointer to pointer but it seems to me I do.

EDIT: Modified as per answer, is this OK?

struct mystruct {
    int offset;
    int * d;
};

int filecheck (struct mystruct *g, int * count) {
    int v, size = 0;
    FILE *f = fopen("/home/pi/schedule/default", "rb");
    if (f == NULL) { 
        return 0; 
    } 
    fseek(f, 0, SEEK_END);
    size = ftell(f);
    fseek(f, 0, SEEK_SET);
    int schedsize = sizeof(int);
    int elementcount = size / schedsize;
    free (g->d);
    if ((g->d = malloc(size))==NULL) return 0; 
    if (elementcount != fread(g->d, schedsize, elementcount, f)) { 
        free(g->d);
        return 0;
    } 
    fclose(f);
    *count = elementcount;  
    return 1;
}

void setp (struct mystruct *g) {
    g->d = NULL;
}

int main (){
    struct mystruct g;
    setp(&g);
    int i, count = 0;
    if (filecheck(&g, &count)==0) {
        printf("Returned 0\n");
        return 0;
    }
    while (1) {
        for (i=0;i<count;i++) {
            printf("%d %d \n", *((g.d)+i), g.d[i]);
        }
        sleep(1);
    }
    return 0;

}

This seems to work but is it correct or am I writing over some part of memory I am not supposed to again with this?

Upvotes: 0

Views: 282

Answers (1)

Sourav Ghosh
Sourav Ghosh

Reputation: 134286

You need to allocate memory to all the pointer elements before using them.

Here, d being a pointer to pointer, first you need to allocate memory for d itself, then you should go on for dereferencing d (using *d).

For example, either

void setp (struct mystruct *g) { 

 g->d = NULL;   // no need to derererence d here, but later need to allocate
}

or, (for better)

void setp (struct mystruct *g) {

 g->d = malloc(32 * sizeof (int *));  // d is allocated
 g->d[i] = malloc(16 * sizeof (int));   // or g->d[i] = NULL; or *(g->d) = NULL;
}

should work fine.

Also, the recommended siganture of main() is int main(void).

Upvotes: 2

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