memory reallocation in c

Question

I am new very interested in c programming now i am writing a c program which for dynamically memory allocation.

These are the the stages of the program:

it gets n numbers from user
it prints them
it gets another n numbers from the user
it prints both sets of numbers

my program works fine for small values of n, but not for large ones, such as 200000 numbers. i.e first I entered 100000 numbers then I entered another 100000 numbers.

It executes fine for the first set but then it gives a segmentation fault. I use gdb for debugging. It shows SIGSEGV error.

Can anybody explain what happened and give me a solution for it?

#include<stdio.h>
#include<malloc.h>
int main(void)
{

    unsigned int *p=NULL;
    unsigned int n;
    unsigned int i;
    unsigned int *a;
    unsigned int *t;
    unsigned int k=0;

    printf("Enter no.of elements...");
    scanf("%d", &n);

    p = (unsigned int*)malloc(n*sizeof(unsigned int));
    a = p;
    t = p;
    for (i=0; i<n; i++, *p++, k++) {
        scanf("%d",p);
    }

    for(i=0;i<n;i++,a++) {
        printf("Element No-%d %d Address->%d\n", i, *a, a);
    }

    a=t;

    printf("next time...how many elements do you enter");
    scanf("%d",&n);

    t=p;
    a = (unsigned int*)realloc((void *)a, n*sizeof(int));
    for (i=0; i<n; i++, *t++ ,k++) {
        scanf("%d",t);
    }

    printf("next time...printing..\n");

    for (i=0; i<k; i++, a++) {
        printf("Element No-%d %d Address->%d\n",i,*a,a);
    }

    free(p);
    free(a);


}

I was with you right up until the "give me a solution." that triggered my auto-close-reflex. — WhozCraig, Nov 11 '12 at 07:48
I took the liberty of making your question and code more legible. Please check that I didn't mess it up. — Nathan Fellman, Nov 11 '12 at 07:49
LOL WhozCraig. You'd think these homework answer hunters would at least disguise their questions a bit better! — Pete855217, Nov 11 '12 at 07:56
The title is wrong, it is not *program* reallocation, but just *memory* reallocation. — Basile Starynkevitch, Nov 11 '12 at 08:00

score 3 · Accepted Answer · edited May 23 '17 at 12:30

3

realloc changes the size of the allocation, it doesn't grow the allocation by the size you specify.

So you need to do

a=realloc(a, original_size + extent_size);

(You don't need to cast the return value of malloc or realloc in C.)

Another note:

t=p;
...
a=t;
...
t=p;
a=realloc(...);

You can't use t after the realloc, since realloc could have changed the block's address. Put t=a; after the realloc. (And use more descriptive names, your code is very hard to follow.)

edited May 23 '17 at 12:30

Community

1
1

answered Nov 11 '12 at 07:43

Mat

202,337
40
393
406

Thanks for responding my program and you said that realloc changes the size of allocation so how can i get back the numbers which are allocated at the previous location and how can add another numbers after at the end of previous memory location ? – prasad Nov 11 '12 at 19:22
There's no standard/portable way to know, you need to keep track of that yourself. – Mat Nov 11 '12 at 19:22

score 1 · Answer 2 · answered Nov 11 '12 at 07:44

You should #include <stdlib.h> and #include <stdio.h>, not <malloc.h> which is deprecated. Please also indent correctly your code

The two lines:

for(i=0;i<n;i++,*p++,k++)
   scanf("%d",p);

seem suspicious. Why the *p? I would make it

for (i=0; i<n; i++, p++)
  if (scanf(" %d", p)<1) 
    exit (EXIT_FAILURE);

Don't forget to enable all warnings and debugging info during compilation (with gcc that means gcc -Wall -g) and to use the debugger (e.g. gdb) and the memory leak detector (e.g. valgrind) to debug your program.

You might want to use calloc instead of malloc, and you definitely should test the result of malloc, calloc, or realloc (all could fail by returning NULL).

And as Mat answered, your realloc call was incorrect.

memory reallocation in c

2 Answers2