Does realloc overwrite old contents?

c realloc
user379888 picture user379888 · Oct 3, 2010 · Viewed 32.2k times · Source

When we reallocate memory via realloc(), are the previous contents over-written? I am trying to make a program which reallocates memory each time we enter the data into it.

Please tell me about memory allocation via realloc, is it compiler dependent for example?

Answer

pmg picture pmg · Oct 3, 2010

Don't worry about the old contents.

The correct way to use realloc is to use a specific pointer for the reallocation, test that pointer and, if everything worked out ok, change the old pointer

int *oldpointer = malloc(100);

/* ... */

int *newpointer = realloc(oldpointer, 1000);
if (newpointer == NULL) {
    /* problems!!!!                                 */
    /* tell the user to stop playing DOOM and retry */
    /* or free(oldpointer) and abort, or whatever   */
} else {
    /* everything ok                                                                 */
    /* `newpointer` now points to a new memory block with the contents of oldpointer */
    /* `oldpointer` points to an invalid address                                     */
    oldpointer = newpointer;
    /* oldpointer points to the correct address                                */
    /* the contents at oldpointer have been copied while realloc did its thing */
    /* if the new size is smaller than the old size, some data was lost        */
}

/* ... */

/* don't forget to `free(oldpointer);` at some time */

Related questions

realloc(): invalid next size when reallocating to make space for strcat on char *

I am getting invalid memory error on following code: printf(" %s\n","FINE 5"); printf("%s LENGTH IS: %d\n","FINE 6",strlen(": ")); buffer = (char *)realloc(buffer, strlen(buffer)* sizeof(char) + (strlen(": ")+1)* sizeof(char)); printf(" %s\n","FINE 7"); strcat(buffer, ": \0"); Output: FINE 5 …

Read More
Dynamic array in C — Is my understanding of malloc and realloc correct?

I am learning how to create dynamic 1D arrays in C. The code below tries to do the following: Using malloc, create a dynamic array of length 10, that holds values of type double. Set each entry of the array to …

Read More
What does malloc(0) return?

What does malloc(0) return? Would the answer be same for realloc(malloc(0),0)? #include<stdio.h> #include<malloc.h> int main() { printf("%p\n", malloc(0)); printf("%p\n", realloc(malloc(0), 0)); return 0; } Output from Linux GCC: manav@manav-workstation:~$ …

Read More