Cant access data after realloc struct vector

Question

I'm using reallocto reduce the size of my vector. I want to loose the last position only. so, if I have 10 positions and I use realloc to allocate enough space for 9 * sizeof(my_struct) will the vector get truncated and keep the old data but the last position ? I thought it was right, but when I try to print the value after the realloc I get Segmentation Fault error. Here is my code:

    //Instanciate the vector
    my_struct *struc;
    my_struct *buffer;

    //Allocate space for 10 slots
    struc = (my_struct *) malloc(10 * sizeof(my_struct));

    //realloc for 9 slots
    buffer = (my_struct *) realloc(struc, 9 * sizeof(my_struct));        
    if(buffer != NULL){
       struc = buffer;
    }

now if I try to use printf to check the elements inside the vector, I get Segmentaition Fault

OBS: The Vector has been filled up with data BEFORE the realloc. I didnt post here because I think it's unnecessary... Imagine the vector already with data in this code.

The code you've posted looks OK. There is probably already heap corruption due to another bug in your program. To get better help, [follow the instructions to create a MCVE](http://stackoverflow.com/help/mcve) — M.M, Mar 20 '15 at 00:03
@MattMcNabb Thanks. What about my thought, is it right? If I reallocate from 10 slots to 9 will I lose only the last one ? The rest of the data will be ok allocated? — PlayHardGoPro, Mar 20 '15 at 00:06
Indeed! I've just tested it with a simple struct with only an integer field, and `realloc` works as expected (although it didn't really change the buffer address which I find logical). — mcleod_ideafix, Mar 20 '15 at 00:21
@mcleod_ideafix SO after all I have to `free(buffer)` right? — PlayHardGoPro, Mar 20 '15 at 00:22
As `buffer` and `struc` point to the same memory region (remember, you just made `struc = buffer`) you can free any of them. — mcleod_ideafix, Mar 20 '15 at 01:31

score 1 · Accepted Answer · answered Mar 20 '15 at 00:06

1

Your data can be moved by realloc(). So if there is any pointer to the data, after the call, such pointers would become invalid.

answered Mar 20 '15 at 00:06

AntoineL

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4
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So after `realloc` I cant access: `my_struct[0].Name` is there was already data on it? Is there a solution for that? Do I have to call a function to fill it again ? – PlayHardGoPro Mar 20 '15 at 00:08
1

@PlayHardGoPro did you mean `struc[0].Name`? Yes you can access that. What you can't do is `my_struct *ptr = &struc[0]; ` , then your realloc code, then access `ptr->Name`. – M.M Mar 20 '15 at 00:15
Got the error. Thank you sir! I wrote it right but the original code I was using the wrong parameter for realloc. ;s – PlayHardGoPro Mar 20 '15 at 00:17

Cant access data after realloc struct vector

1 Answers1