Dynamic Memory Allocation with pointer arrays

Question

I am trying to learn C from tutorials on the web, and I came up with this simple code to try and understand memory allocation on pointers and arrays.

The code compiles and runs flawlessly if size <= 2, but if size > 2 it gives a segmentation error. Can someone please shed a light on how to do this properly?

Thank you.

#include <stdio.h>
#include <stdlib.h>


int main () {


int i, size;

printf("Quantos registos pretende inserir? ");
scanf("%d",&size);
getc(stdin);

typedef struct {
    char nome[81];
    int idade;
    char cargo[81];
} Dados;

Dados *data[(size-1)];

data[(size-1)] = (Dados *)malloc(sizeof(Dados));


for(i=0;i<size;i++) {
    printf("\nInsira os dados do funcionário: ");
    printf("\n\n\tNome: ");
    gets(data[i]->nome);
    printf("\n\tIdade: ");
    scanf("%d",&data[i]->idade);
    getc(stdin);
    printf("\n\tCargo: ");
    gets(data[i]->cargo);

    FILE *fdados;
    if(!(fdados = fopen("dados.txt","a+"))) {
         printf("Impossivel aceder ao ficheiro, verfique o erro ocorrido ...");
    }

    fprintf(fdados, "Funcionário %d:",(i+1));
    fprintf(fdados, "\n\n\tNome: %s",data[i]->nome);
    fprintf(fdados, "\n\tIdade: %d",data[i]->idade);
    fprintf(fdados, "\n\tCargo: %s\n\n",data[i]->cargo);

    fclose(fdados);

}

free(data[(size-1)]);

fflush(stdin);
return(0);
}

Please don't use the word "flawless". The only thing it means is that you have no idea what you're doing. Your code isn't flawless at all, but a horrible car crash for any value of size, since data[size - 1]` is always an out-of-bounds error, since the array is always one element shorter. — Kerrek SB
– Kerrek SB, Commented Aug 13, 2013 at 21:45
(This isn't even considering that variable-length arrays must have strictly positive size, and that size is signed and may well be negative, and that you don't allocate any memory for the bulk of the array elements.) — Kerrek SB
– Kerrek SB, Commented Aug 13, 2013 at 21:46
You are right, poor choice of words. Thank you for your time! — EAmaral
– EAmaral, Commented Aug 13, 2013 at 21:57

Kerrek SB · Accepted Answer · 2013-08-13 21:51:04Z

2

You're giving yourself way too much to do in one go. Break the problem down and test each component:

Parse user input: Produce either a valid size_t integer or abort. Check that the value is neither zero nor too large, or abort.
Suppose you've parsed the value n. Allocate memory for n copies of your structure:
```
Dados * data = malloc(n * sizeof(Dados));
```
At the end, release the memory:
```
free(data);
```
Use proper parsing with error handling to populate each array member data[i].
Practice file operations separately.

answered Aug 13, 2013 at 21:51

Kerrek SB

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2 Comments

EAmaral Over a year ago

Thank you very much, trying to learn from the web can sometimes be hard if you can't find a friendly answer to point you in the right direction. Code is running without segmentation problems

Kerrek SB Over a year ago

@tz0r: That doesn't mean that the code is correct, and it definitely isn't robust. You should always make every part of the code (especially the user input!) well defined and check return values of everything that could possibly fail.

Ion · Accepted Answer · 2013-08-13 22:13:36Z

1

Well you have a struct, and it seems like you want to create multiple copies of that struct in an array. So what you do is this

Dados *data;
data = (Dados *) malloc(sizeof(Dados) * size);

now you have size number of Dados structures located in data.

then at the end make sure to free your memory free(data);

answered Aug 13, 2013 at 22:13

Ion

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Comments

Lidong Guo · Accepted Answer · 2013-08-13 22:07:37Z

0

remove : getc(stdin);

Dados *data[(size-1)];

data[(size-1)] = (Dados *)malloc(sizeof(Dados));// here is wrong . 
                                                  you have data[size-1]
                                             the max index you can use is size-2



for(i=0;i<size;i++) // i < size-1 , since you use data[i] in loop ,

free(data[(size-1)]); // free (data);  just free the pointer you get.

answered Aug 13, 2013 at 22:07

Lidong Guo

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