CODEWITHSUNDEEP
cmemory-management
user2947932
user2947932

Reputation: 11

*** glibc detected *** realloc(): invalid old size 3

This code sometimes give me a Segmentation fault, why could this be?

void concatenarAlBuffer(char **destino, char caracter)
{
char matrizAux[1];
matrizAux[0]=caracter;
matrizAux[1]='\0';

(*destino) = realloc((*destino),2+(strlen((*destino))*sizeof(char)));

if ((*destino)==NULL)   
{                               
    perror("Error al reservar memoria\n");
    exit(-1);                               
}

strcat((*destino),matrizAux);
}

Upvotes: 1

Views: 716

Answers (3)

Daniel Rose
Daniel Rose

Reputation: 17648

char matrizAux[1] hold only one character. Since you are accessing matrizAux[1], i.e. something out-of-bounds, you are corrupting the memory. This can result in the internal data of the runtime getting corrupted, which results in the segfault during the realloc(), even though the realloc itself is correct.

Upvotes: 0

zubergu
zubergu

Reputation: 3706

You make 

char matrizAux[1];

a 1-char long array.
And then you write to its' second element with

matrizAux[1]='\0';

This might or might not crash depending on what's behind your array in memory.
But it still looks like something's missing.
realloc can reallocate only something that was previously malloc- or calloc-ated.
Trying to reallocate something that doesn't fulfill this condition gives you your error. And that's probably the true case here.

Upvotes: 1

Filipe Gonçalves
Filipe Gonçalves

Reputation: 21223

matrizAux can only hold one character. You can't access matrizAux[1] - it's out of bounds.

Declare matrizAux like this:

char matrizAux[2];

Remember that an n-elements array can be indexed in positions 0 up to (and including) n-1.

Also, you don't need sizeof(char), because it it always 1. And parentheses around *destino are useless in this case and make the code harder to read unnecessarily. Here's how I would change your code:

void concatenarAlBuffer(char **destino, char caracter) {
    char matrizAux[2];
    matrizAux[0]=caracter;
    matrizAux[1]='\0';

    *destino = realloc(*destino,2+strlen(*destino));

    if (*destino == NULL) {                               
        perror("Error al reservar memoria\n");
        exit(-1);                               
    }

    strcat(*destino, matrizAux);
}

Upvotes: 1

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