Memory Allocation Error in C

Question

I am trying to get the binary representation of a big integer in GMP. I am storing 1's and 0's in an array called expBinary. I use malloc to allocate a memory of size of "int", then use realloc to increase this memory whenever a new bit is added. The conversion is running perfectly without any issues, but when I try to allocate any more memory using malloc after the while loop, it is giving me segmentation fault when I call the same code a second time, first time it gives me no segmentation fault. I have checked and the "expBinary" is not storing anything out of bounds, I have given the code below

int binarySize = 0;                                                        
int * expBinary = malloc(sizeof(int));                                 
int i = 0;                                                                  

// Run until exp == 0                                                       
while(mpz_cmp_ui(exp,0) != 0)                                               
{                                                                           
    binarySize++;                                                           
    expBinary = (int*) realloc(expBinary,(binarySize));                     
    // Getting LSB of exp                                                   
    if(mpz_even_p(exp) != 0)                                                
        expBinary[i] = 0;                                                   
    else                                                                    
        expBinary[i] = 1;                                                                                                                                 
    // Incrmenting variables                                                
    i++;                                                                    
    // Dividing exponent by 2                                               
    mpz_tdiv_q_ui(exp,exp,2);                                               
}                                                                           

// This line is giving error
int * temp = malloc(sizeof(int));

Its in Mac (Unix) but I guess it will work the same in Linux. I removed the cast but its still showing the same error. — Akshay Gupta
– Akshay Gupta, Commented Jan 19, 2015 at 21:30
@iharob without macports? please do tell or link. And may you live to a hundred healthy years if this pans out =) — WhozCraig
– WhozCraig, Commented Jan 19, 2015 at 21:33

Iharob Al Asimi · Accepted Answer · 2015-01-19 21:40:11Z

5

If you are using an int array, then this is wrong

expBinary = (int*) realloc(expBinary,(binarySize));

it should be

expBinary = realloc(expBinary, binarySize * sizeof(*expBinary));

or equivalently,

expBinary = realloc(expBinary, binarySize * sizeof(int));

I prefer sizeof(*expBinary) for obvious reasons, and also, if realloc() fails you loose reference to the previous pointer so I recommend this

void *tmp;
tmp = realloc(expBinary, binarySize * sizeof(int));
if (tmp == NULL)
    handleFailureHereAndDontContinueToTheNextLineAndFree_expBinary_Please();
expBinary = tmp;

Now, if you want to print the representation using any printf("%s\n", expBinary); you should use char * instead, in that case you should consider that sizeof(char) == 1 always, and you will need one extra byte at the end of the 1's and 0's with the value '\0'.

edited Jan 19, 2015 at 21:40

answered Jan 19, 2015 at 21:29

Iharob Al Asimi

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