Thread: Allocating dynamic memory to a string within a structure

Hi, I have the below code:

Code:

struct fact_entry
{                               
  int n;
  long long int lli_fact;     
  char *str_fact;
};

int main (int argc, char *argv[])
{
   int n;
   int i;
   struct fact_entry *fact_table;
   n = atoi (argv[1]);
   fact_table = (struct fact_entry *)malloc(n*sizeof(fact_table));
   for (i=0; i<n; i++) {
        if(i==0) {
            fact_table[i].n = 0;
            fact_table[i].lli_fact = 1;
            fact_table[i].str_fact = "1";
        }
        else {
            fact_table[i].n = i;
            fact_table[i].lli_fact = i*fact_table[i-1].lli_fact;
            /* the next line seems to be wrong*/
            fact_table[i].str_fact = (char *)malloc(log10(fact_table[i].lli_fact)); 
        }
    }
    for (i=0; i<n; i++) {
        printf("%d    %lld    %s\n", fact_table[i].n, fact_table[i].lli_fact, fact_table[i].str_fact);
    }
  return 0;
}

The program accepts an integer and return 3 columns, the 1st column is the array position, the 2nd column is the factorial and the 3d column is also the factorial but in string format.

The 2 first columns are fine, it's the 3rd column that's failing, so I'm allocating memory the wrong way for the string within the struct. Could someone let me know what I'm doing wrong?

Regards

Well you need to create a string of some sort, not just allocate some space.

> fact_table[i].str_fact = "1";
A mixture of assigning string constants and malloc will be a real PITA later on.

Code:

        if(i==0) {
            fact_table[i].n = 0;
            fact_table[i].lli_fact = 1;
        }
        else {
            fact_table[i].n = i;
            fact_table[i].lli_fact = i*fact_table[i-1].lli_fact;
        }
        fact_table[i].str_fact = malloc(20);  // max of a long long in decimal is 19 digits
        snprintf(fact_table[i].str_fact,20,"%lld",fact_table[i].lli_fact);

> fact_table = (struct fact_entry *)malloc(n*sizeof(fact_table));
This is also wrong.

It should be
fact_table = malloc(n*sizeof(*fact_table));
You weren't allocating anywhere near enough memory, leading to corrupt data.

Also, stop casting malloc in your C programs.
See the FAQ.