Question

I made a small function that fills an allocated block of memory containing every position of a given char within a given string and returns a pointer to the memory block.

The only problem with this function is that there is no way to check the size of the memory block; so I also made a function that counts the occurrence of a given char within a string.

Here is an example of use:

/*count occurences of char within a given string*/
size_t strchroc(const char *str, const char ch)
{ 
    int c = 0;
    while(*str) if(*(str++) == ch) c++;
    return c;
}

/*build array of positions of given char occurences within a given string*/
int *chrpos(const char *str, const char ch)
{
    int *array, *tmp, c = 0, i = 0;

    if(!(array = malloc(strlen(str) * sizeof(int)))) return 0x00;
    while(str[c])
    {
        if(str[c] == ch) array[i++] = c;
        c++;
    }
    if(!(tmp = realloc(array, i * sizeof(int)))) return 0x00;
    array = tmp;
    return array;
}

int main(void)
{
    char *str = "foobar foobar";                //'o' occurs at str[1], str[2], str[8], and str[9]
    int *array, b = 0, d;

    if(!(array = chrpos(str, 'o'))) exit(1);    //array[0] = 1, array[1] = 2, array[2] = 8, array[3] = 9

    /*
     * This is okay since I know that 'o'
     * only occures 4 times in str. There
     * may however be cases where I do not
     * know how many times a given char 
     * occurs so I figure that out before
     * utilizing the contents of array.
     * I do this with my function strchroc.
     * Below is a sample of how I would 
     * utilize the data contained within
     * array. This simply prints out str
     * and on a new line prints the given
     * char's location within the str 
     * array
     */

    puts(str);
    while(b < (int) strchroc(str, 'o'))         //loop once for each 'o' 
    {
        for(d = 0; d < (b == 0 ? array[b] : array[b] - array[b - 1] - 1); d++) putc((int) ' ', stdout);
        printf("%d", array[b]);
        b++;
    }
}

Output:

foobar foobar
 12     89

My only concern is that if one of these two functions fail, there is no way for the data to be used correctly. I was thinking about making the number of occurrences of char within the string an argument for chrpos but even then I would still have to call both functions.

I was wondering if anybody had any suggestions for a way to do this so that I only need one function to build the array.

The only way I can think of is by storing the number of char occurrences into array[0] and having array[1] through array[char_occurences] holding the positions of char.

If anybody has a better idea I would greatly appreciate it.

Answer 1

As stated in my comment the first thing is to save up the data anyway, in case you can’t shrink the allocated memory :

if (!(tmp = realloc(array, i * sizeof(int))))
  return array;
return (tmp); //array = tmp; is useless

If you want to protect a bit more your strchroc function add a if (!str) return 0; at the beginning.