Question

I was wondering if my implementation of an “itoa” function is correct. Maybe you can help me getting it a bit more “correct”, I’m pretty sure I’m missing something. (Maybe there is already a library doing the conversion the way I want it to do, but… couldn’t find any)

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

char * itoa(int i) {
  char * res = malloc(8*sizeof(int));
  sprintf(res, "%d", i);
  return res;
}

int main(int argc, char *argv[]) {
 ...

Answer 1

The only actual error is that you don’t check the return value of malloc for null.

The name itoa is kind of already taken for a function that’s non-standard, but not that uncommon. It doesn’t allocate memory, rather it writes to a buffer provided by the caller:

char *itoa(int value, char * str, int base);

If you don’t want to rely on your platform having that, I would still advise following the pattern. String-handling functions which return newly allocated memory in C are generally more trouble than they’re worth in the long run, because most of the time you end up doing further manipulation, and so you have to free lots of intermediate results. For example, compare:

void delete_temp_files() {
    char filename[20];
    strcpy(filename, "tmp_");
    char *endptr = filename + strlen(filename);
    for (int i = 0; i < 10; ++i) {
        itoa(endptr, i, 10); // itoa doesn't allocate memory
        unlink(filename);
    }
}

vs.

void delete_temp_files() {
    char filename[20];
    strcpy(filename, "tmp_");
    char *endptr = filename + strlen(filename);
    for (int i = 0; i < 10; ++i) {
        char *number = itoa(i, 10); // itoa allocates memory
        strcpy(endptr, number);
        free(number);
        unlink(filename);
    }
}

If you had reason to be especially concerned about performance (for instance if you’re implementing a stdlib-style library including itoa), or if you were implementing bases that sprintf doesn’t support, then you might consider not calling sprintf. But if you want a base 10 string, then your first instinct was right. There’s absolutely nothing “incorrect” about the %d format specifier.

Here’s a possible implementation of itoa, for base 10 only:

char *itobase10(char *buf, int value) {
    sprintf(buf, "%d", value);
    return buf;
}

Here’s one which incorporates the snprintf-style approach to buffer lengths:

int itobase10n(char *buf, size_t sz, int value) {
    return snprintf(buf, sz, "%d", value);
}