Where is the itoa function in Linux?

EDIT: Sorry, I should have remembered that this machine is decidedly non-standard, having plugged in various non-standard libc implementations for academic purposes ;-)

As itoa() is indeed non-standard, as mentioned by several helpful commenters, it is best to use sprintf(target_string,"%d",source_int) or (better yet, because it's safe from buffer overflows) snprintf(target_string, size_of_target_string_in_bytes, "%d", source_int). I know it's not quite as concise or cool as itoa(), but at least you can Write Once, Run Everywhere (tm) ;-)

Here's the old (edited) answer

You are correct in stating that the default gcc libc does not include itoa(), like several other platforms, due to it not technically being a part of the standard. See here for a little more info. Note that you have to

#include <stdlib.h>

Of course you already know this, because you wanted to use itoa() on Linux after presumably using it on another platform, but... the code (stolen from the link above) would look like:

Example

/* itoa example */
#include <stdio.h>
#include <stdlib.h>

int main ()
{
  int i;
  char buffer [33];
  printf ("Enter a number: ");
  scanf ("%d",&i);
  itoa (i,buffer,10);
  printf ("decimal: %s\n",buffer);
  itoa (i,buffer,16);
  printf ("hexadecimal: %s\n",buffer);
  itoa (i,buffer,2);
  printf ("binary: %s\n",buffer);
  return 0;
}

Output:

Enter a number: 1750
decimal: 1750
hexadecimal: 6d6
binary: 11011010110

Hope this helps!

itoa is not a standard C function. You can implement your own. It appeared in the first edition of Kernighan and Ritchie's The C Programming Language, on page 60. The second edition of The C Programming Language ("K&R2") contains the following implementation of itoa, on page 64. The book notes several issues with this implementation, including the fact that it does not correctly handle the most negative number

 /* itoa:  convert n to characters in s */
 void itoa(int n, char s[])
 {
     int i, sign;

     if ((sign = n) < 0)  /* record sign */
         n = -n;          /* make n positive */
     i = 0;
     do {       /* generate digits in reverse order */
         s[i++] = n % 10 + '0';   /* get next digit */
     } while ((n /= 10) > 0);     /* delete it */
     if (sign < 0)
         s[i++] = '-';
     s[i] = '\0';
     reverse(s);
}  

The function reverse used above is implemented two pages earlier:

 #include <string.h>

 /* reverse:  reverse string s in place */
 void reverse(char s[])
 {
     int i, j;
     char c;

     for (i = 0, j = strlen(s)-1; i<j; i++, j--) {
         c = s[i];
         s[i] = s[j];
         s[j] = c;
     }
}