edit:

gotcha!

Code:

 
  s[i++] = n / 2 + '0';

I'm stuck. I'm writing a function called itob which has to convert an integer to a value in the chosen base (I'm testing with base 2). But the s array is getting the value 000\001

validation.h

Code:

 
 int isanumber(char*);

isanumber.c

Code:

 
  int isanumber(char* s) { 
  
  int i = 0; 
  
  while(s[i] != '\n') 
  { 
     if(isalpha((unsigned char) s[i]) || isblank( (unsigned char) s[i]) )
       break;
     
     i++;    
  }        
  
  if(s[i] == '\n')
    return 1; 
  return 0;            
}

itob.c

Code:

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

#include "Validation/isanumber.c"
#include "Headers/validation.h"

#ifndef MAXSIZE 
  #define MAXSIZE 20 
#endif  

static char reversed[MAXSIZE];

void itob(int, char*, int);
void reverse(char*);

void itob( int n, char* s, int b) 
{ 
   int sign, i;
   i = 0;
   
   if( (sign = n) < 0) 
     n = -n; 
   
   if(b == 2) 
   { 
      do { 
        s[i++] = n % 2 + '0';
      
        if(n == 1 || n == 2) 
        {   
           s[i++] = n / 2;
           break;
        }    
     } while( (n /= 2) > 0);          
   }
   if(sign < 0) 
    s[i++] = '-'; 
   
   s[i] = '\0';  
   reverse(s);
} 

void reverse(char* s) 
{ 
   int i, j; 
   for(i = (strlen(s) - 1), j = 0; i >= 0; i--, j++)
   { 
     reversed[j] = s[i];  
   }              
   reversed[j] = '\0';     
}         

int main(void) 
{ 
  char *s = malloc(MAXSIZE * sizeof(char));
  char *strnum = malloc(MAXSIZE * sizeof(char));
  int base; 
  long long num;
  
  
  do { 
   printf("Enter a number: ");
   fgets(strnum, MAXSIZE, stdin);
   num = strtoll(strnum, NULL, 10);
  } while( !(isanumber(strnum)));            
  
  do { 
   printf("Enter a number base: ");      
   scanf("%d", &base);      
  } while(base < 2 || base > 16);       
  
  itob(num, s, base);
  
  printf("The string representation of the number is %s\n", reversed);
  
  free(s);
  free(strnum);
  return 0;    
}

Code:

 

gdb -q itob
Reading symbols from /home/thames/C/itob...done.
(gdb) break 32
Breakpoint 1 at 0x400830: file itob.c, line 32.
(gdb) break 33
Breakpoint 2 at 0x40084e: file itob.c, line 33.
(gdb) run
Starting program: /home/thames/C/itob 
Enter a number: 16
Enter a number base: 2

Breakpoint 1, itob (n=2, s=0x602010 "0000", b=2) at itob.c:32
32               s[i++] = n / 2;
(gdb) cont
Continuing.

Breakpoint 2, itob (n=2, s=0x602010 "0000\001", b=2) at itob.c:33
33               break;
(gdb)

Not sure if your "edit: gotcha!" means you figured it out, but just in case somebody else finds this:

The "\001" is an octal (base 8) escape sequence. The \ starts the escape sequence, and it includes the next 1-3 digits that are valid octal. That means the "\001" represents a single byte with octal representation 001, which is a byte with value 1 (which is different than a byte with the value of the ASCII digit '1', which has a value of 49). This is valid inside a string literals (double quotes) and char literals (single quotes). The octal sequence ends after either 3 digits, or the first character that is not valid octal (letters, punctuation or 8 or 9), whichever is first. The null terminator character, '\0', is an example of a 1-digit octal escape sequence.