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See How to convert int into char* in C (without sprintf) for details to cope with the above issues. (That is a fixed base 10 answer, yet easy enough to modify.)

See How to convert int into char* in C (without sprintf) for details to cope with the above issues. (That is a fixed base 10 answer, yet easy enough to modify.)

include <limits.h>
#define INT_STR_SIZE (sizeof(int)*CHAR_BIT + 2)
static char *my_itoa_base_helper(char *dest, size_t size, int x, int base) {
 if (size == 0) return NULL;
 int digit = -(x%base);
 x /= base;
 if (x) {
 dest = my_itoa_base_helper(dest, size-1, x, base);
 if (dest == NULL) return NULL;
 }
 *dest = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[digit];
 return dest + 1;
}
char *my_itoa_base(char *dest, size_t size, int x, int base) {
 if (base < 2 || base > 36 || size < 2) return NULL;
 char *p = dest;
 if (x < 0) {
 *p++ = '-';
 size--;
 } else {
 x = -x;
 }
 p = my_itoa_base_helper(p, size-1, x, base);
 if (p == NULL) return NULL;
 *p = '0円';
 return dest;
}
// compound literal C99 or later
#define MY_ITOA_BASE(x,base) my_itoa_base((char [INT_STR_SIZE]){""}, INT_STR_SIZE,(x),(base))
// Test code
void testi(int x, int base) {
 printf("%12d: base 10 = %11s, base %2d = %s\n", 
 x, MY_ITOA_BASE(x, 10), base, MY_ITOA_BASE(x, base)); 
 }
include <stdio.h.h>
int main() {
 testi(123, 16);
 testi(-123, 16);
 testi(INT_MIN, 2);
 testi(0, 2);
 testi(INT_MAX, 2);
 testi(INT_MIN, 36);
 return 0;
}

Output

 123: base 10 = 123, base 16 = 7B
 -123: base 10 = -123, base 16 = -7B
 -2147483648: base 10 = -2147483648, base 2 =わ -ひく10000000000000000000000000000000
 0: base 10 = 0, base 2 =わ 0
 2147483647: base 10 = 2147483647, base 2 =わ 1111111111111111111111111111111
 -ひく2147483648: base 10 = -2147483648, base 36 = -ZIK0ZK

include <limits.h>
#define INT_STR_SIZE (sizeof(int)*CHAR_BIT + 2)
static char *my_itoa_base_helper(char *dest, size_t size, int x, int base) {
 if (size == 0) return NULL;
 int digit = -(x%base);
 x /= base;
 if (x) {
 dest = my_itoa_base_helper(dest, size-1, x, base);
 if (dest == NULL) return NULL;
 }
 *dest = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[digit];
 return dest + 1;
}
char *my_itoa_base(char *dest, size_t size, int x, int base) {
 if (base < 2 || base > 36 || size < 2) return NULL;
 char *p = dest;
 if (x < 0) {
 *p++ = '-';
 size--;
 } else {
 x = -x;
 }
 p = my_itoa_base_helper(p, size-1, x, base);
 if (p == NULL) return NULL;
 *p = '0円';
 return dest;
}
// compound literal C99 or later
#define MY_ITOA_BASE(x,base) my_itoa_base((char [INT_STR_SIZE]){""}, INT_STR_SIZE,(x),(base))

Test code & Output

void testi(int x, int base) {
 printf("%12d: base 10 = %11s, base %2d = %s\n", 
 x, MY_ITOA_BASE(x, 10), base, MY_ITOA_BASE(x, base)); 
 }
#include <stdio.h.h>
int main() {
 testi(123, 16);
 testi(-123, 16);
 testi(INT_MIN, 2);
 testi(0, 2);
 testi(INT_MAX, 2);
 testi(INT_MIN, 36);
 return 0;
}
 123: base 10 = 123, base 16 = 7B
 -123: base 10 = -123, base 16 = -7B
 -2147483648: base 10 = -2147483648, base 2 =わ -ひく10000000000000000000000000000000
 0: base 10 = 0, base 2 =わ 0
 2147483647: base 10 = 2147483647, base 2 =わ 1111111111111111111111111111111
 -ひく2147483648: base 10 = -2147483648, base 36 = -ZIK0ZK

[6/21 Edit]
For fun, below is a recursive solution that solves the above issues. Of course a simple loop could be use instead.

include <limits.h>
#define INT_STR_SIZE (sizeof(int)*CHAR_BIT + 2)
static char *my_itoa_base_helper(char *dest, size_t size, int x, int base) {
 if (size == 0) return NULL;
 int digit = -(x%base);
 x /= base;
 if (x) {
 dest = my_itoa_base_helper(dest, size-1, x, base);
 if (dest == NULL) return NULL;
 }
 *dest = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"[digit];
 return dest + 1;
}
char *my_itoa_base(char *dest, size_t size, int x, int base) {
 if (base < 2 || base > 36 || size < 2) return NULL;
 char *p = dest;
 if (x < 0) {
 *p++ = '-';
 size--;
 } else {
 x = -x;
 }
 p = my_itoa_base_helper(p, size-1, x, base);
 if (p == NULL) return NULL;
 *p = '0円';
 return dest;
}
// compound literal C99 or later
#define MY_ITOA_BASE(x,base) my_itoa_base((char [INT_STR_SIZE]){""}, INT_STR_SIZE,(x),(base))
// Test code
void testi(int x, int base) {
 printf("%12d: base 10 = %11s, base %2d = %s\n", 
 x, MY_ITOA_BASE(x, 10), base, MY_ITOA_BASE(x, base)); 
 }
include <stdio.h.h>
int main() {
 testi(123, 16);
 testi(-123, 16);
 testi(INT_MIN, 2);
 testi(0, 2);
 testi(INT_MAX, 2);
 testi(INT_MIN, 36);
 return 0;
}

Output

 123: base 10 = 123, base 16 = 7B
 -123: base 10 = -123, base 16 = -7B
 -2147483648: base 10 = -2147483648, base 2 =わ -ひく10000000000000000000000000000000
 0: base 10 = 0, base 2 =わ 0
 2147483647: base 10 = 2147483647, base 2 =わ 1111111111111111111111111111111
 -ひく2147483648: base 10 = -2147483648, base 36 = -ZIK0ZK