C realizes the conversion of string values into hexadecimal

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

char hex_char(unsigned int n )
{
    
    
    switch (n)
    {
    
    
    case 0:
        return '0';
    case 1:
        return '1';
    case 2:
        return '2';
    case 3:
        return '3';
    case 4:
        return '4';
    case 5:
        return '5';
    case 6:
        return '6';
    case 7:
        return '7';
    case 8:
        return '8';
    case 9:
        return '9';
    case 10:
        return 'a';
    case 11:
        return 'b';
    case 12:
        return 'c';
    case 13:
        return 'd';
    case 14:
        return 'e';
    case 15:
        return 'f';
    }
    return '0';
}

void to_hex(unsigned int n)
{
    
    
    int i = n % 16;
    if (n>= 16)
        to_hex(n / 16);
    printf("%c", hex_char(i));

}

int main(int argc,const char *argv[])
{
    
    
    if(argc < 2)
    {
    
    
        fprintf(stderr, "Usage %s <str>\n", argv[0]);
        return -1;
    }
    
    int a = atoi(argv[1]);
    
    to_hex(a);

    printf("\n");   
    
    return 0;
}

Test Results