Convert String to Hexidecimal

strhex.c

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

char* stringToHex(char *str)
{
    int len = strlen(str);
    char* buf = malloc(len * 2 + 1); 
    char* ret = buf;
    int i;
    char msb = 0, lsb = 0;
    for (; *str != 0; ++str) {
        msb = *str >> 4 & 15; 
        lsb = *str & 15; 

        sprintf(buf, "%x", msb);
        sprintf(buf + 1, "%x", lsb);
        buf += 2;
    }   
    return ret;
}

char htoi(char ch) 
{
    if (ch >= '0' && ch <= '9') {
        return ch - 0x30;
    } else if (ch >= 'a' && ch <= 'f') {
        return ch - 0x61 + 10; 
    } else if (ch >= 'A' && ch <= 'F') {
        return ch - 0x41 + 10; 
    }   

    return -1; 
}

char* hexToString(char *hex)
{
    size_t len = strlen(hex);
    if (len & 1) return NULL;

    char *buf = malloc(len / 2 + 1); 
    char *ret = buf;
    int ch = 0, tmp = 0;
    int i;
    for (i = 0; i < len / 2; i++) {
        char msb = hex[i*2];
        char lsb = hex[i*2+1];
        tmp = htoi(msb);
        if (tmp == -1) {
            goto clean;
        }
        ch = tmp << 4;
        tmp = htoi(lsb);
        if (tmp == -1) {
            goto clean;
        }
        ch |= tmp;
        buf[i] = ch;
    }

    return ret;
clean:
    free(buf);
    return NULL;
}

int main(int argc, char *argv[])
{
    char *ptr = argv[1];
    if (ptr == NULL) return 0;

    char *hex = stringToHex(ptr);
    printf("hex: %s\n", hex);

    char *str = hexToString(hex);
    printf("str: %s\n", str);
    free(hex);
    free(str);

    return 0;
}