bin2hex command in C language

bin2hex.c

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

// $ cat Makefile
// ----------------------------------------
// all: bin2hex
// su:
//   gcc -O3 -o bin2hex bin2hex.c
// clean:
//  rm bin2hex
// ----------------------------------------
// $ make

int main(int argc, char **argv) {
    if (argc<2) {
        printf("Usage: %s <id>\n", argv[0]);
        return 1;
    }
    static char hexconvtab[] = "0123456789abcdef";
    const char* old = argv[1];
    const size_t oldlen = strlen(old);
    char hexed[oldlen * 3 + 1] ;
    memset(hexed, '\0', sizeof(hexed));
    size_t i, j;
    for(i=j=0; i<oldlen; i++) {
        hexed[j++] = hexconvtab[old[i]>>4];
        hexed[j++] = hexconvtab[old[i]&15];
    }
    hexed[j] = '\0';
    printf("hexed str=%s\n", hexed);
    return 0;
}