hex

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

int arg2int(const char *arg)
{
    errno = 0;
    char *tmp;
    long larg = strtol(arg, &tmp, 10);

    if (*tmp == '\0' && errno == 0 && larg >= INT_MIN && larg <= INT_MAX)
        return (int)larg;

    return -1;
}

void dec2hexarr(int dec, char *buf)
{
    int i = 0;
    int rem;

    while (dec)
    {
        rem = dec % 16;
        buf[i++] = (rem < 10) ? rem + '0' : rem + '7';
        dec /= 16;
    }
}

int main(int argc, char **argv)
{
    int i, j;
    for (i = 1; i < argc; ++i)
    {
        char buf[256] = {0};

        dec2hexarr(arg2int(argv[i]), buf);

        for (j = 0; j < 256; ++j)
            printf("%c", buf[j]);

        puts("");
    }

    return 0;
}