simple dec2bin function in C, C (clang)

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//clang 3.8.0

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

/** 
 * parameters: szBin const array of chars already containing the mask 
 * to be printed in (with or without leading/ trailing/ separating blanks)
 * len is the string lenght of the szBin (without evtl. terminating zero)
 * and x is the unsigned number to be expressed in binary '1' and '0'-s
 * return value: the input integer to be expressed
 * no error outputs 
 * note: every characters in input other than '0' is considered blank
 * and will be skipped
 */
int int2bin(char* const szBin, size_t len, const int x);

/**
 * parameters: szBin is the input string; a sequence of '1's and '0's
 * (incl. trailing/ leading/ separating blanks) expressing a binary 
 * number to be expressed as unsigned integer, len is the length of 
 * the szBin (without terminating zero) and x is the offset added to
 * the return value - usually 0
 * note: every other character than '1' will be interpreted as '0' except
 * blanks which will be skipped
 */

int bin2int(const char* const szBin, size_t len, int x);

/**
 * 
 */
int main(void)
{
    char* szBin = (char*)malloc(18);
    int i;
    
    for(i=0x0000; i<0x010; i++)
    {
        memcpy(szBin, " 0000 0000 0000 0000\0", 20);
        int2bin(szBin, 20, 1<<i);
        printf("%04x\t%s\t%d\n", 1<<i, szBin, bin2int(szBin, 20, 0));
    }

memcpy(szBin, " 0000 0000 0000 0000\0", 20);
    int2bin(szBin, 20, 2289);
    printf("%04x\t%s\t%d\n", 2289, szBin, bin2int(szBin, 20, 0));

free(szBin);
    return 0;
}

int int2bin(char* const szBin, size_t len, const int x)
{
    size_t i = 0;
    char* szi = szBin+len;
    while(szi-->szBin)
    {
        if('0'==*szi)
        {
            if((0!=(x&1<<i))) *szi = '1';
            i++;
        }
    }
    return x;
}

int bin2int(const char* const szBin, size_t len, int x)
{
    size_t i = 0;
    const char* szi = szBin+len;
    while(szi-->szBin)
    {
        if('1'==*szi)
        {
            x+=(1<<i);
        }
        if(' '!=*szi) i++;
    }
    return x;
}

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Compilation time: 0.23 sec, absolute running time: 0.11 sec, cpu time: 0.06 sec, memory peak: 3 Mb, absolute service time: 0,35 sec