在C中将十进制数转换为二进制

Question

I was trying to convert a decimal from [0, 255] to a 8 bit binary number, where each bit will be separated by a comma and a space. I tried the following (eventually it worked, except for the last bit does not require any separator ):

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

char* atobin(int input) {
    char *str = malloc(0);
    int index, count = 0;
    if (input > 255| input < 0) {
        printf ("Input out of range. Abort.\n");
        exit(EXIT_FAILURE);
    }

    for (index = 7; index >= 0; index--) {
        *(str + (count++)) = (input >> index & 1) ? '1' : '0';
        *(str + (count++)) = ',';
        *(str + (count++)) = ' ';
    }
    *(str + count) = '\0';
    return str;
}

int main(int argc, char *argv[]) {
    printf("%s\n", atobin(atoi(argv[1])));
    return EXIT_SUCCESS;
}

Now I have a few questions:

1. I used malloc(0) ; as far as I am concerned, it will allocate no memory from the heap. So, how/ why is it working?
2. Is the declaration *(str + count) = '\\0'; necessary?
3. Is there any way to optimize this code?

UPDATE

To carry on this experiment, I have taken the atobin function in to a .h file. This time it creates some problems. Now I add my last question:

What should be minimum integer to be used for the parameter of malloc ? Some trial-and-error method makes me guess it should me 512 . Any idea?

Answer 1

1. From a malloc doc:

If size is zero, the return value depends on the particular library implementation (it may or may not be a null pointer), but the returned pointer shall not be dereferenced.

That it works for you is just luck. Try some random malloc and free afterwards and you will have a high probability - but no guarantee - that you will get a crash.

1. This null-terminates the string. Depends on how you want to use the string if you need it. The printf in your example needs it, because that's the only way for it to know when the string ends.

2. I'm sorry, I don't have time to take a closer look.

Answer 2

malloc(0) is valid to be used. what it returns is implementation defined. but what happens if you access a object through the pointer returned by malloc(0) is undefined behavoiur . you can read this related question.
In C since strings are character arrays terminated by \\0 it is better to use *(str + count) = '\\0'; statement to set the last character of string. It is working in this code but its better to set the end of string in code always. It may work if bits are set to 0 since it is same as terminating a string.