Returning string pointer got wrong string in C

Question

I'm trying to convert decimal to binary. I'm working on function "const char * dtb_16(float a)". in this function, I'm returning the pointer of the string as code below.

OR

const char* dtb_16(float a) {
    char str[17];
    int i = 0;;
    for (i = 0; i < 16; i++) {
        a = a * 2;
        if (a > 1) {
            a = a - 1;
            str[i] = '1';
        }
        else
            str[i] = '0';
    }
    str[16] = 0;
    printf("%s \n", str);;
    return str;
}

and at main

printf("test : %s\n", dtb_16(decimal));

the result is something like this:

starting read files
Please enter binary bit length: 16
0011101011100001
test : 烫烫眺?
1011110101110000
test : 烫烫眺?
1110001111010111
test : 烫烫眺?
1101110000101000
test : 烫烫眺?
1001100110011001

the string looks fine in the function dtb_16 but it goes wrong when back to main. I tried return *str and &str, none of them works.

Don't return strings from a function that have been defined local to that function. — Scott Hunter, Jul 20 '21 at 13:07
Don't post pictures of code but post code as properly formatted text. — Jabberwocky, Jul 20 '21 at 13:08
Does this answer your question? [Returning pointer from a function](https://stackoverflow.com/questions/7754986/returning-pointer-from-a-function) — joH1, Jul 20 '21 at 13:52

czarson · Accepted Answer · 2021-07-21T02:13:11.393

1

You shouldn't return an address of a local variable, instead you will have to create the str variable on the heap. You can do that by swapping char str[17]; to char* str = malloc(sizeof(char) * 17);

edited Jul 21 '21 at 02:13

answered Jul 20 '21 at 13:11

czarson

125
7

That's one possibility, but you should mention that the string needs to be freed at some point. – Jabberwocky Jul 20 '21 at 13:58

Returning string pointer got wrong string in C

1 Answers1