Why printf() prints this mysterious extra text, on Windows?

Question

I recently wrote a simple program to reverse a string, my program simply accepts an input string from user and then reverses it. This is all done with 2 pointers. Concerning the pointer safety in my program I wrote a function to duplicate the give string, This function allocates memory for a new (same length) string, and then copy the character 1-by-1.

My problem is when I run this program, Although It does what I need, It prints some mysterious extra output. It does this every time, before accepting input from the user. This is what happens when I run the program.

C:\Users\0xEDD1E\Desktop\revstr>revstr.exe
[C]
[.]
[revstr.exe]
hello
[hello]
olleh

here the last three lines are Input and Output, It's OK, The problem is in the first 3 lines

[C]
[.]
[revstr]

What are those? any way, here is my program

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

#define swap(a, b) (((a) ^ (b)) && ((a) ^= (b), (b) ^= (a), (a) ^= (b)))

unsigned long strlen_(char *);
char *strdup(char *s);
char *reverseString(char *, int);

int main(void)
{
    //fflush(stdout);
    char *str = (char *) malloc(1024 * sizeof (char));
    scanf("%[^\n]s", str);
    int slen = strlen_(str);
    printf("%s\n", reverseString(str, slen));
    return 0;
}

unsigned long strlen_(char *s)
{
    char *p = s;
    while (*p) p++;
    return p - s;
}

char *strdup(char *s)
{
    char *p = (char *) malloc((size_t) (strlen_(s) + 1) * sizeof (char));

    if (p) {
        char *pp = p;
        while ((*pp++ = *s++)) ;
    }
    printf("[%s]\n", p);
    return p;
}


char* reverseString(char* s, int n) 
{

    char *str = strdup(s);
    char *p = str - 1;
    char *q = str + n;

    while (++p < --q) {
        swap(*p, *q);
    }

    return str;

}

you can see, looking at, strdup() function, those lines are generated by the printf() in the strdup() function (because of printf("[%s]\n, str);).

So I think I found the bug-point. But I can't figure out the reason for this error, and a way to fix it. Especially This happens only on Windows (mine is Windows 10 Pro). I tested this on Ubuntu(64bit), there is no bug like this in Ubuntu.

My instinct says me this error has to do something with pointers used in the functions

UPDATE: I tried fflush(stdout) but that didn't help

What is the reason behind this Bug (or misbehave), could someone explain the reason?

Answer 1

Probably, your system's C library also defines a function strdup, and some code in the program startup calls that function. Then the linker links those calls to your strdup function instead of the system function.

To avoid this, don't name your function strdup, or str anything for that matter: names starting str and a lowercase letter are reserved (C11 7.31.12, 7.31.13); using them as function names in your own code causes undefined behaviour.

NB. char *p = str - 1; also causes undefined behaviour by doing pointer arithmetic outside the bounds of any object. (For example, imagine if str happens to be the first address in the address space). It would be good to restructure your algorithm to not point out of bounds.