suggest me when will the following warning occurs during compilation

Question

temp....is used wrong in call to sprintf or snprintf.

If copying takes place bteween objects that overlap as a result of a call to sprintf() or snprintf(), results are undefined.

Answer 1

This doesn't provoke a warning from gcc, even with -Wall -Wextra -pedantic:

#include "stdio.h"

int main (void) {
    char xx[1000] = "hello";
    sprintf (xx, "xyzzy plugh %s", xx);
    printf ("%s\n", xx);
    return 0;
}

However, the reason why this is considered a bad idea can be seen from the output. Rather than getting:

xyzzy plugh hello

as a normal person may expect, you actually get:

xyzzy plugh xyzzy plugh

but, as with all undefined behaviour, your mileage may vary.

The definitive reference is the C99 standard, section 7.19.6.6 The sprintf function, which states:

The sprintf function is equivalent to fprintf, except that the output is written into an array (specified by the argument s) rather than to a stream. A null character is written at the end of the characters written; it is not counted as part of the returned value. If copying takes place between objects that overlap, the behavior is undefined.

The C++ standard (well, actually the C++0x draft, but it's surely due any day now, hopefully - c'mon guys, get it out there) references this since it incorporates parts of the C standard as deprecated functionality.