Question

#include <stdio.h>
void main()
{
int x=5,y=6;
printf("%d%d%d",x++,(y=x++),(x=y++));
}

Can anyone please explain why does this returns 766?

Answer 1

First, if this question, in this format, was given on a beginner C programming course, the course/teacher is a bad one.

The main problem here is that both ‘x’ and ‘y’ are modified several times before a sequence point, which is undefined behavior (C99/C11 6.5 §2). This is a severe bug, because anything can happen. Before the ++ mess is removed, there is no telling what this code does. Read this then read it again.

Further, the order of evaluation of function arguments is unspecified behavior. (C99/C11 6.5.2.2 §10). That is, the compiler may evaluate them left-to-right or right-to-left, and we cannot know which order that applies. The compiler does not need to document this! But if you are lucky, it could be documented. You must then read the compiler documentation to see which order of evaluation that applies, before attempting to answer the question. Otherwise you must give two answers.

Further, if this is code for a hosted system, such as a Windows PC, main is only allowed to return ‘int’ or this code won’t compile on a C compiler.