Question

Possible Duplicate:
Default values of int when not initialized in c. why do i get different outputs?

Beginner so be lil soft..am compiling a simple code below, I am not assigning any value to my variables but C program generates some random values, why is it so?(Only 2nd variable generates random integers)

So where these values came from?

#include<stdio.h>

main(void) {
    int var1;
    int var2;

    printf("Var1 is %d and Var2 is %d.", var1, var2);
    return 0; //Book says I should use this for getting an output but my compiler anyways compile and return me values whether I use it or not
}

//Output 1st compiled: var1 = 19125, var2 = 8983
//Output 2nd compiled: var1 = 19125, var2 = 9207
//Output 2nd compiled: var1 = 19125, var2 = 9127

Answer 1

Your C program is compiled to some executable program. Notice that if you compile on Linux using gcc -Wall, you’ll get warnings about uninitialized variables.

The var1 and var2 variables get compiled into using some stack slots, or some registers. These contain some apparently random number, which your program prints. (that number is not really random, it is just unpredictable garbage).

The C language does not mandate the implicit initialization of variables (in contrast with e.g. Java).

In practice, in C I strongly suggest to always explicitly initialize local variables (often, the compiler may be smart enough to even avoid emitting useless initialization).

What you observe is called undefined behavior.

You’ll probably observe a different output for var1 if you compiled with a different compiler, or with different optimization flags, or with a different environment (probably typing export SOMEVAR=something before running again your program could change the output for var1, or running your program with a lot of program arguments, etc…).

You could (on Linux) compile with gcc -fverbose-asm -S and add various optimization flags (e.g. -O1 or -O2 …) your source code yoursource.c and look inside the generated yoursource.s assembler code with some editor.