Why am I getting segfault when changing the signature of main?

Question

I am trying to get my feet into C, and wrote this program that displays a kb of my RAM in a random location. Here is the code, and it works fine:

#include <stdio.h>

int main(){
    char *mem;
    for(int i =0; i < 1024; i++){
        mem++;
        printf("%c", *mem);
    }
    return 0;
}

After that, I did the following change in my code, and I get segfaults every time I run my program:

#include <stdio.h>


// Just added this signature
int main(int argc, char *argv[]){
    char *mem;
    for(int i =0; i < 1024; i++){
        mem++;
        printf("%c", *mem);
    }
    return 0;
}

My spider senses tell me that the segfaults I get are random, and should also be caused in the first example, but running the different programs again and again makes it look like predictable behaviour.

$ gcc -v
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/usr/include/c++/4.2.1
Apple LLVM version 7.3.0 (clang-703.0.31)
Target: x86_64-apple-darwin15.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

Answer 1

Both your snippets invoke undefined behavior as you try to

1. Go out of bound (mem++;, with no allocation)
2. use uninitialized values (accessing *mem )

with the current version.

Remember, pointers do not magically inherit (or acquire) memory, you need to make a pointer point to something valid, in general.

Answer 2

The value of mem is undefined (not initialized), but not random. If before main is called, other C runtime functions, are called, then the slot of stack used by mem may have a valid pointer within it. Adding parameters to main changes which slot is used and changes behaviour. This can mean the code doesn't crash, although it is not correct.