为什么不使用malloc就可以使用char指针？

Question

I've programmed something similar and I'm wondering why it works...

char* produceAString(void){
    char* myString;
    while(somethingIsGoingOn){
        //fill myString with a random amountof chars
    }
    return myString;
}

The theory tells me that I should use malloc to allocate space, when I'm using pointers. But in this case I don't know how much space I need for myString, therefore I just skipped it. But why does this work? Is it just bad code, which luckily worked for me, or is there something special behind char pointers?

Answer 1

It worked due to pure chance. It might not work the next time you try it. Uninitialized pointers can point anywhere in memory. Writing to them can cause an instant access violation, or a problem that will manifest later, or nothing at all.

Answer 2

This is generally bad code, yes. Also whatever compiler you use is probably not very intelligent or warnings turned off since they usually throw an error or at least a warning like "variable used uninitialized" which is completely true. You are in ( bad ) luck that when the code runs the point is garbage and somehow the OS allows the write ( or read ), probably you are running in debug mode? My personal experience is that in some cases its predictable what the OS will do, but you should never ever rely on those things, one example is if you build with MinGW in debug mode, the unintialized values are usualy follow a pattern or zero, in release build its usually complete random junk.

Since you "point to a memory location" it must point to a valid location whenever it is an another variable ( pointing to another variable ) or allocating space at run time ( malloc ) what you are doing is neither so you basically read/write a random memory block and because of some black magic the app doesn't crash because of this, are you running on windows? Windows 2000 or XP? since I know those are not as restrictive as windows since Vista, I remember that back in the day I did similar thing under Windows XP and nothing happened when it was supposed to crash.

So generally, allocate or point to a memory block you want to use before you use the pointer in case you dont know how much memory you need use realloc or just simply figure out a good strategy that has the smallest footprint for your specific case.

One way to see what C actually does is to change this line

char* myString;

into

char* myString=(char*)0;

and break before that line with a debugger and watch the myString variable, it'll junk and if it intalizes the variable it'll be 0 then the rest of your code fail with access violation because you point "nowhere". The normal operation would be

char* myString=(char*)malloc(125); // whatever amount you want