malloc doesn't do anything?

I just recently started programming in C.

I'm running into problems with Malloc. It seems like malloc doesn't do anything with my string?

I have the following code:

Code:

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  char *s;
  s = malloc( 5 * sizeof(char));
  strcpy(s, "123451234567890876432123456789678");
 or  s = "123456678";
  printf("String is: %s", s);

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Why can I put a string in that is larger dan 5 bytes?
I've tried many different ways and everytime I reserve a few bytes I can make the string larger.

Maybe i'm using things wrong but i would appreciate it if someone can explain to me why this works...

Malloc guarantees you, in this case, 5 bytes of contiguous memory. There might be more available after your desired block, or there might be garbage which will crash your program when you try to access it. Malloc just returns a pointer to the start of a block of memory that will fit what you want it to. It might be more, but it will never be less.

Quote:

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Why can I put a string in that is larger dan 5 bytes?

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Luck.

Incidentally, s = "12345678" changes what s points to.

Ok, so it's just plain luck now...
Can I determine the size of the newly allocated block (after malloc)?

e.g. sizeof(s) or strlen(s) ? Will it return the allocated space?
When I use these functions now they always return 4 bytes even after mallocing 50

You've defined it, so you already know it. It's 5 bytes.

Doing anything with the data that you "luckily" have access to is dangerous. Don't do it.

If you need more memory than five bytes, allocate more than five bytes.

Quote:

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When I use these functions now they always return 4 bytes even after mallocing 50

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You're probably taking sizeof(s) which returns the size of the pointer s, not the size of what was allocated.

Quote:

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You've defined it, so you already know it. It's 5 bytes.

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Yep. Also, note that sizeof(char) is guaranteed to be 1.

http://cpwiki.sf.net/Buffer_overrun

Thanks that makes it a lot less confusing...