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Why allocating a 0 size char block works in this case? But if I write char *string = NULL; it won't work.

I'm using Visual Studio.

int main()
{   
    char *string = (char *)malloc(0);
    string[0] = 'a';
    string[1] = 'b';
    string[2] = 'c';
    string[3] = 'd';
    string[4] = '\0';

    printf("%s\n",string);
    return 0;
}
Yu Hao
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Viktor Simkó
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3 Answers3

10

First let me state, as per the man page of malloc()

The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().

a call like malloc(0) is valid itself, but then, we need to check the validity of the returned pointer. It can either

  • Return NULL
  • Return a pointer which can be passed to free().

but anyways, dereferencing that pointer is not allowed. It will cause out-of-bound memory access and cause undefined behaviour.

That said, two important things to mention,

  1. Please see why not to cast the return value of malloc() and family in C.

  2. Please check the return value of malloc() before using the returned pointer.

So, to answer your question,

Difference between initializing a string with (char *)malloc(0) and NULL

Do not use malloc(0) in this case, as a NULL check on the pointer may fail, giving the wrong impression of a valid allocation of the memory to the pointer. Always use NULL for initialization.

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Sourav Ghosh
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4

The above code invokes undefined behavior. You have allocated insufficient memory and you are accessing invalid addresses.

According to the specifications, malloc(0) will return either "a null pointer or a unique pointer that can be successfully passed to free()".

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Sadique
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  • `s/random/invalid`. A memory allocator's behavior is pretty un-random, it would make a terrible PRNG. – The Paramagnetic Croissant Aug 06 '15 at 12:20
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    @Quentin but this is a Q&A site for programmers, and if one gives an answer, one is supposed to phrase it correctly. I could as well decide that from now on, I use the word "string" to denote "function", and then I complain every time someone points out that one can't call a string that I just use "string" with "function" as its meaning. Some people are trying to use memory allocation addresses as the source of random numbers. Let's not encourage spreading the infection (what if these "random" numbers end up encrypting your HTTPS traffic? would you like that?) – The Paramagnetic Croissant Aug 06 '15 at 12:32
  • @Quentin grow up. seriously, if you don't see the value in formulating correctly what one says, I don't even want to argue with you further. – The Paramagnetic Croissant Aug 06 '15 at 13:17
  • @TheParamagneticCroissant I'm not even sure why you're arguing in the first place. Of course "invalid" is better than "random" for an indeterminate address. It's just that the thought of someone jumping on the mere presence of the word "random" and trying to make a PRNG out of it made me chuckle. There's no reason to be upset. – Quentin Aug 06 '15 at 13:33
1

malloc definition:

Allocates a block of size bytes of memory, returning a pointer to the beginning of the block.

The content of the newly allocated block of memory is not initialized, remaining with indeterminate values.

If size is zero, the return value depends on the particular library implementation (it may or may not be a null pointer), but the returned pointer shall not be dereferenced.

Taken from here and found this related question.

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Mauker
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