determine length of calloc array of TCHARs (not string length)

Question

I have this code:

TCHAR *sRes;
sRes = (TCHAR *) calloc(16384, sizeof(TCHAR));
DWORD dwRes = sizeof(sRes);

dwRes is always 8, and of course _tcslen(sRes) is always 0.

I am looking for 16384.

score 3 · Answer 1 · answered Apr 18 '11 at 20:29

3

In C there is no way to get the size of a memory block with only the base address of the block.

But when you created the block, you knew the size: just save it and use it afterwards when you need:

TCHAR *sRes;
DWORD dwRes = 16384 * sizeof (TCHAR);
sRes = calloc(16384, sizeof (TCHAR)); /* I prefer `sizeof *sRes` */

/* use `sRes` and `dwRes` as needed ... */

Also, notice I removed the cast from the return value of calloc. Casting here serves no useful purpose and may hide an error.

answered Apr 18 '11 at 20:29

pmg

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But I get a compiler error if I don't cast it. – JeffR Apr 18 '11 at 20:41
2

Then you're probably compiling with a C++ compiler. I prefer to use C compilers for C code, but I understand some people like to compile C code with C++ compilers for extra (*redundant and possibly erroneous*) diagnostics. – pmg Apr 18 '11 at 20:44
Yes, I am compiling with a C++ compiler. @Blagovest: I'm using Visual Studio 2008. I get a "void *" compiler error. – JeffR Apr 18 '11 at 20:47
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Using a C++ compiler for C code is like [speaking British english in the USA or vice-versa](http://en.wikipedia.org/wiki/American_and_British_English_differences#Words_and_phrases_with_different_meanings). Do it at your own risk. I think it is possible to configure Visual Studio as a C (C89) compiler. – pmg Apr 18 '11 at 20:54

score 0 · Accepted Answer · answered Apr 18 '11 at 20:09

0

There is no supported mechanism for obtaining size of a block allocated with malloc or calloc. If you call HeapAlloc instead you may call HeapSize thereafter.

answered Apr 18 '11 at 20:09

bmargulies

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I read HeapAlooc will get the memory from the process default heap. calloc will get the memory from CRT heap. What's the implications of usage of either one? – JeffR Apr 18 '11 at 20:16
You to free HeapAlloc memory with HeapFree. I also suspect that it's slower, but it's been a long time. – bmargulies Apr 18 '11 at 20:18

score 0 · Answer 3 · answered Apr 18 '11 at 20:36

The operating system and the underlying memory allocator implementation keep track of that number, but there is no standard facility to obtain this value in application code.

sizeof is a static operator and hence cannot be used to return the size of anything that's determined at runtime.

Your only option is to create a custom struct where you manually keep both the returned pointer and the size which was allocated.

determine length of calloc array of TCHARs (not string length)

3 Answers3