Obtaining memory block size

[DustyMx]

HI guys

Does someone know how can I retrieve the size of a memory block that had been reserved using malloc/calloc?

char *chuck = (char*) malloc(sizeof(char) * 150);

Having declared the last statement how can I get back 150?

Thanks in advance.

 

[Salem]

You don't.
If you need to remember the size, then use another variable.

Also, casting the result of malloc in C is a bad thing.


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[hunghsun]

>> Also, casting the result of malloc in C is a bad thing.

How is that a bad thing? From what I can see, he/she is assigning the memory space using malloc to a char pointer. perfectly fine there.

Regardless, he's probably right on having to memorize the size somewhere else.

 

[DustyMx]

Thanks for the advice.

Just to let you know the only function that I found related to this topics was malloc_size() which return the all size of the block you have reserved which include the housekeeping information(which as far as I could notice it varies its size) and therefore it is not useful for me.

Regards

 

[ArkM]

More precisely: NO C standard library function to obtain allocated (by malloc) memory block size via pointer. May be any C implementations include such extented heap managementfunctions (it's true), but it's not a portable code, avoid this feature.

Sometimes it may be useful to design new codes (from the beginning) with user-defined memory allocator (kind of malloc wrapper). It's not a common way and as usually you can't use such wrappers with legacy codes.

No need to cast malloc returned pointer in C because of (void*) to (type*) conversion is legal in C (but not in C++.

 

[Salem]

> How is that a bad thing?
Because casting suppresses a much more serious error you would otherwise get if you failed to include stdlib.h in the first place.

Since C quietly converts the void* returned by malloc into the char* of the assigned pointer, the cast at best does nothing.

If you're getting complaints about void* conversions, then stop using C++ to compile your code, or start using new/delete for memory management.

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[marsd]

Well said Salem.
Too much C++ thinking in a C forum.
Check the archives:
We have gone round and round on this before.