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Dynamic memory allocation in C

mikkel250 on December 25, 2019

Overview This post deals with pointers because dynamic memory allocation only works with pointers. Whenever you define a variable in C, ...

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Stefano Prosperi • Dec 26 '19

Casting the result of malloc is unnecessary and very bad practice.

Bad:

int * pNumber = (int*) malloc(25*sizeof(int));

Good:

int * pNumber = malloc(25*sizeof(int));

therselman • Dec 26 '19 • Edited

I think you are wrong about casting malloc is bad practice. You will get a compiler warning if you don't! It's total rubbish that it's bad practice! If you are not getting compiler warnings, then you aren't using a high enough warning level! It's more an inconvenience than anything else, that's why C++ created the Auto keyword, but even then you would need to cast it so that auto can infer the correct type.

Stefano Prosperi • Dec 26 '19 • Edited

stackoverflow.com/a/605858/9815377

It's important to stress that we're talking about C, not C++

Andrew Clayton • Dec 27 '19

Indeed.

I think it is also worth explicitly stating that these days C and C++ are very different languages. (All too often I see people conflating the two or just saying C/C++, when they really mean one or the other...)

Paul Hewlett • Dec 27 '19

The realloc example, whilst correct, is bad practice. What if realloc fails? In that case you will lose reference to the previously allocated str. The original memory will be orphaned.

mikkel250 • Dec 27 '19

Can you elaborate and/or provide an example of what is considered best practice?

Paul Hewlett • Dec 28 '19

Use a dummy ptr something like this...

Void *ptr = realloc(str, new size)
If (!ptr) {
.... Handle error...
}
str = ptr

This way if realloc fails the reference to str is not lost.

Of course in a normal Linux environment it is rare for any of the allocation functions such as malloc, calloc etc.. to fail. See anything about overcommit for an explanation.. This would make good subject for a followup article.