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When I use malloc in a C program, I get a warning:
warning: incompatible implicit declaration of built-in function 'malloc' [enabled by default] I can then include <malloc.h> or <stdlib.h> to get rid of the warning although it works without it as well.
So I was wondering, what's the difference between these headers and which one does gcc links when I don't include anything?
(I'm using ubuntu 12.04 64-bit with gcc 4.6.3)
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In computing, malloc is a subroutine for performing dynamic memory allocation. malloc is part of the standard library and is declared in the stdlib. h header. Many implementations of malloc are available, each of which performs differently depending on the computing hardware and how a program is written.
The header file stdlib. h stands for Standard Library. It has the information of memory allocation/freeing functions.
One easy way to differentiate these two header files is that “<stdio. h>” contains declaration of printf() and scanf() while “<stdlib. h>” contains declaration of malloc() and free(). In that sense, the main difference in these two header files can considered that, while “<stdio.
h is the header of the general purpose standard library of C programming language which includes functions involving memory allocation, process control, conversions and others. It is compatible with C++ and is known as cstdlib in C++. The name "stdlib" stands for "standard library".
The <malloc.h> header is deprecated (and quite Linux specific, on which it defines non-standard functions like mallinfo(3)). Use <stdlib.h> instead if you simply need malloc(3) and related standard functions (e.g. free, calloc, realloc ....). Notice that <stdlib.h> is defined by C89 (and later) standards, but not <malloc.h>
Look into /usr/include/malloc.h you'll find there some non-standard functions (e.g. malloc_stats(3), etc...) - in addition of malloc....
And gcc don't link header files, but libraries. Read Levine's book about linkers & loaders for more.
If you don't include any headers (and dont explicitly declare malloc yourself, which would be a bad idea), malloc is implicitly declared as returning some int value (which is wrong). I do invite you to pass at least the -Wall flag to gcc when using it.
You might also pass -v to gcc to understand the actual programs involved: cc1 is the compiler proper (producing assembly code), as the assembler, ld the linker, and collect2 an internal utility which invokes the linker.
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