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In C, the library function malloc is used to allocate a block of memory on the heap. The program accesses this block of memory via a pointer that malloc returns. When the memory is no longer needed, the pointer is passed to free which deallocates the memory so that it can be used for other purposes.
The malloc() call itself is much less expensive than brk(), because it is a library call, not a system call. Symmetric behavior is adopted when memory is freed by the process. Memory blocks are not immediately returned to the system, which would require a new brk() call with a negative argument.
maaloc can fail as it cant allocate memory bigger than default heap. not all system memory is available to malloc. The in MS-Windows the default heap is not static -- the os will expand it as needed.
The malloc function allocates a memory block of at least size bytes. The malloc function guarantees alignment in Windows CE in the same way as LocalAlloc in the Win32 API. The block can be larger than size bytes because of space required for alignment and maintenance information.
Memory management is a service provided by underlying operating system. When we call malloc()/free() and there's no operating systems running(for example a bare metal embedded system), how is the memory allocation and tracking handled?
There should be an entity that tracks which addresses are free and which are not. That's OS memory management unit. malloc()/free() will then have to call OS system calls. So no OS means no malloc()/free(). Am I wrong in assuming this?
Update:
All answers pointed out that malloc/free can use either static pool allocation(when no OS is available) or use sbrk/brk which are kernel system calls. Question is how does the malloc/free knows if there's a kernel beneath or not?
Answer(see comment by "Kuba Ober" under his answer below):
malloc doesn't need to know anything, because the C library that you link your project with is specific to the target: if you develop for Linux, you use a C library for Linux, different than when you develop for OS X, or Windows, or bare bones ARM Cortex M0. Or, heck, barebones x86. It's people who write the C library that know how to implement it so that it works on the desired target. For example, a barebones C library for x86 would use EFI and ACPI to query the list of available blocks of RAM unused by hardware nor BIOS, and then use those in fulfilling allocation requests.
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