Elsewhere there are things that we all miss, yet it takes just one to notice...

Static memory management

Static memory pool

alternative to malloc()

In a recent project working with the FFMpeg API, I needed a way of allocating memory blocks and freeing them from a static block of memory. The main requirement was that when I closed the stream, I could clean up everything much easier.

When working with FFMpeg you really need to do a lot of memory management manually for video buffers, audio buffers and subtitle buffers. By using a static memory pool for each section, you reduce the chance of memory fragmentation.

After finishing this, I am now using this in other areas too. I will get round to optimising the code so that one version will not rely on the c++ vector class and will just use standard ansi c.

This class can be used as an alternative to malloc() and free(). And there’s no need to worry about freeing up the blocks when it comes to finishing up using the memory.

The simple test to show how it is used:

#include "memorymanager.h"

// written off the cuff (hopefully it helps)

void test_mm() {
    char my_buffer[2 * MBYTE];
    MemoryManager mem;
    mem.init(my_buffer, sizeof(my_buffer));
    char *block1 = (char*)mem.alloc(60000);
    char *block2 = (char*)mem.alloc(12000);
    mem.free(block1);
    mem.clean();
}

The header file:

//
// Created by wlgfx on 5/18/16.
//

#ifndef FFPLAYER2_MEMORYMANAGER_H
#define FFPLAYER2_MEMORYMANAGER_H

#define MBYTE 1048576

#include <sys/types.h>
#include 
#include 

// use a mutex lock is using multi threaded
#define MM_USE_MUTEX

struct MBlock {
    uint ptr;
	uint size;
	MBlock(uint p, uint s) {ptr = p; size = s;}
};

class MemoryManager {
public:
	void    init(void* ptr, uint size);	// Initialise the static block of memory
	void*   alloc(uint size);			// return address or NULL if no memory available
	void    free(void* ptr);			// return the block back to the pool
	void    clean();					// resets the memory pool

	uint    total_free();
	uint    total_used();

	void    debug();

private:
	std::vector  free_blocks;
	std::vector  used_blocks;

	uint    data;   // pointer to static block
	uint    size;   // size of static block
	int     last;   // last pos allocated from

#ifdef MM_USE_MUTEX
	pthread_mutex_t mem_mutex = PTHREAD_MUTEX_INITIALIZER;  // TODO: swap for std::mutex
#endif
};

#endif //FFPLAYER2_MEMORYMANAGER_H

The cpp file:

//
// Created by wlgfx on 5/18/16.
//

#include "MemoryManager.h"

#include 

// Remove the Android stuff if not using the platform
#include <android/log.h>

#define  LOG_TAG    "WLGFX"
#define  LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)

void MemoryManager::init(void* in, uint size) {
    pthread_mutex_init(&mem_mutex, NULL);

	uint ptr = (uint)in;
	data = ptr;
	this->size = size;
	clean();
}

void *MemoryManager::alloc(uint size) {
#ifdef MM_USE_MUTEX
	while (!pthread_mutex_trylock(&mem_mutex)) sched_yield();
	//pthread_mutex_lock(&mem_mutex);
#endif

	bool found = false;
	int l = 0;

	// first try last position to allocate from

	if (free_blocks[last].ptr + size <=
			free_blocks[last].ptr + free_blocks[last].size) {
		l = last;
		found = true;
	} else {

		// scan through free blocks

		do {
			if (free_blocks[l].ptr + size <=
					free_blocks[l].ptr + free_blocks[l].size) {
				found = true;
			} else {
				l++;
			}
		} while (!found && l < free_blocks.size());
	}

	if (found) {
		used_blocks.push_back(MBlock(free_blocks[l].ptr, size));

		free_blocks[l].size -= size;
		free_blocks[l].ptr += size;

		if (free_blocks[l].size <= 0) { // need to remove block?
			free_blocks.erase(free_blocks.begin() + l);
			if (free_blocks.size() == l) l = 0;
		}

		last = l;   // set last position as next marker

		void *result = (void*)used_blocks[used_blocks.size() - 1].ptr;
#ifdef MM_USE_MUTEX
		pthread_mutex_unlock(&mem_mutex);
#endif
		return result;
	}

#ifdef MM_USE_MUTEX
	pthread_mutex_unlock(&mem_mutex);
#endif

	return nullptr; // to be checked by the user
}

void MemoryManager::free(void *in) {
	uint ptr = (uint)in;

	int     used = 0;
	bool    got = false;

#ifdef MM_USE_MUTEX
	while (!pthread_mutex_trylock(&mem_mutex)) sched_yield();
	//pthread_mutex_lock(&mem_mutex);
#endif

	size_t  usize = used_blocks.size();

	// locate which used block to free

	while (used < usize) {
		if (used_blocks[used].ptr == ptr) {
			got = true;
			break;
		}

		used++;
	}

	if (!got) {
#ifdef MM_USE_MUTEX
		pthread_mutex_unlock(&mem_mutex);
#endif
		return;
	}       // block not found

	int     c = free_blocks.size(); // current free block to test against

	uint    ub_beg = used_blocks[used].ptr;             // used blocks beginning
	uint    ub_siz = used_blocks[used].size;            // used blocks size
	uint    ub_end = ub_beg + ub_siz;                   // used blocks end

	while (c) {
		c--;    // reverse through list

		bool    done = false;   // flagged if merged with free block

		uint fb_beg = free_blocks[c].ptr;               // free blocks beginning
		uint fb_siz = free_blocks[c].size;              // free blocks size
		uint fb_end = fb_beg + fb_siz;                  // free blocks end

		// used_end == free_begin? merge with free blocks beginning

		if (ub_end == fb_beg) {
			fb_beg -= ub_siz;   // move free blocks pointer backwards
			fb_siz += ub_siz;   // increase free blocks size (fb_end stays same)

			ub_end  = fb_end;
			ub_siz  = fb_siz;   // increase used blocks size

			done = true;
		}

		// used beginning == free end? merge at end of free block

		if (ub_beg == fb_end) {
			fb_end += ub_siz;   // move end pointer
			fb_siz += ub_siz;

			ub_beg  = fb_beg;
			ub_siz  = fb_siz;

			done = true;
		}

		// if a merge has occurred then clean up free blocks data

		if (done)
			free_blocks.erase(free_blocks.begin() + c);
	}

	// add the finished with block to free list

	free_blocks.push_back(MBlock(ub_beg, ub_siz));

	// remove from used blocks list

	used_blocks.erase(used_blocks.begin() + used);

#ifdef MM_USE_MUTEX
	pthread_mutex_unlock(&mem_mutex);
#endif
}

// puts the memory block back to beginning invalidating previous allocations

void MemoryManager::clean() {
	while (!pthread_mutex_trylock(&mem_mutex)) sched_yield();
	//pthread_mutex_lock(&mem_mutex);
	free_blocks.clear();
	used_blocks.clear();
	free_blocks.push_back(MBlock(data, size));
	last = 0;
	pthread_mutex_unlock(&mem_mutex);
}

uint MemoryManager::total_free() {
	while (!pthread_mutex_trylock(&mem_mutex)) sched_yield();
	//pthread_mutex_lock(&mem_mutex);

	uint result = 0;
	size_t pos = 0;
	size_t size = free_blocks.size();

	while (pos < size)
		result += free_blocks[pos++].size;

	pthread_mutex_unlock(&mem_mutex);

	return result;
}

uint MemoryManager::total_used() {
	while (!pthread_mutex_trylock(&mem_mutex)) sched_yield();
	//pthread_mutex_lock(&mem_mutex);

	uint result = 0;
	size_t pos = 0;
	size_t size = used_blocks.size();

	while (pos < size)
		result += used_blocks[pos++].size;

	pthread_mutex_unlock(&mem_mutex);

	return result;
}

void MemoryManager::debug() {
	LOGD("MEMORY MANAGER");

	while (!pthread_mutex_trylock(&mem_mutex)) sched_yield();
	//pthread_mutex_lock(&mem_mutex);

	std::stringstream ss;

	ss << "Free Blocks: " << total_free() << " - ";

	for (size_t c = 0; c < free_blocks.size(); c++) {
		ss << "(" << free_blocks[c].ptr - data << ", " << free_blocks[c].size << ") ";
	}

	LOGD(ss.str().c_str());

	std::stringstream ts;

	ts.clear();

	ts << "Used Blocks: " << total_used() << " - ";

	for (size_t c = 0; c < used_blocks.size(); c++) {
		ts << "(" << used_blocks[c].ptr - data << ", " << used_blocks[c].size << ") ";
	}

	pthread_mutex_unlock(&mem_mutex);

	LOGD(ts.str().c_str());
}
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