请用C语言编写 malloc realloc free 这三个函数？题目如下所示

Question

malloc（）realloc（）free（）
最好不要复制 C库里的
只是C语言。

Answer 1

//IBM的实现，仅供参考/* Include the sbrk function */#include <unistd.h>int has_initialized = 0;void *managed_memory_start;void *last_valid_address;void malloc_init(){/* grab the last valid address from the OS */last_valid_address = sbrk(0);/* we don't have any memory to manage yet, so*just set the beginning to be last_valid_address*/managed_memory_start = last_valid_address;/* Okay, we're initialized and ready to go */has_initialized = 1;}struct mem_control_block {int is_available;int size;};void free(void *firstbyte) {struct mem_control_block *mcb;/* Backup from the given pointer to find the* mem_control_block*/mcb = firstbyte - sizeof(struct mem_control_block);/* Mark the block as being available */mcb->is_available = 1;/* That's It! We're done. */return;}void *malloc(long numbytes) {/* Holds where we are looking in memory */void *current_location;/* This is the same as current_location, but cast to a* memory_control_block*/struct mem_control_block *current_location_mcb;/* This is the memory location we will return. It will* be set to 0 until we find something suitable*/void *memory_location;/* Initialize if we haven't already done so */if(! has_initialized) {malloc_init();}/* The memory we search for has to include the memory* control block, but the user of malloc doesn't need* to know this, so we'll just add it in for them.*/numbytes = numbytes + sizeof(struct mem_control_block);/* Set memory_location to 0 until we find a suitable* location*/memory_location = 0;/* Begin searching at the start of managed memory */current_location = managed_memory_start;/* Keep going until we have searched all allocated space */while(current_location != last_valid_address){/* current_location and current_location_mcb point* to the same address. However, current_location_mcb* is of the correct type so we can use it as a struct.* current_location is a void pointer so we can use it* to calculate addresses.*/current_location_mcb =(struct mem_control_block *)current_location;if(current_location_mcb->is_available){if(current_location_mcb->size >= numbytes){/* Woohoo! We've found an open,* appropriately-size location.*//* It is no longer available */current_location_mcb->is_available = 0;/* We own it */memory_location = current_location;/* Leave the loop */break;}}/* If we made it here, it's because the Current memory* block not suitable, move to the next one*/current_location = current_location +current_location_mcb->size;}/* If we still don't have a valid location, we'll* have to ask the operating system for more memory*/if(! memory_location){/* Move the program break numbytes further */sbrk(numbytes);/* The new memory will be where the last valid* address left off*/memory_location = last_valid_address;/* We'll move the last valid address forward* numbytes*/last_valid_address = last_valid_address + numbytes;/* We need to initialize the mem_control_block */current_location_mcb = memory_location;current_location_mcb->is_available = 0;current_location_mcb->size = numbytes;}/* Now, no matter what (well, except for error conditions),* memory_location has the address of the memory, including* the mem_control_block*//* Move the pointer past the mem_control_block */memory_location = memory_location + sizeof(struct mem_control_block);/* Return the pointer */return memory_location;}