Inter-Process-Communication-IPC-/Shared Memory/shm_demo.c

#include <stdio.h>
#include <memory.h>
#include <sys/shm.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <errno.h>

int
create_and_write_shared_memory(char* mmap_key, char* value, unsigned int size) {

	int shm_fd;

	/*
		Create a shared memory object in kernel space. If shared memory
		already exists it will truncate it to zero bytes again.
	*/

	shm_fd = shm_open(mmap_key, O_CREAT | O_RDWR | O_TRUNC, 0660);

	if (shm_fd < 0) {
		printf("Failure on shm_open on shm_fd, errcode = %d\n", errno);
		return -1;
	}
	
	if (ftruncate(shm_fd, size) == -1) {
		printf("Error on ftruncate to allocate size of shared memory region\n");
		return -1;
	}

	/*Now map the shared memory in kernel space into process's Virtual address space.*/
	void* shm_reg = mmap(NULL, // let the kernel chose to return the base address of shm memory
		size,				   // size of the shared memory to map to virtual address of 
		PROT_READ | PROT_WRITE,// shared memory is Read and Writable
		MAP_SHARED,			   // shared memory is accessible by different processes
		shm_fd,				   // file descriptor of the shared memory
		0);					   // offset from the base address of the physical/shared memory to be mapped


	/*
		shm_reg is the address in process's Virtual address space, just like any other address.
		The Linux paging mechanism maps this address to starting address of the shared memory region
		in kernel space. Any operation performed by process on shm_reg address is actually the operation
		performed in shared memory which resides in kernel.
	*/

	memset(shm_reg, 0, size);
	memcpy(shm_reg, value, size);
	munmap(shm_reg, size);

	/*
		Reader Process will not able to read shm if writer unlink
		the name below.
	*/

	//shm_unlink(mmap_key);
	close(shm_fd);
	return size;

}

int
read_from_shared_memory(char* mmap_key,
	char* buffer,
	unsigned int buff_size,
	unsigned int bytes_to_read) {

	int shm_fd = 0, rc = 0;

	shm_fd = shm_open(mmap_key, O_CREAT | O_RDONLY, 0660);

	if (shm_fd < 0) {

		printf("Failure on shm_open on shm_fd, error code = %d\n", errno);
		return -1;

	}

	void* shm_reg = mmap(NULL, bytes_to_read, PROT_READ, MAP_SHARED, shm_fd, 0);

	if (shm_reg == MAP_FAILED) {
		printf("Error on mapping\n");
		return -1;
	}

	memcpy(buffer, shm_reg, bytes_to_read);
	rc = munmap(shm_reg, bytes_to_read);

	if (rc < 0) {
		printf("munmap failed\n");
		return -1;
	}

	close(shm_fd);
	return bytes_to_read; /* Return the number of bytes read */
}