Shared Memory for IPC

Shared Memory/shm_demo.c

@@ -0,0 +1,97 @@
+#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 */
+}