initial commit

Makefile

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+main: main.c
+	gcc -pedantic -Wall -o main main.c -lrt -pthread

README.md

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+# shared memory
+Simple example of inter process communication with shm_open().
+
+The main process forks itself and then waits for the child to finish. The child writes into a shared memory and exits. After that the main process prints what the child wrote into the shared memory.
+
+## Note
+The file noerror.c does the same as main.c, but no error handling.
+## compile
+```bash
+make
+```
+## run 
+```bash
+./main
+```

main.c

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+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/stat.h> 
+#include <fcntl.h>           
+#include <string.h>
+#include <semaphore.h>
+
+#define SHARED_MEM "/SHARED_MEM"
+#define NUM_CALLS 5
+
+struct shared_str {
+	sem_t sem;
+	size_t len;
+	char buf[256];
+};
+
+void errExit(char* msg){
+	perror(msg);
+	exit(EXIT_FAILURE);
+}
+
+int main(){
+	int fd = shm_open(SHARED_MEM, O_CREAT | O_RDWR, S_IRUSR);
+	if (fd < 0){
+		errExit("failed to open shared memory object");
+	}
+	if (ftruncate(fd, sizeof(struct shared_str))){
+		errExit("failed to truncate");
+	}
+	struct shared_str *sstr = mmap(NULL, sizeof(struct shared_str),
+			PROT_READ | PROT_WRITE,
+			MAP_SHARED, fd, 0);
+	if (sstr == MAP_FAILED)
+		errExit("mmap");
+	if (sem_init(&sstr->sem, 1, 0) == -1)
+		errExit("sem_init-sem1");
+	pid_t pid;
+	if ((pid = fork()) < 0) {
+		errExit("failed to fork");
+	}
+	if ( pid == 0 ){
+		printf("child is running...\n");
+		for (int i =0 ; i <NUM_CALLS ; i++) {
+			sleep(1);
+			sstr -> len = snprintf((char*)&sstr->buf,256,"hallo %d",i);
+			if (sem_post(&sstr->sem) == -1)
+				errExit("sem_post");
+		}
+		close(fd);
+		exit(EXIT_SUCCESS);
+	}
+	for (int i =0 ; i<NUM_CALLS ; i++) {
+		if (sem_wait(&sstr->sem) == -1)
+			errExit("sem_wait");
+		pread(fd,sstr, sizeof(struct shared_str),0);	
+		printf("child: %s\n",sstr -> buf);
+	}
+	int status;
+	pid = wait(&status);
+	printf("child process %d terminated with status %d\n", pid, WEXITSTATUS(status));
+	shm_unlink(SHARED_MEM);
+	exit(EXIT_SUCCESS);
+}

noerror.c

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+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/stat.h> 
+#include <fcntl.h>           
+#include <string.h>
+#include <semaphore.h>
+
+#define SHARED_MEM "/SHARED_MEM"
+#define NUM_CALLS 5
+
+struct shared_str {
+	sem_t sem;
+	size_t len;
+	char buf[256];
+};
+
+void errExit(char* msg){
+	perror(msg);
+	exit(EXIT_FAILURE);
+}
+
+int main(){
+	int fd = shm_open(SHARED_MEM, O_CREAT | O_RDWR, S_IRUSR);
+	ftruncate(fd, sizeof(struct shared_str));
+	struct shared_str *sstr = mmap(NULL, sizeof(struct shared_str),
+			PROT_READ | PROT_WRITE,
+			MAP_SHARED, fd, 0);
+	sem_init(&sstr->sem, 1, 0);
+	pid_t pid = fork();
+	if ( pid == 0 ){
+		printf("child is running...\n");
+		for (int i =0 ; i <NUM_CALLS ; i++) {
+			sleep(1);
+			sstr -> len = snprintf((char*)&sstr->buf,256,"hallo %d",i);
+			sem_post(&sstr->sem);
+		}
+		close(fd);
+		exit(EXIT_SUCCESS);
+	}
+	for (int i =0 ; i<NUM_CALLS ; i++) {
+		sem_wait(&sstr->sem);
+		pread(fd,sstr, sizeof(struct shared_str),0);	
+		printf("child: %s\n",sstr -> buf);
+	}
+	int status;
+	pid = wait(&status);
+	printf("child process %d terminated with status %d\n", pid, WEXITSTATUS(status));
+	shm_unlink(SHARED_MEM);
+	exit(EXIT_SUCCESS);
+}