Shared memory demo

README.md

Based on MAN PAGE for shm_open: https://man7.org/linux/man-pages/man3/shm_open.3.html

Compile

clang sender.c -o sender
clang receiver.c -o receiver

Run

./sender /shmem1
./receiver /shmem1

Makefile

CC = clang
CFLAGS = -pedantic -pedantic-errors -Wall -Werror -Wextra
LDFLAGS =

.PHONY: all
all: build_sender build_receiver
	./sender /shmem1
	./receiver /shmem1

.PHONY: build_sender
build_sender:
	$(CC) $(CFLAGS) $(LDFLAGS) sender.c -o sender

.PHONY: build_receiver
build_receiver:
	$(CC) $(CFLAGS) $(LDFLAGS) receiver.c -o receiver

.PHONY: reformat
reformat:
	clang-format -i --style=Google receiver.c sender.c

receiver.c

#include <fcntl.h> /* for O_* consts */
#include <stdio.h> /* print */
#include <stdlib.h>
#include <string.h>   /* memcpy */
#include <sys/mman.h> /* shm_open ... */
#include <sys/stat.h> /* For mode consts */
#include <unistd.h>   /* ftruncate */

#define BUF_SIZE 1024

typedef struct Payload_ {
  int a;
  int b;
  char buf[BUF_SIZE];
} Payload;

int main(int argc, char* argv[]) {
  if (argc != 2) {
    fprintf(stderr, "Usage: %s /shm-path\n", argv[0]);
    exit(EXIT_FAILURE);
  }

  const char* shm_path = argv[1];

  int fd = shm_open(shm_path, O_RDWR, 0);
  if (fd == -1) {
    perror("shm_open");
    exit(EXIT_FAILURE);
  }

  Payload* payload =
      mmap(NULL, sizeof(Payload), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

  if (payload == MAP_FAILED) {
    perror("mmap");
    exit(EXIT_FAILURE);
  }

  printf("SHMEM content: %s\n", payload->buf);

  shm_unlink(shm_path);
  return 0;
}

sender.c

#include <fcntl.h> /* for O_* consts */
#include <stdio.h> /* print */
#include <stdlib.h>
#include <string.h>   /* memcpy */
#include <sys/mman.h> /* shm_open ... */
#include <sys/stat.h> /* For mode consts */
#include <unistd.h>   /* ftruncate */

#define BUF_SIZE 1024

typedef struct Payload_ {
  int a;
  int b;
  char buf[BUF_SIZE];
} Payload;

int main(int argc, char* argv[]) {
  if (argc != 2) {
    fprintf(stderr, "Usage: %s /shm-path\n", argv[0]);
    exit(EXIT_FAILURE);
  }

  const char* shm_path = argv[1];
  int fd = shm_open(shm_path, O_CREAT | O_EXCL | O_RDWR, S_IRUSR | S_IWUSR);
  if (fd == -1) {
    perror("shm_open");
    exit(EXIT_FAILURE);
  }

  if (ftruncate(fd, sizeof(Payload)) == -1) {
    perror("truncate");
    exit(EXIT_FAILURE);
  }
  Payload* payload =
      mmap(NULL, sizeof(Payload), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

  if (payload == MAP_FAILED) {
    perror("mmap");
    exit(EXIT_FAILURE);
  }

  memcpy(&payload->buf, "HELLO\0", 6);

  /**
   * Unlink the shared memory object. Even if the peer process
   *  is still using the object, this is okay. The object will
   *  be removed only after all open references are closed.
   */

  // shm_unlink(shm_path);

  return 0;
}