add shared memory demo code

README.md

@@ -1,2 +1,5 @@
 # c-showcase
 c code showcase
+
+## shared memory
+shared memory write and read demo.

shared_memory/read.c

@@ -0,0 +1,88 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/ipc.h>
+#include <sys/shm.h>
+
+#define BUFSZ 512
+
+int main(int argc, char *argv[])
+{
+    int shmid;
+    int ret;
+    key_t key;
+    char *shmadd;
+
+    // create key value
+    key = ftok("../", 2015);
+    if (key == -1)
+    {
+        perror("ftok");
+    }
+
+    system("ipcs -m"); // check shared memory
+
+    /*
+        #include <sys/ipc.h>
+        #include <sys/shm.h>
+
+        int shmget(key_t key, size_t size, int shmflg); // create or open shared memory
+    */
+    // open shared memory
+    shmid = shmget(key, BUFSZ, IPC_CREAT|0666);
+    if (shmid < 0)
+    {
+        perror("shmget");
+        exit(-1);
+    }
+
+    /*
+        #include <sys/types.h>
+        #include <sys/shm.h>
+
+        void *shmat(int shmid, const void *shmaddr, int shmflg); // map shared memory with process
+    */
+    // mapping
+    shmadd = shmat(shmid, NULL, 0);
+    if (shmadd < 0)
+    {
+        perror("shmat");
+        exit(-1);
+    }
+
+    // read shard memory data
+    printf("data = [%s]\n", shmadd);
+
+    /*
+        #include <sys/types.h>
+        #include <sys/shm.h>
+
+        int shmdt(const void *shamaddr); detach shared memory with process
+    */
+    // detach shared memory and current process
+    ret = shmdt(shmadd);
+    if (ret < 0)
+    {
+        perror("shmdt");
+        exit(1);
+    }
+    else
+    {
+        printf("deleted shared-memory.\n");
+    }
+
+    /*
+        #include <sys/ipc.h>
+        #include <sys/shm.h>
+
+        int shmctl(int shmid, int cmd, struct shmid_ds *buf); // control shared meory
+    */
+    // delete shared memory
+    shmctl(shmid, IPC_RMID, NULL);
+
+    return 0;
+}
+
+/* compile command: gcc read.c -o read */

shared_memory/write.c

@@ -0,0 +1,61 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/ipc.h>
+#include <sys/shm.h>
+
+#define BUFSZ 512
+
+int main(int argc, char *argv[])
+{
+    int shmid;
+    int ret;
+    key_t key;
+    char *shmadd;
+
+    // create key value
+    key = ftok("../", 2015);
+    if (key == -1)
+    {
+        perror("ftok");
+    }
+
+    /* 
+        #include <sys/ipc.h>
+        #include <sys/shm.h>
+
+        int shmget(key_t key, size_t size, int shmflg); // create or open shared memory
+    */
+    // create shared memory
+    shmid = shmget(key, BUFSZ, IPC_CREAT|0666);
+    if (shmid < 0)
+    {
+        perror("shmget");
+        exit(-1);
+    }
+
+    /*
+        #include <sys/types.h>
+        #include <sys/shm.h>
+
+        void *shmat(int shmid, const void *shmaddr, int shmflg); // map shared memory with process
+    */
+    // mapping
+    shmadd = shmat(shmid, NULL, 0);
+    if (shmadd < 0)
+    {
+        perror("shmat");
+        _exit(-1);
+    }
+
+    // copy data to shared memory
+    printf("copy data to shared memory.\n");
+    bzero(shmadd, BUFSZ); // clear shared memory
+    strcpy(shmadd, "how are you?");
+
+    return 0;
+}
+
+/* compile command: gcc write.c -o write */