Linux IPC (Shared memory)

一個簡單的程式，利用Child算好fabonacci數列後，還回給parent print。

在這之間是利用shared memory來傳遞資料的！

#include                                             
#include                                            
#include                                           
#include                                          
#include "../lib/string.c"                                    
#define MAX_SEQUENCE 10                                       
#define DEBUG 1                                               

typedef struct {
        long seq[MAX_SEQUENCE];
        int seq_size;          
} shared_data;                 

int main(int argc, char* argv[]){
        int segment_id, pid;     
        char *shared_memory;     
        shared_data fib_seq;     
        const int size = 4096;   

        if(argc != 2){
                 fprintf(stderr, "Usage: %s seq_size\n", argv[0]);
                 return -1;                                       
        }                                                         
        fib_seq.seq_size =  atoi(argv[1]);                        
        if(fib_seq.seq_size > MAX_SEQUENCE){                      
                 fprintf(stderr, "seq_size overflow.\n");         
                 return -1;                                       
        }                                                         

        segment_id = shmget(IPC_PRIVATE, size, S_IRUSR | S_IWUSR);

        if(!(pid = fork())){
                shared_memory = (char *) shmat(segment_id, NULL, 0);

                fib_seq.seq[0] = 0;
                fib_seq.seq[1] = 1;

                int i;
                if(DEBUG) fprintf(stderr, "Counting Fabonacii...\n");
                for(i = 2; i < fib_seq.seq_size; i++)
                        fib_seq.seq[i] = fib_seq.seq[i-2] + fib_seq.seq[i-1];

                if(DEBUG) fprintf(stderr, "Converting each int to string...\n");
                char* fib_str[fib_seq.seq_size];
                for(i = 0; i < fib_seq.seq_size; i++)
                        fib_str[i] = itoa(fib_seq.seq[i]);

                if(DEBUG) fprintf(stderr, "Join the seq to string...\n");
                char* return_str = join_strings(fib_str, fib_seq.seq_size, ',');

                sprintf(shared_memory, "%s\n", return_str);
                shmdt(shared_memory);
                return 0;
        }else{
                wait(NULL);
                if(DEBUG) fprintf(stderr, "Back to parent...\n");
                shared_memory = (char *) shmat(segment_id, NULL, 0);
                printf("%s", shared_memory);
                shmdt(shared_memory);
                shmctl(segment_id, IPC_RMID, NULL);

                return 0;
        }
}

在DEBUG訊息中都有寫上註解了。
簡單說明一下sharedmem的一些system call
segment_id = sgmget(....) //分配shared memory的空間
shared_memory = (char *) shmat(....) //使程式取得該shared mem的pointer
shmdt(shared_memory) //中斷與shared mem的連結
shmctl(...) //清除shared mem region