父子进程通过mmap进行通信

本来打算使用pipe进行父子进程之间的数据交互（应用场景是父进程向多个子进程分发数据，子进程进行处理）；但是担心pipe的性能，转而使用mmap实现。

废话少叙，上代码。

#include <stdio.h>
#include <sys/types.h>
#include <signal.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <fcntl.h>

#define SHMFILE "shm.data"

typedef struct cell_t {
    int state;
    #define CELL_READY  1
    #define CELL_CLEAR  0

    int size;
    char    data[64];
} cell_t;

int main(void) {

    signal( SIGCHLD, SIG_IGN );
    
    int i, ret = 0;
    int fd = open(SHMFILE, O_CREAT | O_RDWR | O_TRUNC, 0600);
    if ( fd < 0 ) {
        fprintf(stderr, "fail to open %s -- %s\n", SHMFILE, strerror(errno));
        return  2;
    }
    int n_cells = 8;
    int shm_size = sizeof(cell_t) * n_cells;
    off_t off = lseek(fd, shm_size, SEEK_END);
    if ( off != (off_t)shm_size ) {
        fprintf(stderr, "fail to seek %s (got:%ld want:%d) -- %s\n", SHMFILE, off, shm_size, strerror(errno));
        return  3;
    }
    ret = write(fd, "", 1);
    if ( ret < 0 ) {
        fprintf(stderr, "fail to write %s -- %s\n", SHMFILE, strerror(errno));
        return  5;
    }


    cell_t* cell = (cell_t *) mmap(NULL, shm_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if ( cell == NULL ) {
        fprintf(stderr, "fail to mmap %s -- %s\n", SHMFILE, strerror(errno));
        return  7;
    }
    for (i=0; i<n_cells; i++) {
        memset( cell + i, 0, sizeof(cell_t) );
    }

    int id = 3;
    int pid = fork();
    if (pid < 0) {
        fprintf(stderr, "fail to fork child -- %s\n", strerror(errno));
        return  1;
    }
    else if (pid == 0) {
        // child process
        pid = getpid();
        fprintf(stdout, "\tC#%d I am here ...\n", pid);
        
        cell_t* c = &cell[id];
        
        for ( i=0; i<5; i++ ) {
            if ( c->state == CELL_READY ) {
                fprintf(stdout, "\tC#%d size=%d data='%s'\n", pid, c->size, c->data);
                c->state = CELL_CLEAR;
                break;
            }
            sleep(1);
        }

        fprintf(stdout, "\tC#%d byebye\n", pid);

        munmap(cell, shm_size);
        close(fd);
        exit(0);
    }

    pid = getpid();
    fprintf(stdout, "P#%d I am here ...\n", pid);

    sleep(1);
    {
        cell_t* c = &cell[id];

        char*   data = "Hello,World.";
        c->size = strlen(data);
        memmove( c->data, data, c->size );
        c->state = CELL_READY;
        fprintf(stdout, "P#%d cell->state=%d\n", pid, c->state);

        sleep(5);
        fprintf(stdout, "P#%d cell->state=%d\n", pid, c->state);
    }

    fprintf(stdout, "P#%d byebye\n", pid);

    munmap(cell, shm_size);
    close(fd);
    return  0;
}