Linux下的进程间通信：共享存储

在向文件写入内容后，生产者改变锁结构中的 l_type 域为 unlock 值：

lock.l_type = F_UNLCK;

并调用 fcntl 来执行解锁操作。最后程序关闭了文件并退出。

示例 2. 消费者程序

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
 
#define FileName "data.dat"
 
void report_and_exit(const char* msg) {
  [perror][4](msg);
  [exit][5](-1); /* EXIT_FAILURE */
}
 
int main() {
  struct flock lock;
  lock.l_type = F_WRLCK;    /* read/write (exclusive) lock */
  lock.l_whence = SEEK_SET; /* base for seek offsets */
  lock.l_start = 0;         /* 1st byte in file */
  lock.l_len = 0;           /* 0 here means 'until EOF' */
  lock.l_pid = getpid();    /* process id */
 
  int fd; /* file descriptor to identify a file within a process */
  if ((fd = open(FileName, O_RDONLY)) < 0)  /* -1 signals an error */
    report_and_exit("open to read failed...");
 
  /* If the file is write-locked, we can't continue. */
  fcntl(fd, F_GETLK, &lock); /* sets lock.l_type to F_UNLCK if no write lock */
  if (lock.l_type != F_UNLCK)
    report_and_exit("file is still write locked...");
 
  lock.l_type = F_RDLCK; /* prevents any writing during the reading */
  if (fcntl(fd, F_SETLK, &lock) < 0)
    report_and_exit("can't get a read-only lock...");
 
  /* Read the bytes (they happen to be ASCII codes) one at a time. */
  int c; /* buffer for read bytes */
  while (read(fd, &c, 1) > 0)    /* 0 signals EOF */
    write(STDOUT_FILENO, &c, 1); /* write one byte to the standard output */
 
  /* Release the lock explicitly. */
  lock.l_type = F_UNLCK;
  if (fcntl(fd, F_SETLK, &lock) < 0)
    report_and_exit("explicit unlocking failed...");
 
  close(fd);
  return 0;
}

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