读书笔记：第9章 记录上锁 (2)
《UNIX网络编程：卷2》P155-P156：图9-2
fcntl上锁的加1测试
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/*
* lockmain.c
* 图9-2 文件上锁例子的main函数
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)
#define MAXLINE 1024
#define SEQFILE "seqno"
void my_lock(int fd);
void my_unlock(int fd);
int main(int argc, char *argv[])
{
int fd;
long i, seqno;
pid_t pid;
ssize_t n;
char line[MAXLINE + 1];
pid = getpid(); // 获取进程ID
// 打开文件
if ((fd = open(SEQFILE, O_RDWR, FILE_MODE)) < 0) {
fprintf(stderr, "open %s error: %s\n", SEQFILE, strerror(errno));
exit(1);
}
for (i = 0; i < 20; i++) {
my_lock(fd); // 锁定文件
lseek(fd, 0L, SEEK_SET); //定位到文件开始位置
if ((n = read(fd, line, MAXLINE)) < 0) { // 读取一行
fprintf(stderr, "read error: %s\n", strerror(errno));
exit(1);
}
line[n] = '\0';
n = sscanf(line, "%ld\n", &seqno); // 将字符转换成数字
printf("%s: pid = %d, seq# = %ld\n", argv[0], pid, seqno);
seqno++; // 加1
snprintf(line, sizeof(line), "%ld\n", seqno); // 将数字写回缓冲区
lseek(fd, 0L, SEEK_SET); // 定位到文件开始处
if (write(fd, line, strlen(line)) < 0) { // 将缓冲区内容写回文件
fprintf(stderr, "write error: %s\n", strerror(errno));
exit(1);
}
my_unlock(fd); // 解锁文件
}
exit(0);
}
/*
* lockfcntl.c
* P161 图9-3 fcntl上锁
*/
void my_lock(int fd)
{
struct flock lock;
lock.l_type = F_WRLCK; // 写锁
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
// 加锁整个文件
if (fcntl(fd, F_SETLKW, &lock) < 0) {
fprintf(stderr, "fcntl error: %s\n", strerror(errno));
}
return; // 不做任何事情，直接返回
}
void my_unlock(int fd)
{
struct flock lock;
lock.l_type = F_UNLCK; // 解锁
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
// 解锁整个文件
if (fcntl(fd, F_SETLK, &lock) < 0) {
fprintf(stderr, "fcntl error: %s\n", strerror(errno));
}
return; // 不做任何事情，直接返回
}
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创建文件seqno，并写入1。运行程序：
$ ./lockmain & ./lockmain & [1] 2513 [2] 2514 $ ./lockmain: pid = 2513, seq# = 1 ./lockmain: pid = 2513, seq# = 2 ./lockmain: pid = 2513, seq# = 3 ./lockmain: pid = 2513, seq# = 4 ./lockmain: pid = 2513, seq# = 5 ./lockmain: pid = 2513, seq# = 6 ./lockmain: pid = 2513, seq# = 7 ./lockmain: pid = 2513, seq# = 8 ./lockmain: pid = 2513, seq# = 9 ./lockmain: pid = 2513, seq# = 10 ./lockmain: pid = 2513, seq# = 11 ./lockmain: pid = 2513, seq# = 12 ./lockmain: pid = 2513, seq# = 13 ./lockmain: pid = 2513, seq# = 14 ./lockmain: pid = 2513, seq# = 15 ./lockmain: pid = 2513, seq# = 16 ./lockmain: pid = 2513, seq# = 17 ./lockmain: pid = 2513, seq# = 18 ./lockmain: pid = 2513, seq# = 19 ./lockmain: pid = 2513, seq# = 20 ./lockmain: pid = 2514, seq# = 21 ./lockmain: pid = 2514, seq# = 22 ./lockmain: pid = 2514, seq# = 23 ./lockmain: pid = 2514, seq# = 24 ./lockmain: pid = 2514, seq# = 25 ./lockmain: pid = 2514, seq# = 26 ./lockmain: pid = 2514, seq# = 27 ./lockmain: pid = 2514, seq# = 28 ./lockmain: pid = 2514, seq# = 29 ./lockmain: pid = 2514, seq# = 30 ./lockmain: pid = 2514, seq# = 31 ./lockmain: pid = 2514, seq# = 32 ./lockmain: pid = 2514, seq# = 33 ./lockmain: pid = 2514, seq# = 34 ./lockmain: pid = 2514, seq# = 35 ./lockmain: pid = 2514, seq# = 36 ./lockmain: pid = 2514, seq# = 37 ./lockmain: pid = 2514, seq# = 38 ./lockmain: pid = 2514, seq# = 39 ./lockmain: pid = 2514, seq# = 40 [1]- 完成 ./lockmain [2]+ 完成 ./lockmain
该运行结果看上去是正确的，但这不足以告诉我们程序是否正常工作。只循环20次，两个进程中途不进行切换，我们可能永远也发现不了错误。需要更多循环次数的测试。