锁设置子函数：
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/*
* 设置记录锁子函数lock_set.c
*
* 记录锁分为读取锁和写入锁，其中读取锁又称为共享锁，可以使多个
* 进程都能够在文件的同一部分建立读取锁。而写入锁又称为互斥锁，
* 在任何时刻只能有一个进程在文件的某个部分建立写入锁。当然，在
* 文件的同一部分不能同时建立读取锁和写入锁。
*
* fcntl的lock结构如下所示：
* struct flock {
* short l_type;
* off_t l_start;
* short l_whence;
* off_t l_len;
* pid_t l_pid;
* }
*
* 技巧：为加锁整个文件，通常的方法是将l_start说明为0，l_whence
* 说明为SEEK_SET，l_len说明为0。
*/
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
void print_lock(struct flock lock)
{
printf(" -----------------------------\n");
if (lock.l_type == F_RDLCK) {
printf("\tl_type: F_RDLCK\n");
}
else if (lock.l_type == F_WRLCK) {
printf("\tl_type: F_WRLCK\n");
}
else if (lock.l_type == F_UNLCK) {
printf("\tl_type: F_UNLCK\n");
}
printf("\tl_start: %d\n", (int)lock.l_start);
if (lock.l_whence == SEEK_SET) {
printf("\tl_whence: SEEK_SET\n");
}
else if (lock.l_whence == SEEK_CUR) {
printf("\tl_whence: SEEK_CUR\n");
}
else if (lock.l_whence == SEEK_END) {
printf("\tl_whence: SEEK_END\n");
}
printf("\tl_len: %d\n", (int)lock.l_len);
printf(" -----------------------------\n");
}
void lock_set(int fd, int type)
{
struct flock lock;
/*赋值lock结构体，加锁整个文件*/
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
while (1) {
lock.l_type = type;
/*
* 根据不同的type来给文件加锁或解锁，
* 如果成功，则返回0，失败则返回1。
* 举例：如果一个文件原来已经建立了互斥锁，那么再调用fcntl
* 建立锁就会失败，返回-1。
*/
if ((fcntl(fd, F_SETLK, &lock)) == 0) {
/*如果是共享锁*/
if (lock.l_type == F_RDLCK) {
printf("read only set by %d\n", getpid());
}
/*如果是互斥锁*/
else if (lock.l_type == F_WRLCK) {
printf("write lock set by %d\n", getpid());
}
else if (lock.l_type == F_UNLCK) {
printf("release lock by %d\n", getpid());
}
print_lock(lock);
return;
}
else {
/*
* 获得lock的描述，也就是将文件fd的加锁信息存入到lock结构中
* 如果成功则返回0
* 如果失败则返回-1
*/
if ((fcntl(fd, F_GETLK, &lock)) == 0) {
if (lock.l_type != F_UNLCK) {
if (lock.l_type == F_RDLCK) {
printf("read lock already set by %d\n", lock.l_pid);
}
else if (lock.l_type == F_WRLCK) {
printf("write lock already set by %d\n", lock.l_pid);
}
getchar();
}
}
else {
printf("cannot get the description of struck flock.\n");
}
}
}
}
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读取锁：
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/*
* 测试文件读取锁
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
void lock_set(int, int);
void die(char *msg)
{
perror(msg);
exit(1);
}
int open_file(void)
{
int fd;
if ((fd = open("/tmp/hello.c", O_CREAT | O_TRUNC | O_RDWR, 0666)) < 0) {
die("open error");
}
else {
printf("Open file: hello.c %d\n", fd);
}
return fd;
}
void close_file(int fd)
{
if (close(fd) < 0) {
die("close error");
}
else {
printf("Close file: hello.c\n");
}
}
int main(void)
{
int fd;
fd = open_file();
/*给文件加写入锁*/
lock_set(fd, F_RDLCK);
/*等待键盘任意键触发*/
getchar();
/*给文件解锁*/
lock_set(fd, F_UNLCK);
getchar();
close_file(fd);
return 0;
}
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写入锁：
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/*
* 测试文件写入锁
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
void lock_set(int, int);
void die(char *msg)
{
perror(msg);
exit(1);
}
int open_file(void)
{
int fd;
if ((fd = open("/tmp/hello.c", O_CREAT | O_TRUNC | O_RDWR, 0666)) < 0) {
die("open error");
}
else {
printf("Open file: hello.c %d\n", fd);
}
return fd;
}
void close_file(int fd)
{
if (close(fd) < 0) {
die("close error");
}
else {
printf("Close file: hello.c\n");
}
}
int main(void)
{
int fd;
fd = open_file();
/*给文件加写入锁*/
lock_set(fd, F_WRLCK);
/*等待键盘任意键触发*/
getchar();
/*给文件解锁*/
lock_set(fd, F_UNLCK);
getchar();
close_file(fd);
return 0;
}
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Makefile源文件：
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CC = gcc
OBJS = fcntl_write fcntl_read
CFLAGS = -Wall -g
all: $(OBJS)
fcntl_write: fcntl_write.c lock_set.c
$(CC) $(CFLAGS) $^ -o $@
fcntl_read: fcntl_read.c lock_set.c
$(CC) $(CFLAGS) $^ -o $@
.PHONY: clean
clean:
rm -rf $(OBJS)
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测试结果：
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[armlinux@lqm fcntl]$ ./fcntl_read
Open file: hello.c 3
read only set by 12398
-----------------------------
l_type: F_RDLCK
l_start: 0
l_whence: SEEK_SET
l_len: 0
-----------------------------
release lock by 12398
-----------------------------
l_type: F_UNLCK
l_start: 0
l_whence: SEEK_SET
l_len: 0
-----------------------------
Close file: hello.c
[armlinux@lqm fcntl]$ ./fcntl_write
Open file: hello.c 3
write lock set by 12399
-----------------------------
l_type: F_WRLCK
l_start: 0
l_whence: SEEK_SET
l_len: 0
-----------------------------
release lock by 12399
-----------------------------
l_type: F_UNLCK
l_start: 0
l_whence: SEEK_SET
l_len: 0
-----------------------------
Close file: hello.c
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