Flock 函數 互斥鎖 線程可用， 多進程可用，但fork進程不行
Fcntl 函數 多進程可用 fork進程可用 但線程不行
Fcntl函數的記錄鎖所用來設置對文件進行加密，所以無論是fork還是多個程序對同一個文件均可以起到加鎖的作用。但對線程則沒有作用。簡單使用可以，涉及到多進程特別是fork的話就需要注意了。因爲fork繼承此文件描述符，在fork中釋放鎖，那麼也就是主進程釋放鎖了。
Flock函數只能用來對整個文件加鎖，多個線程對同一個文件也可以，但是fork的進程不行。
所以總的來說對進程加鎖採用fcntl，需要線程安全的話就還是選擇線程鎖，比如互斥鎖，或者讀寫鎖(這個是共享互斥鎖)等。
下面是我的測試代碼。Wrapfcntl這兩個文件是對fcntl函數的包裝，採用<unix環境高級編程>數中的例子。這本書裏的代碼很經典的，不但瞭解函數的使用，還介紹了環境，使用注意等。比百度到的要了解的透徹多了，就像我這裏寫的，只是個筆記或者總結而已。
******************************** wrapfcntl.h *********************************
#ifndef IRIDIUMFCNTL_H
#define IRIDIUMFCNTL_H
#include <stdio.h>
#include <string.h>
#include <sys/types.h> /* about files */
#include <sys/stat.h>
#include <sys/file.h>
#include <fcntl.h>
#include <unistd.h>
#include <pthread.h>
/* fcntl函數的包裝，帶w表示阻塞調用*/
#define read_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len))
#define readw_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len))
#define write_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len))
#define writew_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len))
#define un_lock(fd, offset, whence, len) \
lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len))
int lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len);
#endif
******************************** wrapfcntl.c *********************************
#include "wrapfcntl.h"
int lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len)
{
struct flock lock;
lock.l_type = type; /* F_RDLCK, F_WRLCK, F_UNLCK */
lock.l_start = offset; /* byte offset, relative to l_whence */
lock.l_whence = whence; /* SEEK_SET, SEEK_CUR, SEEK_END */
lock.l_len = len; /* #bytes (0 means to EOF) */
return (fcntl(fd, cmd, &lock));
}
******************************** main.c *********************************
/*gcc main.c wrapfcntl.c 測試多進程的fcntl鎖 */
/*gcc main.c wrapfcntl.c -DTEST_FLOCK 測試多進程的flock鎖 */
/*gcc main.c wrapfcntl.c -DTEST_PTHREAD -lpthread 測試多線程的fcntl鎖 */
/*gcc main.c wrapfcntl.c -DTEST_FLOCK -lpthread 測試多線程的flock鎖 */
#include "wrapfcntl.h"
#define THREAD_NUM 1
void *thread_function(void *arg);
const char main_process[] = "++++++++++ this is the main process \n";
const char child_process[] = "---------- this is the child process \n";
const char pthread[] = "======== this is the child thread \n";
//#define TEST_FLOCK
int fd;
const sleep_s = 10;
int main()
{
int res;
pthread_t thread_group[THREAD_NUM];
void *(*thread_func_group[THREAD_NUM])(void *);
int i;
char info[THREAD_NUM][20];
char temp[20];
for(i = 0; i< THREAD_NUM; i++){
sprintf(temp, "%d%d%d%d%d", i, i, i, i, i);
strcpy(info[i], temp);
}
for( i = 0; i < THREAD_NUM; i++)
thread_func_group[i] = thread_function;
void *thread_result;
int n = 0;
fd = open("/var/log/iridium/test.log", O_CREAT|O_APPEND|O_RDWR, 0744);
if(fd < 0){
printf("open error fd = %d\n", fd);
return -1;
}
#ifdef TEST_PTHREAD
printf("Testing pthread =========================\n");
#elif defined TEST_FLOCK
printf("Testing flock ==========================\n");
#else
printf("Testing multi process ==================\n");
#endif
#ifdef TEST_PTHREAD
for(i = 0 ; i < THREAD_NUM; i++){
res = pthread_create(&(thread_group[i]), NULL, thread_func_group[i], (void*)(info[i]));
if (res != 0)
{
perror("Thread creation failed!");
return -1;
}
}
#else
if(fork() == 0){
while(1){
printf("child process waiting lock\n");
#ifdef TEST_FLOCK
flock(fd, LOCK_EX);
#else
writew_lock(fd, 0, SEEK_SET, 0);
#endif
n = write(fd, child_process, strlen(child_process));
if(n < 0)
printf("-----------child write error\n");
fprintf(stderr, "%s ********** after %d s unlock\n", child_process, sleep_s);
sleep(sleep_s);
#ifdef TEST_FLOCK
flock(fd, LOCK_UN);
#else
un_lock(fd, 0 ,SEEK_SET, 0);
#endif
usleep(1);
}
}
#endif
while(1){
printf("main\n");
#ifdef TEST_FLOCK
flock(fd, LOCK_EX);
#else
writew_lock(fd, 0, SEEK_SET, 0);
#endif
n = write(fd, main_process, strlen(main_process));
if(n < 0)
printf("++++++++++++++main write error\n");
fprintf(stderr, "%s ********** after %d s unlock\n", main_process, sleep_s);
sleep(sleep_s);
#ifdef TEST_FLOCK
flock(fd, LOCK_UN);
#else
un_lock(fd, 0, SEEK_SET, 0);
usleep(1);
#endif
}
/* 忽略多個線程同時終止的問題，代碼不會執行到此，線程隨進程終止 */
for(i = 0; i < THREAD_NUM; i++){
pthread_join(thread_group[i], NULL);
}
close(fd);
return 0;
}
void *thread_function(void *arg)
{
#if 1
int n = 0;
while(1){
#ifdef TEST_FLOCK
flock(fd, LOCK_EX);
#else
writew_lock(fd, 0, SEEK_SET, 0);
#endif
n = write(fd, pthread, strlen(pthread));
if(n < 0)
printf("=================child write error\n");
fprintf(stderr, "%s ********** after %d s unlock\n", pthread, sleep_s);
sleep(sleep_s);
#ifdef TEST_FLOCK
flock(fd, LOCK_UN);
#else
un_lock(fd, 0 ,SEEK_SET, 0);
#endif
}
close(fd);
#endif
return ;
}