file lock and thread sync operations (I)

第一点，多线程，每个线程都open flock write，不必explicitly sync threads（mutex、rdlock等），每个线程操作锁的顺序内部有序，并且，如果文件以append方式打开，文件写入不会混乱，如果以trunc方式打开，则文件内容可能混乱（write offset的原因，引起覆盖）。

第二点，flock，是进程层面的锁（第一点可印证），同时，这个锁与kernel维护的open file table关联。可联想下process table -- open file table -- vnode/inode table。

第三点，只主线程中open，然后把fd传给各子线程，各线程通过explicit thread sync来操作共享的fd。这个方法也可行。与flock相比，哪个更好，待确认。
第四点，flock vs. lockf vs. flockfile。

#include <pthread.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/file.h>

#ifndef MYNB
# ifdef LOCK_NB
# undef LOCK_NB
# endif
#define LOCK_NB 0
#endif

#ifdef __cplusplus
extern "C" {
#endif

#if 0
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
#endif

void *thr_fn(void *data)
{
pthread_t tid = pthread_self();
const char *fn = (const char *)data;
int fd, len;
char buf[32];

fprintf(stderr, "in thr... %lu\n", tid);

// fd = open(fn, O_RDWR | O_CREAT | O_APPEND, 0666);
fd = open(fn, O_RDWR | O_CREAT, 0666);

if (fd == -1) {
char errbuf[64];
strerror_r(errno, errbuf, sizeof(errbuf));
fprintf(stderr, "in thr %lu: %s\n", tid, errbuf);
return (void *)-1;
}

fprintf(stderr, "in thr %lu fd: %d\n", tid, fd);

if (flock(fd, LOCK_EX | LOCK_NB) == -1) {
char errbuf[64];
close(fd);
strerror_r(errno, errbuf, sizeof(errbuf));
fprintf(stderr, "in thr %lu: %s\n", tid, errbuf);
return (void *)-1;
} else {
fprintf(stderr, "in thr %lu: flock OK\n", tid);
}

len = snprintf(buf, sizeof(buf), "%lu\n", tid);
fprintf(stderr, "in thr %lu, to be written: %d\n", tid, len);

#if 0
pthread_mutex_lock(&mutex);
#endif

len = write(fd, buf, len);
#if 0
fsync(fd);
#endif

#if 0
pthread_mutex_unlock(&mutex);
#endif

fprintf(stderr, "in thr %lu, written: %d\n", tid, len);

if (flock(fd, LOCK_UN | LOCK_NB) == -1) {
char errbuf[64];
close(fd);
strerror_r(errno, errbuf, sizeof(errbuf));
fprintf(stderr, "in thr %lu: %s\n", tid, errbuf);
return (void *)-1;
} else {
fprintf(stderr, "in thr %lu: flock un\n", tid);
}

close(fd);

fprintf(stderr, "leaving thr... %lu\n", tid);

return (void *)0;
}

int main(void)
{
const char *fn = "./foo.txt";
pthread_t tid1, tid2, tid = pthread_self();
int fd, len;
char buf[32];

pthread_create(&tid1, NULL, thr_fn, (void *)fn);

fprintf(stderr, "main %lu\n", tid);

#if 0
sleep(2);
#endif

// fd = open(fn, O_RDWR | O_CREAT | O_APPEND, 0666);
fd = open(fn, O_RDWR | O_CREAT, 0666);

if (fd == -1) {
char errbuf[64];
strerror_r(errno, errbuf, sizeof(errbuf));
fprintf(stderr, "main: %s\n", errbuf);
}

fprintf(stderr, "main fd: %d\n", fd);

if (flock(fd, LOCK_EX | LOCK_NB) == -1) {
char errbuf[64];
close(fd);
strerror_r(errno, errbuf, sizeof(errbuf));
fprintf(stderr, "main: %s\n", errbuf);
} else {
fprintf(stderr, "main: flock OK\n");
}

#if 0
sleep(2);
#endif

pthread_create(&tid2, NULL, thr_fn, (void *)fn);

len = snprintf(buf, sizeof(buf), "%lu\n", tid);
fprintf(stderr, "main, to be written: %d\n", len);

#if 0
pthread_mutex_lock(&mutex);
#endif

len = write(fd, buf, len);
#if 0
fsync(fd);
#endif

#if 0
pthread_mutex_unlock(&mutex);
#endif

fprintf(stderr, "main, written: %d\n", len);

if (flock(fd, LOCK_UN | LOCK_NB) == -1) {
char errbuf[64];
close(fd);
strerror_r(errno, errbuf, sizeof(errbuf));
fprintf(stderr, "main: %s\n", errbuf);
} else {
fprintf(stderr, "main: flock un\n");
}

close(fd);

pthread_join(tid1, NULL);
pthread_join(tid2, NULL);

fprintf(stderr, "leaving main...\n");
return 0;
}

#ifdef __cplusplus
}
#endif