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 __cplusplusextern "C" {#endif#if 0pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;#endifvoid *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