Question

i want to watch a file fd is readable when not eof in linux c.
now i have to loop until !eof,i think it’s waste of cpu time.

static int rs_eof_read_binlog(rs_master_info_t *m, void *buf, 
    size_t size) {

FILE *fp;
size_t n;

fp = m->bfp;
for( ;; ) {
    n = fread(buf, size, 1, fp);
    if(n * size < size) {
        if(ferror(fp) != 0) {
            return RS_ERR;
        }

        if(feof(fp) != 0) {
            fseek(fp, -n * size, SEEK_CUR);
            sleep(1);
        }
    }
    else {
        break;
    }
}

clearerr(fp);
return RS_OK;

}

i checkd the select,when a file eof ,select returns.

Those listed in readfds will be watched to see if characters become avail‐
   able  for reading (more precisely, to see if a read will not block; in particular, a file descriptor is also ready on end-of-file)

fixed!!!!
sample code use inotify

int main(int argc, char *argv[])

{

char p, buf[1024];
int fd = open("/home/terry/1.txt", O_RDONLY, "00666");
struct inotify_event * event;
size_t len;

read(fd, &p, 1);

int ifd = inotify_init();
int wd = inotify_add_watch(ifd, "/home/terry/1.txt", IN_MODIFY);
while(len = read(ifd, buf, 1024)) {
    event = (struct inotify_event *) buf;
    if(event->mask & IN_MODIFY) {
        printf("changed\n");
        break;
    }
}



return 0;

}

Answer 1

You can try the inotify mechanism.
Use inotify_add_watch to listen to the IN_MODIFY event on the file you want to watch.