Question

in unistd.h

using open() with the O_APPEND flag gives atomic writes always to the end of the file…

this is great and all, but what if i need to know the offset at which it atomically appended to the file…?

i realize O_APPEND is often used for log files, but I’d actually like to know at what offset in the file it atomically appended.

I don’t see any obvious way to do this..? Does anyone know?

Thanks

Answer 1

To get the current position in a file descriptor, use lseek() with offset 0 and whence SEEK_CUR.

int fd = open(...);
if (fd) {
    off_t positionWhereAppendingBegins = lseek(fd, 0, SEEK_CUR);
    write(...);
    close(fd);
}

Note that this will not give you reliable results if the descriptor was opened some other way, i.e. via socket().