Two Writers One Offset

Two independent processes append audit lines to the same file. Each runs this, once per line:

int fd = open("audit.log", O_WRONLY | O_CREAT, 0644);
off_t end = lseek(fd, 0, SEEK_END);
pwrite(fd, line, len, end);
close(fd);

Under load the log ends up with fewer lines than were written, and some lines are half-overwritten by others. What is the defect and the fix?

Both processes create the file, so each gets its own inode and the second open() replaces the first. Add O_EXCL so only one writer owns the file.

Reading the end offset and writing at it are two separate steps, so both processes can observe the same end and then write over each other. Open with O_APPEND and use write(): the kernel repositions to end-of-file and writes as one atomic step.

The writes are only in the page cache, so one process's data is evicted before the other's lseek can see it. Add O_SYNC to the open() so each line is on disk before the next offset is computed.

pwrite() ignores the file offset the process just established, so it always writes at offset 0. Use write() after the lseek() instead.