Read a file in a while loop line-by-line while another function updates it

Question

I am reading a file in a while loop from start to end:

FILE *file;
file = fopen(path_to_file), "r");
char *line = NULL;
size_t len = 0;

while (getline(&line, &len, file) > 0) {
        delete_line_from_file(line);
}

fclose(file);

The function delete_line_from_file() removes the line passed to it from the file. It reads in the whole file via open(fd, O_RDONLY | O_CLOEXEC) + read() + close(), then removes the line from the buffer and writes the whole buffer to the same file via open(fd, O_WRONLY | O_TRUNC | O_CLOEXEC) + write() + close(). The read() is locked in an advisory read-lock via struct flock lk and the write() is locked in an advisory write-lock.

When I read the file there are lines that get missed which has something to do with me reading the file from start to finish in one loop while writing to it. If I read in the whole file and go through the buffer line-by-line no lines get missed. (This is my preferred solution so far.) There are also no mistakes made when truncating and writing the file. The missed lines are still in the file after the loop finishes.

Can I make sure that my while-loop does not miss a line and cleanly empties the file? The file needs to be emptied line-by-line. It cannot be just truncated.

Here is one possible solution I had in mind. Mirror the file via fstat(file &fbuf) and check it's size with if (fbuf.st_size !=0) fseek(file, 0, SEEK_SET); but that seems inefficient.

Answer 1

So is the goal to empty the file completely?

Why don't you open the file as such:

open("file", O_TRUNC | O_WRONLY);

This will open the file with truncation. Alternatively, and perhaps a better solution, you can do this:

fopen("file", "w");

fopen with the "w" option delete the original file and replaces it with the new file of name "file".

Answer 2

Use fseek and ftell inside your loop.

Two processes modifying the same file is a recipe for problems. May be you need to use a pipe(2).