Using flock() to lock a file opened with fopen()

Question

I am working on old C/C++ code someone in my company wrote for Linux. The code opens a file using fopen() and then locks the file using flock():

FILE *fp=fopen("text.txt", "wt");
flock( fp, LOCK_EX );

The problem is that when I compile the code using Eclipse on Linux, the compiler throws an error:

    invalid conversion from ‘FILE*’ {aka ‘_IO_FILE*’} to ‘int’

flock() obviously takes an int as the 1st parameter.

What function can I use to convert "FILE *" into int?

Possible duplicate of [How do I lock files using fopen()?](https://stackoverflow.com/q/7573282/608639), [How to block file when you reading (fopen) it](https://stackoverflow.com/q/26313856/608639), [File locking in Linux using C program](https://stackoverflow.com/q/13468238/608639), etc. — jww, Apr 01 '19 at 23:47
I don't understand how this could ever have worked; `flock` has taken an `int` (file descriptor) and not a `FILE *` for its entire history. — zwol, Apr 02 '19 at 00:59
Beats me :-( the original code was in C, not C++, so perhaps the compiler used then (20 years ago) allowed it. — yaronkl, Apr 02 '19 at 02:22
@yaronki There certainly have been C compilers that would _compile_ your sample fragment without complaint, but, at runtime, the file would not have been locked. Possibly the `flock` has always failed (setting `errno` to `EBADF`, I would expect) and nobody noticed. — zwol, Apr 02 '19 at 16:43

score 3 · Answer 1 · answered Apr 01 '19 at 23:43

3

You'll want fileno(3):

flock( fileno(fp), LOCK_EX);

answered Apr 01 '19 at 23:43

Tanktalus

1 Answers1