CODEWITHSUNDEEP
clinux
Jimm
Jimm

Reputation: 8505

how to check if the file is closed

I have file descriptor and inside my signal handler, i close the file. But due to other conditions, the file could have been closed earlier. Is there a way to check if the file descriptor points to an open file in c and linux?

UPDATE: Is it possible to determine filename associated with a file descriptor? This way if the fd gets recycled, app can detect it.

Upvotes: 6

Views: 9905

Answers (3)

alk
alk

Reputation: 70921

After you closed the file descriptor fd assign -1 to it, so you later could test fd against -1 to see if you already closed it.

You could lookup the filename a (valid) file descriptor referrs to by calling readlink() on /proc/<pid>/fd/<file descriptor>.

Upvotes: 2

rashok
rashok

Reputation: 13414

Try to do any file operation like lseek on the file descriptor. If it returns -1. Then check errno, if its EBADF then the file descriptor is already closed.

Try to do lseek in an unaffected manner like below.

lseek(fd, 0, SEEK_CUR);

Usually while opening the first file in a program we always get the file descriptor as 3. After this file is closed, if we try to open some other file we will get the same file descriptor as 3. Because always we will get the lowest available number. If we are closing and reopening many files in a program, then we need to improve our code to track of file descriptors list to check whether its closed or not.

Upvotes: 5

Xeor
Xeor

Reputation: 1866

When you open a file, it always get the minimal available fd assigned. So if you close your fd, and then open another file somewhere in your code, you could easily have the same fd reassigned to this new file. So there is no reliable way to tell that the file descriptor is closed, because it can now point to another opened file.

Upvotes: 2

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