CODEWITHSUNDEEP
linux
Adelina
Adelina

Reputation: 45

Linux Implement open file descriptors C

1) Is it any alternative to looping through /proc in order to get the total number of open file descriptors? I used the following dirs:

/proc/PID/fd/* 
/proc/PID/maps 
/proc/PID/cwd 
/proc/PID/root 
/proc/PID/exe

2) The number is different from lsof | wc -l and cat /proc/sys/fs/file-nr

3) Loaded dynamically linked libraries and current working directories can be counted as open file descriptors? Implementation all open file descriptors in C for Linux

Upvotes: 2

Views: 440

Answers (2)

Brian Campbell
Brian Campbell

Reputation: 332736

How you count this depends on what information you are interested in.

Looking through /proc/PID/fd/* will give you the number of open file descriptors. However, one caveat is that two processes may actually share a file descriptor, if you fork then the child process inherits the file descriptor from its parent, and this method will then count it twice, once for each process.

/proc/PID/maps will show you the memory map of the process, which can include the loaded executable itself and dynamically linked libraries, but also includes things that don't correspond to files like the heap, the stack, the vdso section which is a virtual shared object exported by the kernel, and so on.

lsof will list a variety of ways that files can be in use, which includes more than just file descriptors; it also includes the executable and shared libraries, but does not include the memory regions that don't correspond to files that show up in /proc/PID/maps like the stack, heap, vdso section, etc.

/proc/sys/fs/file-nr will report the number of open kernel file handles. A kernel file handle is different than a file descriptor; there can be more than one file descriptor open that point to the same file handle, for instance, by calling dup or dup2.

These differences explain why you're getting different numbers from these different ways of counting. The question is, what purpose are you using this count for? That will help answer which way of counting you should actually use.

Upvotes: 1

user4822941
user4822941

Reputation:

1) no, but it seems you are confused as to what constitutes an open file descriptor, as suggested by your second question

2) see http://codingtragedy.blogspot.com/2015/04/nofile-ulimit-n-rlimitnofile-most.html - while it explains handling of a resource limits which may seem irrelevant, it also explains the difference between a file descriptor and a 'struct file' which you most likely want, and it even covers your lsof usage.

3) Again, it is unclear what is your actual question. current working directory is not a file descriptor and is only represented with an inode. A process may or may not keep fd for a linked library around, but mapping itself occupies a 'struct file'.

Upvotes: 1

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