I have been running a C program which connects to a Redis database every once in a while. After running the program for a few hours it was displaying "Cannot create socket: too many files open". I'm pretty sure that I'm closing the connection every time after I'm done with the logic part at the database. So I wanted to know if there is any way to know how many sockets have been created by a C program, while it is running? I'm using Ubuntu 16.04.

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    On linux: find /proc/PID/fd -lname 'socket:*' | wc -l. But unless your C program is the next big thing, it has no business having open more than a couple sockets at the same time; go fix it so it stops leaking fds -- every open file or socket should be closed when no longer in use. – mosvy Mar 14 '19 at 8:04

You did not specify an operating system, and /proc/*/fd/ does not exist on all operating systems.

You actually need to list all of the currently open file descriptors for the process, from which you can determine what is actually being leaked. It is not necessarily a socket file descriptor that is being leaked.

On FreeBSD, NetBSD, OpenBSD …

… and their various derivatives (e.g. TrueOS, DragonFly BSD).

Use the fstat command with the -p option and the relevant process ID:

fstat -p 718

  • fstat. NetBSD General Commands Manual. 2013-12-15.


Use the procfiles command similarly, with the -n option to print names:

procfiles -n 6679

On OpenSolaris …

… and thus on Illumos, Schillix, et al.

Use the pfiles command:

pfiles 13253

  • pfiles. User Commands. SunOS 10.5 Manual. 2008-12-10.

On Linux

Use the lsof command similarly:

lsof -p 41467


In my C++ programs, a simple class that owns a file descriptor and closes it in its destructor works wonders. Unfortunately, you'll have to resort to GNU-specific language extensions (i.e. __attribute__(__cleanup__)) to get that in C programs.

You can of course go further with other tools. With DTrace and a suitable script, you can monitor the process as it opens and closes file descriptors. (You can do this with truss, or ktrace or strace, too; although the their mechanisms for selecting subsets of the system calls made are either fairly basic or nonexistent. You can also do this with a debugger and some suitable breakpoints.)

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At a system level at does not better what language the program is written in.

Looking in /proc/«pid»/fd will tell you total number of file descriptors, that the process has open (not just sockets), but will be a close approximation. Maybe some one else can tell you for sockets.

There is also strace you can use this to trace system calls that your process makes. strace «my-program» «program args…»

in the program

Add some logging: printf when you open and close a socket, to a logfile.

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  • Does the count of files in /proc/«pid»/fd show the total number of sockets open at current time or does the count specify the total number of sockets created over time. – yash Mar 14 '19 at 9:35
  • No. There is no count summary, there are just files. Each file represents an open file ( where file is a file, socket, pipe, … that is open at the time). Therefore there will be a few more than the number of open sockets. (3 plus one for every socket, file, pipe that the process opened). – ctrl-alt-delor Mar 14 '19 at 16:45
  • @JdeBP`s answer is fuller, and shows how to get just the sockets. – ctrl-alt-delor Mar 14 '19 at 16:47

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