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I know that most devices are presented in the /dev directory, but only now have I actually noticed two very strange, yet familiar names in there...

I'm talking about stdin@ and stdout@ down there:

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What are these for? Are they used by functions of the C language?

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1 Answer 1

up vote 9 down vote accepted

These files are actually called stdin, stdout and stderr. The @ character is added by ls to tell you that they are symbolic links. ls -l would reveal that the targets of these symbolic links are /proc/self/fd/0, /proc/self/fd/1 and /proc/self/fd/2.

/proc is a virtual filesystem provided by the kernel that shows information about the operating system. Among other things, there are directories like /proc/1234 that contain information about the process with ID 1234. /proc/self is a symbolic link to the directory for whatever process is accessing it (the kernel returns a different target to different processes).

/proc/self/fd (which is also symlinked at /dev/fd) contains one entry per file that the process has open. Each entry corresponds to a file descriptor and is a symbolic link to the file (if possible — for pipes, sockets and deleted files, the symbolic link gives information that's useful for debugging but does not point to a file that you could reopen by name). These file descriptors are the same that you manipulate with C functions such as open, close, dup, read, write, etc. They are not used by the C functions, they're other ways to interact with the same objects.

The reason /dev/stdin and friends exist is that sometimes a program requires a file name, but you want to tell it to use a file that's already open (a pipe, for instance). So you can pass /dev/stdin to tell the program to read its standard input.

Further reading:

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