CODEWITHSUNDEEP
unixstreamfile-descriptor
imagineerThat
imagineerThat

Reputation: 5553

Unix: What are stdin/out/err REALLY?

Assuming the following are correct...

  1. stdin, stdout, and stderr are streams
  2. streams are file descriptors
  3. file descriptors are numbers/indexes in the kernel representing open files

Questions:

a. Does it follow by transition that stdin/out/err involve open files? So if I do ls /dir, does ls output the results to a file referred to by stdout(2)?
b. Where does above file live? in a /proc//? OR is that where the FD lives?
c. What is /dev/stdout? If I do vim /dev/stdout, vim tells me it is not a file. I see there's a series of links that lead to /dev/pts/27. What is going on? I tried to cat /dev/stdout but nothing happens.
d. In general, how is it that "files" in linux are actually NOT files?

Upvotes: 0

Views: 295

Answers (1)

Keith Thompson
Keith Thompson

Reputation: 263567

Some of your assumptions are incorrect. For example, stdin is of type FILE*; it's not a "file descriptor".

stdin, stdout, and stderr are macros defined in <stdio.h>. (Yes, they're required to be macros, not just variable names). They expand to expressions of type FILE*, and they point to the FILE objects associated with the standard input, output, and error streams.

A "file descriptor" is a small integer value representing a POSIX stream. On UNIX-like systems, FILE* values are generally associated with file descriptors (you can use the fileno and fdopen functions to go from one to the other), but they're not the same thing.

Basically, there are two distinct I/O systems, one built on top of the other. The lower level system uses numeric file descriptors, manipulated via the open, read, write, and close functions, and so forth. The higher level, as defined by the ISO C standard, uses pointers of type FILE*, manipulated with fopen, fread, fwrite, fprintf, putchar, fclose, and so forth.

As I mentioned, on UNIX-like system, the C standard layer is generally implemented on top of the POSIX layer. On non-POSIX systems (like MS Windows), the C standard layer may be implemented on top of some other system-specific interface.

Linux and other UNIX-like systems try (incompletely) to follow an "everything is a file" philosophy. There are a number of file-like entities under /proc. These are not physical files stored on disk; they're entities that can be accessed using either the POSIX or ISO C I/O layers. Neither layer requires the "files" it deals with to be actual disk files, so there's nothing inconsistent about this.

man proc for more information on what's under the /proc directory (there's far more detail than I can put in this answer).

Upvotes: 2

Related Questions