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Full Discussion: file descriptors
Top Forums UNIX for Dummies Questions & Answers file descriptors Post 46809 by oombera on Tuesday 27th of January 2004 12:14:54 AM
Old 01-27-2004
Short File Descriptors Tutorial
Here's a nice little (basic) tutorial: http://www.livefirelabs.com/unix_tip...3/06092003.htm.

This explains what "info" about a file they hold ... as for the more technical side, you'll have to dig deeper.
Quote:
The Standard UNIX File Descriptors - Standard Input (stdin), Standard Output (stdout), and Standard Error (stderr)

If you are familiar with UNIX I/O redirection, syntax similar to the following should not be new to you:
  • command > file 2>&1
Briefly, when command runs it sends "normal" output to file, and any error messages generated by command are also written to file. "2>&1" handles the latter.

Have you ever wondered where the numbers 2 and 1 come from? This may be common knowledge to our more experienced readers, buy may need some explaining to those who are relatively new to the UNIX operating system environment. The 2 and 1 are file descriptors. Okay, so what's a file descriptor?

When a UNIX program wants to use a file, it must first open that file. When it does so, UNIX will associate a number with the file. This number, which is used by the program when reading from and writing to the file, is the file descriptor.

A typical UNIX program will open three files when it starts. These files are:
  • standard input (also known as stdin)
  • standard output (also known as stdout)
  • standard error (also known as stderr)
Standard input has a file descriptor of 0, standard output uses 1, and the number 2 is used by standard error. Are you starting to see where this is headed?

Looking at our command again,
  • command > file 2>&1
you should now recognize that 2>&1 instructs the shell to send messages headed to stderr (2) to the same place messages to stdout (1) are sent. In our example, that place is file.

If you are wondering, > is equivalent to 1>, and < is short for <0.
 

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LEARN ABOUT HPUX

sem_destroy

sem_destroy(2)							System Calls Manual						    sem_destroy(2)

NAME

       sem_destroy - destroy an unnamed POSIX semaphore

SYNOPSIS

DESCRIPTION

       is  used  to  destroy  an unnamed semaphore.  A successful call to will invalidate the unnamed semaphore referred to by sem and removes all
       descriptors referencing it.  The semaphore should have been created by a previous call to and there should not be any processes blocked	on
       it.

       To use this function, link in the realtime library by specifying on the compiler or linker command line.

EXAMPLES

       The following call to will destroy an unnamed semaphore referred to by sem and remove all descriptors referencing it.

RETURN VALUE

       If the semaphore was destroyed and the descriptors referencing it were removed, returns 0 to the caller.

       If the semaphore could not be destroyed, the call returns -1 and sets to indicate the error.

ERRORS

       fails and does not perform the requested operation if any of the following conditions are encountered:

	      [EBUSY]	     There are threads currently blocked on the semaphore or there are outstanding locks held on the semaphore.

	      [EINVAL]	     The argument sem is not a valid unnamed semaphore.

SEE ALSO

       sem_init(2), sem_open(2), <semaphore.h>.

STANDARDS CONFORMANCE

																    sem_destroy(2)
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