wht do these codes mean?

jackandking · 12-22-2004, 12:13 AM

for(int i=0;i< NOFILE;++i)
close(i);

I found these from a daemon example.

RandomLinuxNewb · 12-22-2004, 12:36 AM

It looks like i would be the handle to a file, and NOFILE would be the number of files that are opened by the script. So I'm guessing that it's closing all files that have been opend by the script.

bm17 · 12-22-2004, 12:38 AM

The daemon is shutting down any file descriptors that it has open. NOFILES is probably #defined to be the maximum number of simultaneous files/connections handled.

If you want to play at this level, buy a book on the C programming language.

jackandking · 12-23-2004, 07:17 AM

after testing, I found:
NOFILE=256

and when running to these codes , program exit normally.

Proud · 12-23-2004, 07:57 AM

We're assuming you understand the basic syntax/sematics of a for( ; ; ) loop... is that you're also asking about?

jackandking · 12-23-2004, 08:05 AM

of course not,

mainly, there are two points confusing me:
NOFILE refers to what?
close(i) operate on what ?

itsme86 · 12-23-2004, 09:38 AM

Quote:

NOFILE refers to what?

The number of files used in the program.

Quote:

close(i) operate on what ?

A file descriptor.

If that's not what you're looking for then you're really going to need to be a little more specific with your questions. It's hard to answer a question "What does this code mean?" Say right off what's confusing you about it and what you'd like to know.

jackandking · 12-23-2004, 07:18 PM

I am sorry for my illegibility statement.

please allow me to try again:

I don't define NOFILE in my program,so is it defined by system or compiler?
why executing close(i) exit my program?
what's more ,I have not opened any files before that codes.

btmiller · 12-23-2004, 07:58 PM

Perhaps NOFILE is defines in a header you're including. It's not, to my knowledge, a standard ANSI C or POSIX macro. There is a MAX_OPEN_FILES or something similar defined in POSIX, but I don't think there's a NOFILE. In any case, if the program compiles, it's being defined somewhere, so check the header files included in the program.

The close system call does not terminate your program. That is done by returning from main, calling exit or (sometimes) _exit. What makes you think the close call is causing your program to terminate?

jschiwal · 12-23-2004, 08:23 PM

From 'Advanced Programming in the Unix Environment' on page 42:

Code:

A common sequence of code in a daemon process is one that closes all open files.
Some programs have the code sequence

  #include
  
  for (i = 0; i < NOFILE; i++)
      close(i);

assuming the constant NOFILE was defined in the <sys/param.h> header.
Other programs use the constant _NFILE that some versions of <stdio.h>
provide as the upper limit.

For your own programs, use the POSIX.1 value OPEN_MAX to determine the maximum open files value portably.

This value may be indeterminate at compile time. Using the function call sysconf(_SC_OPEN_MAX) can also determine this value. The value is referred to as 'run-time invariant'. This means that it won't change during the lifetime of the process.

jackandking · 12-25-2004, 11:14 AM

ok, that's all, thank all of you peoples very much!
I think i should read more books and study harder.