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Top Forums Programming Writing c code for who | sort | lp Post 302123714 by porter on Tuesday 26th of June 2007 09:52:54 PM
Old 06-26-2007
Quote:
Originally Posted by the_learner
I really dont understand this pipe() and fork() and dup() and fnctl()... I think I need to buy that book which Perderabo suggested.. Advanced Unix Programming.
Okay, what you need to understand is what a process is in UNIX. Basically a process is

1. a memory image

2. a set of file descriptors 0,1,2 etc.

3. a point of execution

The memory image exists outside the kernel, the file descriptors live inside the kernel. A single file descriptor points to a file. That file may be referenced by multiple file descriptors.

Fact 1. All close() does is closes a single file descriptor for the current process. That drops the usage count on the actual file, when the usage count on the file drops to zero then it is really closed.

Fact 2. When a process terminates, all it's file descriptors that are still open are closed.

Fact 3. All "dup" and "dup2" do are allocate another file descriptor to point to the same file and increase the usage count on the underlying file.

Fact 4. Fork() duplicates the entire memory image AND the file descriptor table. Both processes now continue executing at the same point, the only difference being the parent got the pid of the child, the child got nothing. Duplicating the file descriptor table is like dup() has been called on each open file descriptor.

Fact 5. Exec() replaces the entire memory image with the new program but the file descriptors do not change EXCEPT for the file descriptors marked as FD_CLOEXEC, which are closed.

Fact 6. Pipe() returns two file descriptors, one is readable, one is writeable. A pipe will remain readable until all writeable file descriptors have been closed. Remember if you create a pipe then call fork there are two descriptors that are writeable.

Hints:

Get into the habit of putting -1 in variables that hold file descriptors when they are not holding a value, when you close it, put -1 back in the file descriptor variable.

Code:
int fd=-1;

fd=open(.....)

....stuff...

close(fd);

fd=-1;

 

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Perl::Critic::Policy::Subroutines::ProhibitReturnSort(3pUser Contributed Perl DocumentatPerl::Critic::Policy::Subroutines::ProhibitReturnSort(3pm)

NAME
Perl::Critic::Policy::Subroutines::ProhibitReturnSort - Behavior of "sort" is not defined if called in scalar context. AFFILIATION
This Policy is part of the core Perl::Critic distribution. DESCRIPTION
The behavior of the builtin "sort" function is not defined if called in scalar context. So if you write a subroutine that directly "return"s the result of a "sort" operation, then you code will behave unpredictably if someone calls your subroutine in a scalar context. This Policy emits a violation if the "return" keyword is directly followed by the "sort" function. To safely return a sorted list of values from a subroutine, you should assign the sorted values to a temporary variable first. For example: sub frobulate { return sort @list; # not ok! @sorted_list = sort @list; return @sort # ok } KNOWN BUGS
This Policy is not sensitive to the "wantarray" function. So the following code would generate a false violation: sub frobulate { if (wantarray) { return sort @list; } else{ return join @list; } } CONFIGURATION
This Policy is not configurable except for the standard options. CREDITS
This Policy was suggested by Ulrich Wisser and the <http://iis.se> team. AUTHOR
Jeffrey Ryan Thalhammer <jeff@imaginative-software.com> COPYRIGHT
Copyright (c) 2005-2011 Imaginative Software Systems. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. The full text of this license can be found in the LICENSE file included with this module. perl v5.14.2 2012-06-07 Perl::Critic::Policy::Subroutines::ProhibitReturnSort(3pm)
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