Very simple Unix Prog in C, could use advice/help Post: 16824

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Top Forums Programming Very simple Unix Prog in C, could use advice/help Post 16824 by Perderabo on Thursday 7th of March 2002 09:04:43 AM

03-07-2002

Administrator Emeritus

The particular error that you are searching for is here:
if( fd = open( FILENAME, O_RDWR ) == -1 )
If the open system call succeeds, it will not return -1. Thus you will set fd=(open() == -1) to zero! You will not attempt the creat. Then you stat() the real file. Eventually you attempt a read from fd 0. You could just type something since you're reading from the terminal.

You gotta use:
if ((fd = open() ) == -1)
to get what you want. Don't feel too bad, this one still nails me.

You have several other bugs, but I'll let you find them for the most part. One bug is very subtle though. First you open() the file, if that fails then you creat() it. That is a race condition. Suppose the file is not there, so the open() fails. Just then, another process creates it. Then your creat() call must now fail too. Whenever you check to see if something is possible and then do it if looks possible, you are setting yourself up for this situation. It's ok to do a check first, but then you must check for success to see if it really worked.

Perderabo

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explain_creat_or_die

explain_creat_or_die(3) 				     Library Functions Manual					   explain_creat_or_die(3)

NAME

       explain_creat_or_die - create and open a file creat and report errors

SYNOPSIS

       #include <libexplain/creat.h>
       void explain_creat_or_die(const char *pathname, int mode);

DESCRIPTION

       The  explain_creat_or_die function is used to call the creat(2) system call.  On failure an explanation will be printed to stderr, obtained
       from explain_creat(3), and then the process terminates by calling exit(EXIT_FAILURE).

       This function is intended to be used in a fashion similar to the following example:
	      explain_creat_or_die(pathname, mode);

       pathname
	       The pathname, exactly as to be passed to the creat(2) system call.

       mode    The mode, exactly as to be passed to the creat(2) system call.

       Returns:
	       This function only returns on success.  On failure, prints an explanation and exits.

SEE ALSO

       creat(2)
	       open and possibly create a file or device

       explain_creat(3)
	       explain creat(2) errors

       exit(2) terminate the calling process

COPYRIGHT

       libexplain version 0.52
       Copyright (C) 2008 Peter Miller

															   explain_creat_or_die(3)