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Full Discussion: C Brain Teaser
Top Forums Programming C Brain Teaser Post 302117287 by Perderabo on Friday 11th of May 2007 11:41:41 PM
Old 05-12-2007
There are some other approaches to this. A smart perl script could call time() to get the current time. Then it would output a c program that would refuse to run after the next two seconds pass. That should be enough time to compile and run it once. After the output, the c prgram would sleep for 2 seconds to ensure that a second run is impossible. The perl script could also obtain the pid of the current login shell. The C program would refuse to run if the login shell has a different pid. The C program would output the string, sleep a few seconds, then kill the login shell. For added security these approaches could be combined.
 

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SLEEP(1)						    BSD General Commands Manual 						  SLEEP(1)

NAME
sleep -- suspend execution for an interval of time SYNOPSIS
sleep seconds DESCRIPTION
The sleep command suspends execution for a minimum of seconds. If the sleep command receives a signal, it takes the standard action. IMPLEMENTATION NOTES
The SIGALRM signal is not handled specially by this implementation. The sleep command will accept and honor a non-integer number of specified seconds (with a '.' character as a decimal point). This is a non- portable extension, and its use will nearly guarantee that a shell script will not execute properly on another system. EXIT STATUS
The sleep utility exits 0 on success, and >0 if an error occurs. EXAMPLES
To schedule the execution of a command for x number seconds later (with csh(1)): (sleep 1800; sh command_file >& errors)& This incantation would wait a half hour before running the script command_file. (See the at(1) utility.) To reiteratively run a command (with the csh(1)): while (1) if (! -r zzz.rawdata) then sleep 300 else foreach i (`ls *.rawdata`) sleep 70 awk -f collapse_data $i >> results end break endif end The scenario for a script such as this might be: a program currently running is taking longer than expected to process a series of files, and it would be nice to have another program start processing the files created by the first program as soon as it is finished (when zzz.rawdata is created). The script checks every five minutes for the file zzz.rawdata, when the file is found, then another portion processing is done courteously by sleeping for 70 seconds in between each awk job. SEE ALSO
nanosleep(2), sleep(3) STANDARDS
The sleep command is expected to be IEEE Std 1003.2 (``POSIX.2'') compatible. HISTORY
A sleep command appeared in Version 4 AT&T UNIX. BSD
April 18, 1994 BSD
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