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  • GCC, -O2, and bitfields - is this a bug or a feature?

    - by Rooke
    Today I discovered alarming behavior when experimenting with bit fields. For the sake of discussion and simplicity, here's an example program: #include <stdio.h> struct Node { int a:16 __attribute__ ((packed)); int b:16 __attribute__ ((packed)); unsigned int c:27 __attribute__ ((packed)); unsigned int d:3 __attribute__ ((packed)); unsigned int e:2 __attribute__ ((packed)); }; int main (int argc, char *argv[]) { Node n; n.a = 12345; n.b = -23456; n.c = 0x7ffffff; n.d = 0x7; n.e = 0x3; printf("3-bit field cast to int: %d\n",(int)n.d); n.d++; printf("3-bit field cast to int: %d\n",(int)n.d); } The program is purposely causing the 3-bit bit-field to overflow. Here's the (correct) output when compiled using "g++ -O0": 3-bit field cast to int: 7 3-bit field cast to int: 0 Here's the output when compiled using "g++ -O2" (and -O3): 3-bit field cast to int: 7 3-bit field cast to int: 8 Checking the assembly of the latter example, I found this: movl $7, %esi movl $.LC1, %edi xorl %eax, %eax call printf movl $8, %esi movl $.LC1, %edi xorl %eax, %eax call printf xorl %eax, %eax addq $8, %rsp The optimizations have just inserted "8", assuming 7+1=8 when in fact the number overflows and is zero. Fortunately the code I care about doesn't overflow as far as I know, but this situation scares me - is this a known bug, a feature, or is this expected behavior? When can I expect gcc to be right about this? Edit (re: signed/unsigned) : It's being treated as unsigned because it's declared as unsigned. Declaring it as int you get the output (with O0): 3-bit field cast to int: -1 3-bit field cast to int: 0 An even funnier thing happens with -O2 in this case: 3-bit field cast to int: 7 3-bit field cast to int: 8 I admit that attribute is a fishy thing to use; in this case it's a difference in optimization settings I'm concerned about.

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  • automating write conflict messages

    - by archer
    is there any method to avoid the annoying write conflict messages by automating and hiding the process so that it doesn't appear as if the program is defective? there doesn't seem to be any point to these messages anyway as there is only one real choice which is to drop the changes.

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  • Share data in LAN

    - by eWolf
    I'm building a music library program, and I want to have the ability to share the library in the LAN. How can I discover others who share their library? I'd like to find others' libraries without typing in IPs and stuff.

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  • What's the diff between VSS 6.0 and VSS 2005?

    - by Peter Turner
    We've been using VSS 6.0 since time began, but yesterday I nabbed VSS2005 off of our MSDN subscription, it wouldn't let me install it off the ISO through Daemon Tools (not sure why, but I submitted error report to MS...). I noticed it had a program files directory right on the ISO, so I just copied the folder onto my hard drive. Well, I opened up the client and behold, a glamorous version of VSS 6.0 connected to the exact same DB. Anyone know if I'm going to destroy everything by using it?

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  • "Hello World" in less than 20 bytes

    - by xelurg
    We have had an interesting competition once, where everyone would write their implementation of hello world program. One requirement was that is should be less than 20 bytes in compiled form. the winner would be the one whose version is the smallest... What would be your solution? :) Platform: 32bit, x86 OS: DOS, Win, GNU/Linux, *BSD Language: Asm, C, or anything else that compiles into binary executable (i.e. no bash scripts and stuff ;)

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  • python simple function error ?

    - by abhilashm86
    Here's a simple function to do simple math operations, when i call this from other program using import, output i get is none. When i remove def function, everything is working fine. What's the problem with defining this function? I'm new to python. def calci(a, op, b): if op == '+': c = a + b elif op == '-': c = a-b elif op == '*': c= a*b elif op =='/': if(b == 0): print('can\'t divide') c = a/b print('value is',c) return c result = calci(12,'+', 12) print(result) nter code here

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  • How to put Java source inside IDE

    - by karikari
    I have a Java program source files, complete with its subdirectories. My problem is, how can I put all of this inside an IDE (ie. Netbeans, Eclipse) , so that I can easily, modify, compile and package it to become a *.jar file.

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  • Unable to compile output of lex

    - by dbarker
    When I attempt to compile the output of this trivial lex program: # lex.l integer printf("found keyword INT"); using: $ gcc lex.yy.c I get: Undefined symbols: "_yywrap", referenced from: _yylex in ccMsRtp7.o _input in ccMsRtp7.o "_main", referenced from: start in crt1.10.6.o ld: symbol(s) not found collect2: ld returned 1 exit status lex --version tells me I'm actually using 'flex 2.5.35' although ls -fla `which lex` isn't a symlink. Any ideas why the output won't compile?

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  • MS-Access to SQL Server 2000 problem with datetime

    - by 0plus1
    I'm trying to transfer some data from Access to SQL Server 2000 (there are no other way to do this sadly), I'm using Access to mssql to do the job. The program fails everytime it finds a datetime in this fashion: 03/07/2009 10.17.00 While it imports date like this: 29/12/2008 I don't understand where this problem come from so I ask you for your help. Thank you very much.

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  • execute python file from another file

    - by ariel
    Hi I have a python file that has functions and classes. now I am writting another program (in another file). and I want to start the new file with running the old file (with the function and classes). I have tried using exec(path_2_oldFile.pyw) but it didn't work. thanks for any help Ariel

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  • What is a good programming language for beginners? [closed]

    - by user122401
    Possible Duplicates: Best ways to teach a beginner to program? What is the easiest language to start with? What is a good programming language for beginners? I am 15 and had learned C++ before never got into it though? But is this like learning a new language? like having to do it over and over and every day having new stuff to learn ?

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  • Streaming output with commons-exec?

    - by Stefan Kendall
    Can anyone give me an example of how to stream the output of an external program executed with DefaultExecutor? I'm not finding any documentation describing how to do this. My external process will run for several hours, so just grabbing all output data isn't feasible; it must be streamed.

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  • Getting started with JavaMe

    - by Jeroen from Belgium
    Hello, I'm trying to make a very simple program run on my Nokia 5000. On the Nokia website, there is a SDK (Series 40). I included this SDK in my IntelliJ project. The problem is, when I try to run the helloWorld app on my phone, I get an error message. (http://wiki.forum.nokia.com/index.php/Hello_World_in_Java_ME) Can anyone help me out? Thanks in advance!

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  • how to find the active thread count?

    - by DayOne
    Hi, i have a c# program which calls into a c++ library. The c# programs process has a high thread count 50 - 60. Most seem to be created in c++ and i supect most are suspended/waiting. How do i find how many of these threads are active at a given point in time? thanks

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  • die $template->error() produces no line number

    - by Kinopiko
    In the following short program: use Template; my $template = Template->new (INCLUDE_PATH => "."); $template->process ("non-existent-file") or die $template->error (); why does "die" not produce a line number and newline? Output looks like this: $ perl template.pl file error - non-existent-file: not found ~ 503 $

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  • psexec premissions

    - by user311130
    Hey, I wrote a c# program to run a remote exe using Psexec.exe *via domain-admin user - works fine *via loca-admin user - 'access denied error' you know what could solve this?

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