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• C++ shared libraries

  - by saminny

  Hi, I am trying to get my head around the way shared libraries work in the c++ unix environment. I understand we only need header files and no shared libraries specification when compiling code. But if I want to create an executable or shared library from my compiled files, do I need to specify shared library dependencies (those are dynamic)? And do the paths of shared libraries need to match the path at runtime loading? I am using Linux 2.6.18-164.11.1.el5 #1 SMP x86_64 GNU/Linux I am having a problem where my code is not able to pick up a library at runtime. I have tried setting LD_LIBRARY_PATH and PATH. But at runtime when I run the executable, I get the following error: Error: librc.so: cannot open shared object file: No such file or directory Sam

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• What Should be the Structure of a C++ Project?

  - by Ell

  I have recently started learning C++ and coming from a Ruby environment I have found it very hard to structure a project in a way that it still compiles correctly, I have been using Code::Blocks which is brilliant but a downside is that when I add a new header file or c++ source file, it will generate some code and even though it is only a mere 3 or 4 lines, I do not know what these lines do. First of all I would like to ask this question: What do these lines do? #ifndef TEXTGAME_H_INCLUDED #define TEXTGAME_H_INCLUDED #endif // TEXTGAME_H_INCLUDED My second question is, do I need to #include both the .h file and the .cpp file, and in which order. My third question is where can I find the GNU GCC Compiler that, I beleive, was packaged with Code::Blocks and how do I use it without Code::Blocks? I would rather develop in a notepad++ sort of way because that is what I'm used to in Ruby but since C++ is compiled, you may think differently (please give advice and views on that as well) Thanks in advance, ell.

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• Are volatile data members trivially copyable?

  - by Lightness Races in Orbit

  Whilst writing this answer I realised that I'm not as confident about my conclusions as I usually would ensure before hitting Post Your Answer. I can find a couple of reasonably convincing citations for the argument that the trivial-copyability of volatile data members is either implementation defined or flat-out false: https://groups.google.com/forum/?fromgroups=#!topic/comp.std.c++/5cWxmw71ktI http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48118 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3159.html#496 But I haven't been able to back this up in the standard1 itself. Particularly "worrying" is that there's no sign of the proposed wording change from that n3159 issues list in the actual standard's final wording. So, what gives? Are volatile data members trivially copyable, or not? 1   C++11

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• Postgresql error: could not open segment 1 of relation base/20983/2416

  - by kokonut

  I'm running a Postgresql query and getting the following error: ActiveRecord::StatementInvalid (PGError: ERROR: could not open segment 1 of relation base/20983/24161 (target block 5046584): No such file or directory The query is of the format 'SELECT "locations".* FROM "locations" WHERE ("locations"."id" IN (115990, 78330, 77891, 78248, ...)' with about 600 ids in the IN clause - not an optimal query I know but it's what I have to work with for the moment! The server is running PostgreSQL 8.4.6 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real (Ubuntu 4.4.1-4ubuntu9) 4.4.1, 64-bit. Postgis 1.5 is also installed, and the locations table contains a geometry column. Anyone have any idea what could be causing the error? Thanks!

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• why C clock() returns 0

  - by eddy ed

  I've got something like this: clock_t start, end; start=clock(); something_else(); end=clock(); printf("\nClock cycles are: %d - %d\n",start,end); and I always get as an output "Clock cycles are: 0 - 0" Any idea why this happens? (Just to give little detail, the something_else() function performs a left-to-right exponentiation using montgomery represantation, moreover I don't know for certain that the something_else() function does indeed take some not negligible time.) This is on Linux. The result of uname -a is: Linux snowy.*****.ac.uk 2.6.32-71.el6.x86_64 #1 SMP Fri May 20 03:51:51 BST 2011 x86_64 x86_64 x86_64 GNU/Linux

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• Delete the first line that matches a pattern

  - by gioele

  How can I use sed to delete only the first line that contains a certain pattern? For example, I want to remove the first line matching FAA from this document: 1. foo bar quuz 2. foo FAA bar (this should go) 3. quuz quuz FAA (this should remain) 4. quuz FAA bar (this should also remain) The result should be 1. foo bar quuz 3. quuz quuz FAA (this should remain) 4. quuz FAA bar (this should also remain) A solution for POSIX sed would be greatly appreciated, GNU sed is OK.

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• Can my employer force me to backup my personal machine? [closed]

  - by Eric B

  Here's the background: Approximately 1.25 years ago, the company I work for was acquired by a larger 400 person company. Before acquisition (and today still) we are all remote employees using our own personal hardware for work-related duties (coding, email, etc). We are approximately 15 employees within the larger organization. Some time after acquisition, the now owning company was slapped with a civil lawsuit. Part of this lawsuit (discovery) is requiring them to retrieve & store from us any related information. Because we were a separate company up until acquisition, there is a high probability that our personal machines might contain information about what the lawsuit alleges (email, documents, chat logs?, etc). Obviously, this depends largely on the person's job function (engineer vs. customer support vs. CEO). All employees are being required to comply. Since acquisition (1.25 yrs), the new company has not provided us with company laptops/desktops. We continue to use personal hardware, licenses, etc for work. Email is via POP3s and not hanging around on the mail server - it's on everyone's client. Documents are spread across personal machines. So, now they want us each to backup our complete personal machines. They are allowing us to create a "personal" folder where we can place personal documents. That single folder will be excluded from backup. Of course, that means total re-arrangement of documents, etc. For most of us, 99% of the data on the machine is NOT related to work. So, what's the consensus? Should we comply? What is their recourse if we do not?

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• Display path of file in status bar

  - by robUK

  Hello GNU Emacs 23.1.1 I am wondering is there a way to display the path of the file in the status bar, instead of just the filename. I have to open many files in many directories, and sometimes I forget what directory they are in. Just easier to display the some of the path in the status bar if that is possible. Sometimes my directory paths are very long. for example /projects/clientserver/trunk/src/client/client.c like this in the status bar, just display the last two directories. /src/client/client.c Many thanks for any suggestions,

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• Can I make a pointer to the code, and pass to the next instruction?

  - by drigoSkalWalker

  Like this link http://gcc.gnu.org/onlinedocs/gcc-3.3.1/gcc/Labels-as-Values.html I can get the memory address of an label, so if I declare a label, get your address, and add your address, i will pass to next instruction? some ilustration int main () { void *ptr; label: instruction 1; instruction 2; ptr = &&label; // So if I do it... ptr = ptr + 1; // I will get the instruction 2 correct?? Thanks for all answers.

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• Are the atomic builtins provided by gcc actually translated into the example code, or is that just f

  - by Jared P

  So I was reading http://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html, and came across this: type __sync_and_and_fetch (type *ptr, type value, ...) type __sync_xor_and_fetch (type *ptr, type value, ...) type __sync_nand_and_fetch (type *ptr, type value, ...) These builtins perform the operation suggested by the name, and return the new value. That is, { *ptr op= value; return *ptr; } { *ptr = ~*ptr & value; return *ptr; } // nand Is this code literal? or is it just to explain what gcc is doing atomically using c-like syntax? And if this is the direct translation, can someone explain how it is atomic?

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• I have to manually change the DNS suffix order every time I connect to VPN. Can I change this permanently or fix the problem somehow?

  - by CarlB

  Sorry in advance but I'm a programmer, not a network engineer, so I'm a noob at this stuff. Anyway, when I am not connected to VPN from my work PC at home, I have the following DNS suffixes listed (real domain names substituted): enterprise.org network.org company.com us.enterprise.org After connecting to VPN, one more DNS suffix is added to the very top of the list: problem-domain.com At this point, most network functions that I can normally perform when actually connected to the LAN in the office are unusable. I get error messages about the network paths not being found and what-not. Anyway, I played around with the suffixes and realized that if I just moved problem-domain.com down one spot to the second in the list, all the problems went away. Unfortunately, it returns to the top spot every time I reconnect, and I tend to get disconnected frequently. Is there something else I can do about this or should I just contact the IT department? I've had this problem before and they weren't able to resolve it but I suppose it would be worth trying again if I could get a different person on the job. What I don't understand is that I thought it didn't matter what order the suffixes were in? Isn't Windows supposed to go through each suffix until it finds a match (or has gone through all the suffixes)? Why is it quitting after the first one? Thanks in advance.

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• Emacs: print key binding for a command or list all key bindings

  - by Yktula

  In Emacs (GNU 23.2, *nix), how can I: list the key sequences bound to a particular command? For example, how can we list all the key sequences that execute save-buffers-kill-emacs, with the output of key sequences bound to it? Assuming we can do this, listing the key sequences bound to goto-line should print the output: M-g g on a default install. list all key-bindings? Does C-h b do this? Would it print my own bindings? I am aware that executing the command directly can print a key sequence it can be activated with, but it doesn't always do so, and a few things happen, including: (1) the output doesn't remain for long, (2) the command is executed. I want a command that lists for me (preferably all) the bindings attached to a given command, without executing the command, or something like that.

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• Precision problems of real numbers in Fortran

  - by saladoil

  I've been trying to use Fortran for my research project, with the GNU Fortran compiler (gfortran), latest version, but I've been encountering some problems in the way it processes real numbers. If you have for example the code: program test implicit none real :: y = 23.234, z z = y * 100000 write(*,*) y, z end program You'll get as output: 23.23999 2323400.0. I find this really strange. Can someone tell me what's exactly happening here? Looking at z I can see that y does retain its precision, so for calculations that shouldn't be a problem I suppose. But why is the output of y not exactly the same as the value that I've specified, and what can I do to make it exactly the same?

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• send() always interrupted by EPIPE

  - by Manuel Abeledo

  I've this weird behaviour in a multithreaded server programmed in C under GNU/Linux. While it's sending data, eventually will be interrupted by SIGPIPE. I managed to ignore signals in send() and treat errno after each action because of it. So, it has two individual sending methods, one that sends a large amount of data at once (or at least tries to), and another that sends a nearly similar amount and slices it in little chunks. Finally, I tried with this to keep it sending data. do { total_bytes_sent += send(client_sd, output_buf + total_bytes_sent, output_buf_len - total_bytes_sent, MSG_NOSIGNAL); } while ((total_bytes_sent < output_buf_len) && (errno != EPIPE)); This ugly piece of code does its work in certain situations, but not always. I'm pretty sure it's not a hardware or ISP problem, as this server is running in six european servers, four in Germany and two in France. Any ideas? Thanks in advance.

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• Output current with Teensy++ 2.0 (arduino-based hardware)

  - by omtinez

  I am working on a project with a Teensy++ 2.0 for testing, eventually the goal is to use a Teensy 2.0 (info on both available here) and mount it onto a robot R/C car along with a Raspberry Pi. I have been able to use and test one of the very cheap distance sensors that use ultrasound, which requires very little current. I was trying to power a motor, I don't know exactly what kind of motor but I assume a very low-power one which is what comes with the R/C car cheapo, but nothing is happening. When I plug the motor to GROUND and +5V it runs fine, but when I use GROUND and one of the GPIO pins then nothing happens with the motor. The same GPIO pins were tested to successfully power and run the ultrasound sensor, so the board is fine. My suspicion is that the GPIO pins don't output enough current to power the motor, but my knowledge of electronics is rather scarce (I am a computer scientist, not an electrical engineer). So please forgive me if I am asking something obvious or plain stupid, but does the board not have enough power to power the motor? If so, I could try to use a second power supply that would go straight into the motor and use the GPIO as a gate to turn that power on and off; would such thing work? Is there a better design that could be used?

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• What configure options were used when building gcc / libstdc++?

  - by OK

  After reading about the problem of passing empty std::string objects between DLLs and EXEs, I am concerned about the configure options used to build my gcc / libstdc++. More specific I want to know if --enable-fully-dynamic-string was used during ./configure. I'm using MinGW 4.4.0 on Windows XP. Does anybody know the configuration used to build this release? Is there a general way to find this information for any installation of GNU gcc? The gcc manual gives me no hint on this topic. Thanks for your input!

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• tmux: create session if none exists

  - by rampion

  I'm switching from using gnu-screen to tmux, and I'm still trying to wrap my head around changes. One thing I'd like to be able to do is start tmux and have it automatically connect to a session if one exists, and otherwise create a new session. tmux attach attaches to an automatically existing session - but errors out if no session exists tmux new creates a new session - but it does so every time, so I can't leave it in my .tmux.conf tmux has-session tests whether a session exists - but I don't know how to stitch it together with the other commands Can anyone give me some configure file advice?

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• Potential impact of large broadcast domains

  - by john

  I recently switched jobs. By the time I left my last job our network was three years old and had been planned very well (in my opinion). Our address range was split down into a bunch of VLANs with the largest subnet a /22 range. It was textbook. The company I now work for has built up their network over about 20 years. It's quite large, reaches multiple sites, and has an eclectic mix of devices. This organisation only uses VLANs for very specific things. I only know of one usage of VLANs so far and that is the SAN which also crosses a site boundary. I'm not a network engineer, I'm a support technician. But occasionally I have to do some network traces for debugging problems and I'm astounded by the quantity of broadcast traffic I see. The largest network is a straight Class B network, so it uses a /16 mask. Of course if that were filled with devices the network would likely grind to a halt. I think there are probably 2000+ physical and virtual devices currently using that subnet, but it (mostly) seems to work. This practise seems to go against everything I've been taught. My question is: In your opinion and  From my perspective - What measurement of which metric would tell me that there is too much broadcast traffic bouncing about the network? And what are the tell-tale signs that you are perhaps treading on thin ice? The way I see it, there are more and more devices being added and that can only mean more broadcast traffic, so there must be a threshold. Would things just get slower and slower, or would the effects be more subtle than that?

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• printf'ing a matrix

  - by Flavius

  I'm trying to implement an all-purpose function for printing 2D data. What I've come up with is: int mprintf(FILE* f, char* fmt, void** data, size_t cols, size_t rows) The challenge is determining how many bits to read at once from data, based on fmt. The format fmt is going to be the stdlib's-specific format for printf() and alike. Do you have any knowledge of already-existing features from stdlibc (GNU GCC C's) I could use to ease this up? I try avoiding having to do it all manually, because I know "I am stupid" (I don't want to introduce stupid bugs). Thus, reusing code would be the bug-freest way. Thanks Addendum I see there's a /usr/include/printf.h. Can't I use any of those functions to do it right and ease my job at the same time?

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• Generating link-time error for deprecated functions

  - by R..

  Is there a way with gcc and GNU binutils to mark some functions such that they will generate an error at link-time if used? My situation is that I have some library functions which I am not removing for the sake of compatibility with existing binaries, but I want to ensure that no newly-compiled binary tries to make use of the functions. I can't just use compile-time gcc attributes because the offending code is ignoring my headers and detecting the presence of the functions with a configure script and prototyping them itself. My goal is to generate a link-time error for the bad configure scripts so that they stop detecting the existence of the functions.

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• Gnuplot, yerrorbar, text as x axis

  - by The Flying Dutchman

  I have the following data: t4.8k 1.84 1.86 1.83 t5.8k 1.82 1.84 1.8 t7.10k 1.79 1.8 1.77 t8.8k 1.8 1.84 1.76 I need to plot this in GNU plot using yerror bars. Column1 - dataset name. This is the xaxis scale. Column2 - Y-Mean Column3 - Y-Max Column4 - Y-Min Here is the plotting code that I use: plot "chameleonConfidence.dat" using xtic(1):2:4:3 title "Ratio of Time Taken" with yerrorbars But this gives me the following error Warning: empty x range [4.94066e-324:4.94066e-324], adjusting to [4.94066e-324:4.94066e-324] "chameleonConfidence.gplot", line 15: x_min should not equal x_max! Can someone help me with this?

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• Running script constantly in background: daemon, lock file with crontab, or simply loop?

  - by Mauritz Hansen

  I have a Perl script that queries a database for a list of files to process processes the files and then exits Upon startup this script creates a file (let's say script.lock), and upon exit it removes this file. I have a crontab entry that runs this script every minute. If the lockfile exists then the script exits, assuming that another instance of itself is running. The above process works fine but I am not very happy with the robustness of this approach. Specifically, if for some reason the script exits prematurely and the lockfile is not removed then a new instance will not execute properly. I would appreciate some advice on the following: Is using the lock file a good approach or is there a better/more robust way to do this? Is using crontab for this a good idea or could I better write an endless loop with sleep()? Should I use the GNU 'daemon' program or the Perl Proc::Daemon module (or some other equivalent) for this?

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• a macro question for c language (#define)

  - by Daniel

  I am reading source code of hoard memory allocator, and in the file of gnuwrapper.cpp, there are the following code #define CUSTOM_MALLOC(x) CUSTOM_PREFIX(malloc)(x) What's the meaning of CUSTOM_PREFIX(malloc)(x)? is CUSTOM_PREFIX a function? But as a function it didn't defined anywhere. If it's variable, then how can we use variable like var(malloc)(x)? more code: #ifndef __GNUC__ #error "This file requires the GNU compiler." #endif #include <string.h> #include <stdlib.h> #include <stdio.h> #include <malloc.h> #ifndef CUSTOM_PREFIX ==> here looks like it's a variable, so if it doesn't define, then define here. #define CUSTOM_PREFIX #endif #define CUSTOM_MALLOC(x) CUSTOM_PREFIX(malloc)(x) ===> what's the meaning of this? #define CUSTOM_FREE(x) CUSTOM_PREFIX(free)(x) #define CUSTOM_REALLOC(x,y) CUSTOM_PREFIX(realloc)(x,y) #define CUSTOM_MEMALIGN(x,y) CUSTOM_PREFIX(memalign)(x,y)

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• read after lseek always return 0

  - by Wins

  I'm having an issue reading byte in file after performing lseek long cur_position = lseek(fd, length * -1, SEEK_CUR ); // Rewind file reading by length bytes. printf("cur_position: %i\n", cur_position); int num_byte = read(fd, &byte, 1); printf("num_byte = %i\n", num_byte); With the initial variable length set to 34, the above code would produce cur_position 5 (so there are definitely at least 34 bytes after the lseek function returns), but the variable num_byte returned from function read always returns 0 even though there are still more bytes to read. Does anyone know the reason num_byte always return 0 or do I make mistake in the above code? Just for information, the above code was run on the following machine $ uname -srvpio Linux 3.2.0-24-generic #39-Ubuntu SMP Mon May 21 16:52:17 UTC 2012 x86_64 x86_64 GNU/Linux

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• An increase to 3 Gig of RAM slows down Ubuntu 10.04 LTS

  - by williepabon

  I have Ubuntu 10.04 running from an external hard drive (installed on an enclosure) connected via USB port. Like a month or so ago, I increased RAM on my pc from 2 Gigs to 3 Gigs. This resulted on extremely long boot times and slow application loads. While I was understanding the nature of my problem, I posted various threads on this forum ( Questions # 188417, 188801), where I was advised to gather speed tests, and other info on my machine. I was also suggested that I might have problems with the RAM installed. Initially, I did not consider that possibility because: 1) I did a memory test with a diagnostic program from DELL (My pc is from Dell) 2) My pc works fine with Windows XP (the default OS), no problems with memory 3) My pc works fine when booting with Ubuntu 10.10 memory stick, no speed problems 4) My pc works fine when booting with Ubuntu 11.10 memory stick, no speed problems Anyway, I performed the memory tests suggested. But before doing it, and to check out any possibility of hardware issues on the hard drive, I did the following: (1) purchased a new hard drive enclosure and moved my hard drive to it, (2) purchased a new USB cable and used it to connect my hard drive/enclosure setup to a different USB port on my pc. Then, I performed speed tests with 1 Gig, 2 Gigs and 3 Gigs of RAM with my Ubuntu 10.04 OS. Ubuntu 10.04 worked well when booted with 1 Gig or 2 Gigs of RAM. When I increased to 3 Gigs, it slowed down to a crawl. I can't understand the relationship between an increase of 1 Gig and the effect it has in Ubuntu 10.04. This doesn't happen with Ubuntu 10.10 and 11.10. Unfortunately for me, Ubuntu 10.04 is my principal work operating system. So, I need a solution for this. Hardware and system information: DELL Precision 670 2 internal SATA Hard drives Audigy 2 ZS audio system Factory OS: Windows XP Professional SP3 NVidia 8400 GTS video card More info: williepabon@WP-WrkStation:~$ uname -a Linux WP-WrkStation 2.6.32-38-generic #83-Ubuntu SMP Wed Jan 4 11:13:04 UTC 2012 i686 GNU/Linux williepabon@WP-WrkStation:~$ lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 10.04.4 LTS Release: 10.04 Codename: lucid Speed test with the 3 Gigs of RAM installed: williepabon@WP-WrkStation:~$ sudo hdparm -tT /dev/sdc [sudo] password for williepabon: /dev/sdc: Timing cached reads: 84 MB in 2.00 seconds = 41.96 MB/sec Timing buffered disk reads: 4 MB in 3.81 seconds = 1.05 MB/sec This is a very slow transfer rate from a hard drive. I will really appreciate a solution or a work around for this problem. I know that that there are users that have Ubuntu 10.04 with 3 Gigs or more of RAM and they don't have this problem. Same question asked on Launchpad for reference.

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