C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Segmentation faultwhy???          C++Talk.NET Forum Index -> C++ Language (Moderated) View previous topic :: View next topic   Author Message Cristian Guest Posted: Wed May 10, 2006 12:21 am    Post subject: Segmentation faultwhy??? OGGETTO: Segmentation fault…why??? I work on Linux system Fedora Core 5 x86_64 Why this ( you see under) program doesn't work ? The program interrupts him visualizing as error: segmentation fault I have also made tests on other bases (Ubuntu) but I have always met the same problem It perfectly works if I reduce the dimension of dim1 to 1000 !!!!!! On WindowsXP32Bit it works ok also for dim1=dim2=10000 for example. #include <vector> #include <iostream> #include <cmath> #include <string> #include <time.h> using namespace std; int main( ) { long i, j ; long dim1=10000, dim2=1000 ; double m[dim1][dim2]; for ( i=0,n=0 ; i < dim1 ; i++ ) { for ( j=0 ; j < dim2 ; j++) { m[i][j] = (double) ( (j+1) + 3.0 *1/(j+1)*(j+1)*(j+1)*(j+1) + 50* (i+1) ); } } cout << "v(" << 100 << "," << 99 << ")=" << m[99][98] << endl; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top red floyd Guest Posted: Wed May 10, 2006 1:21 pm    Post subject: Re: Segmentation faultwhy??? Cristian wrote: Quote: OGGETTO: Segmentation fault…why??? I work on Linux system Fedora Core 5 x86_64 Why this ( you see under) program doesn't work ? The program interrupts him visualizing as error: segmentation fault We've told you already. You're probably overflowing your stack, by putting 8MB (or 80MB for the 10000 case) onto the stack. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top rolkA Guest Posted: Wed May 10, 2006 7:21 pm    Post subject: Re: Segmentation faultwhy??? Cristian <cristian (AT) yahoo (DOT) com> wrote in news:4t3162psocq5j0268s080dnvs6n7rtjcht (AT) 4ax (DOT) com: Quote: OGGETTO: Segmentation fault why??? I work on Linux system Fedora Core 5 x86_64 Why this ( you see under) program doesn't work ? The program interrupts him visualizing as error: segmentation fault My guess is that you use too memory for the stack. Try using the heap instead by: - using a std::vector - or allocating dynamically -- Sam / rolkA [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Wed May 10, 2006 9:21 pm    Post subject: Re: Segmentation faultwhy??? Cristian wrote: Quote: OGGETTO: Segmentation fault?why??? I work on Linux system Fedora Core 5 x86_64 Why this ( you see under) program doesn't work ? The program interrupts him visualizing as error: segmentation fault I have also made tests on other bases (Ubuntu) but I have always met the same problem It perfectly works if I reduce the dimension of dim1 to 1000 !!!!!! Which strongly suggests that you are exceeding the stack limits on your machine. You did set the stack limits high enough, didn't you? ("ulimit -s unlimited" on most Unix and Unix-look-alikes). Quote: On WindowsXP32Bit it works ok also for dim1=dim2=10000 for example. #include <vector #include <iostream #include <cmath #include <string #include <time.h using namespace std; int main( ) { long i, j ; long dim1=10000, dim2=1000 ; double m[dim1][dim2]; Which fails to compile with a C++ compiler -- array dimensions must be compile time constants. (G++ is not, by default, a C++ compiler. For it to even attempt to compile C++, you need the -pendantic -std=c++98 options. And when those are given, the code doesn't compile.) Quote: for ( i=0,n=0 ; i < dim1 ; i++ ) { My compilers all complain about n not being defined here as well. In C++, it would be far more idiomatic to not declare i (and j) before the loop, but rather in the for statement, e.g.: for ( size_t i = 0 ; i < dim1 ; ++ i ) { for ( size_t j = 0 ; j < dim2 ; ++ j ) { // ... Quote: for ( j=0 ; j < dim2 ; j++) { m[i][j] = (double) ( (j+1) + 3.0 *1/(j+1)*(j+1)*(j+1)*(j+1) + 50* (i+1) ); } } cout << "v(" << 100 << "," << 99 << ")=" << m[99][98] << endl; } After declaring dim1 and dim2 as long, and removing the initialization of n -- and, of course, setting ulimit to an unlimited stack size, it works on every machine I have access to. Still... I'd be very leary of using variables of that size on the stack. The problem is that stack overflow is undefined behavior -- you can't do much about it. If you used dynamic allocation (e.g. std::vector), you will get an error you can trap and handle if there is not enough memory. (Supposing your OS supports this -- in the past, many Linux were delivered with support for a working malloc/operator new turned off.) -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Sebastian Redl Guest Posted: Wed May 10, 2006 9:21 pm    Post subject: Re: Segmentation faultwhy??? Cristian wrote: Quote: #include <vector #include <iostream #include <cmath #include <string #include <time.h Why not <ctime>? Quote: using namespace std; int main( ) { long i, j ; long dim1=10000, dim2=1000 ; double m[dim1][dim2]; Declaring an array with non-const bounds is invalid and does not compile. In addition, if it did compile, the array would be (assuming an 8-byte double, as is typical) 80 megabytes large, and allocated on the stack. Many operating systems will not allow this, including Windows and, I think, Linux. Quote: for ( i=0,n=0 ; i < dim1 ; i++ ) { n is not declared. It's not used, either. Quote: for ( j=0 ; j < dim2 ; j++) { m[i][j] = (double) ( (j+1) + 3.0 *1/(j+1)*(j+1)*(j+1)*(j+1) + 50* (i+1) ); No need for the cast (and it should be a static_cast anyway), but you should really think about operator precedence and types within the expression here. In particular, the entire middle expression will always yield 0. In conclusion, I suspect the segfault is a stack overflow. I'm really surprised that it runs in Windows, as you say it does. Aside from that, there's still a lot of stuff wrong. -- Sebastian Redl [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Gene Bushuyev Guest Posted: Wed May 10, 2006 9:21 pm    Post subject: Re: Segmentation faultwhy??? "Cristian" <cristian (AT) yahoo (DOT) com> wrote in message news:4t3162psocq5j0268s080dnvs6n7rtjcht (AT) 4ax (DOT) com... Quote: OGGETTO: Segmentation fault.why??? I work on Linux system Fedora Core 5 x86_64 Why this ( you see under) program doesn't work ? The program interrupts him visualizing as error: segmentation fault I have also made tests on other bases (Ubuntu) but I have always met the same problem It perfectly works if I reduce the dimension of dim1 to 1000 !!!!!! On WindowsXP32Bit it works ok also for dim1=dim2=10000 for example. #include <vector #include <iostream #include <cmath #include <string #include <time.h using namespace std; int main( ) { long i, j ; long dim1=10000, dim2=1000 ; double m[dim1][dim2]; C++ doesn't have dynamic arrays; dim1 and dim2 must be constants. Assuming that's fixed, the above line tries to create about 80MB stack frame. On Windows stack size is defined by linker. Check the linker options for that. It can also be changed by EDITBIN /STACK utility. On Unix, check unlimit/limit commands. Other platforms may have other ways of controlling program stack, you need to read documentation. Quote: From a practical standpoint, it's not a good idea to make stack too big. It's better for performance if your program stack can fully fit into a processor cache. Instead use heap for large objects: double* m = new double[dim1 * dim2]; -- Gene Bushuyev (www.gbresearch.com) ---------------------------------------------------------------- There is no greatness where there is no simplicity, goodness and truth. ~ Leo Tolstoy [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top ThosRTanner Guest Posted: Wed May 10, 2006 9:21 pm    Post subject: Re: Segmentation faultwhy??? Cristian wrote: Quote: OGGETTO: Segmentation fault...why??? I work on Linux system Fedora Core 5 x86_64 Why this ( you see under) program doesn't work ? The program interrupts him visualizing as error: segmentation fault Knowing /where/ your program segfaulted would be helpful - debugging the resulting core file and doing 'where' would show you which line. Quote: I have also made tests on other bases (Ubuntu) but I have always met the same problem It perfectly works if I reduce the dimension of dim1 to 1000 !!!!!! On WindowsXP32Bit it works ok also for dim1=dim2=10000 for example. #include <vector #include <iostream #include <cmath #include <string #include <time.h using namespace std; int main( ) { long i, j ; long dim1=10000, dim2=1000 ; double m[dim1][dim2]; That is a flipping big array. 10 million doubles is around 80Mb. I'd reckon you have a stack overflow going on here. I'm not sure what the default stack size is on linux, but I'd be surprised if it was THAT big. Quote: for ( i=0,n=0 ; i < dim1 ; i++ ) { for ( j=0 ; j < dim2 ; j++) { m[i][j] = (double) ( (j+1) + 3.0 *1/(j+1)*(j+1)*(j+1)*(j+1) + 50* (i+1) ); } } cout << "v(" << 100 << "," << 99 << ")=" << m[99][98] << endl; } [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top eden Guest Posted: Wed May 10, 2006 9:21 pm    Post subject: Re: Segmentation faultwhy??? Cristian wrote: Quote: OGGETTO: Segmentation fault...why??? I work on Linux system Fedora Core 5 x86_64 Why this ( you see under) program doesn't work ? The program interrupts him visualizing as error: segmentation fault I have also made tests on other bases (Ubuntu) but I have always met the same problem It perfectly works if I reduce the dimension of dim1 to 1000 !!!!!! On WindowsXP32Bit it works ok also for dim1=dim2=10000 for example. [...] long dim1=10000, dim2=1000 ; double m[dim1][dim2]; I am not sure about the segmentation fault, but It is interesting how you could dimension the array that way, without constant expresions. Goran [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Eugene Alterman Guest Posted: Thu May 11, 2006 1:21 pm    Post subject: Re: Segmentation faultwhy??? "Cristian" <cristian (AT) yahoo (DOT) com> wrote in message news:4t3162psocq5j0268s080dnvs6n7rtjcht (AT) 4ax (DOT) com... Quote: long dim1=10000, dim2=1000 ; double m[dim1][dim2]; This is not a valid C++ code. C++ does not support variable size arrays as C99 does. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Cristian Guest Posted: Fri May 12, 2006 12:21 pm    Post subject: Re: Segmentation faultwhy??? On 11 May 2006 19:09:02 -0400, "kanze" <kanze@gabi-soft.fr> wrote: Quote: Sebastian Redl wrote: Quote: In sum, the expression is (almost) the same as the much simpler: 3.0 * (j+1) * (j+1 ) and the fact that he didn't write it this was suggests that it isn't actually the expression he wanted. I vaguely suspect that what he was looking for was something like: 3.0 / pow( j+1, 4 ) But I'm not really sure -- that's so much simpler than what he wrote as well, it's surprising that he wouldn't use it. Yes, I want to perform the following calculation: 3.0 / pow( j+1, 4 ) The problem is however that I come from the windows'world and in windows the use of pow() provoked me notable decelerations and at times problems of overflow on these calculations (great matrixes. Dimensions: 20000 x 20000 double) In windows the management of the stack is probably different considering that up to coarsely 10000x10000 I succeeded in compiling without problems. I have resolved using std::vector The problem is that using const long dim1=...,dim2=...; <vector<vector< double>> m(dim1,vector<double>(dim2) ) I get strange behaviors as it regards the speed : for example: m--> 10^4 * 10^4 run-time of the calculation: 3 sec "-> 20^4 * 10^4 " : more than 100 sec Why? in windows I get a doubling of the run-time instead in reference to the said example above Thanks to every all Cristian [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top kanze Guest Posted: Sat May 13, 2006 1:21 am    Post subject: Re: Segmentation faultwhy??? Cristian wrote: Quote: On 11 May 2006 19:09:02 -0400, "kanze" <kanze@gabi-soft.fr> wrote: Sebastian Redl wrote: In sum, the expression is (almost) the same as the much simpler: 3.0 * (j+1) * (j+1 ) and the fact that he didn't write it this was suggests that it isn't actually the expression he wanted. I vaguely suspect that what he was looking for was something like: 3.0 / pow( j+1, 4 ) But I'm not really sure -- that's so much simpler than what he wrote as well, it's surprising that he wouldn't use it. Yes, I want to perform the following calculation: 3.0 / pow( j+1, 4 ) The problem is however that I come from the windows'world and in windows the use of pow() provoked me notable decelerations and at times problems of overflow on these calculations (great matrixes. Dimensions: 20000 x 20000 double) Curious. I would expect the speed to be roughly the same, and I certainly cannot see the results overflowing unless you were picking up a pow(int,int) as an extension; casting the first argument to double before the call should have fixed that. (In earlier times, floating point on Intel machines was very slow, and offering such functions as an extension made sense. Today, I think the Intel processors are like most others, with floating point as fast, if not faster, than integral arithmetic.) Quote: In windows the management of the stack is probably different It is. But I don't know too much about it. Quote: considering that up to coarsely 10000x10000 I succeeded in compiling without problems. I have resolved using std::vector For a two dimensional array, I'm not sure that that is the ideal solution -- unless you wrap it in a class which allows [][]. (The first [] is a member of the class, and returns a helper object -- double* actually works fine -- which supports the second [].) Quote: The problem is that using const long dim1=...,dim2=...; vector<vector< double>> m(dim1,vector<double>(dim2) ) I get strange behaviors as it regards the speed : I can imagine -- for a 20000x20000 matrice, you're doing 200001 allocations (plus the initializations). Quote: for example: m--> 10^4 * 10^4 run-time of the calculation: 3 sec "-> 20^4 * 10^4 " : more than 100 sec Why? Well, you've doubled the number of allocations. In all likely hood, you've stumbled across some undocumented internal boundary which has led to the initialisations not being more or less contiguous, have ended up with bad locality, and have started paging. Quote: in windows I get a doubling of the run-time instead in reference to the said example above It depends on a lot of external factors, over which you have very little control. Using std::vector is, per se, a good idea, but I'd allocate a single large array, and do the vector arithmetic myself. Something like: class Matrix { public: Matrix( long dim1, long dim2 ) : myDim1( dim1 ) , myDim2( dim2 ) , myData( dim1 * dim2 ) { } double* operator[]( long i ) { return &myData[ 0 ] + (j * myDim1) ; } // ... private: long myDim1 ; long myDim2 ; std::vector< double > myData ; } -- James Kanze GABI Software Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Cristian Guest Posted: Sat May 13, 2006 12:21 pm    Post subject: Re: Segmentation faultwhy??? On 12 May 2006 07:47:23 -0400, Cristian <cristian (AT) yahoo (DOT) com> wrote Quote: I get strange behaviors as it regards the speed : for example: m--> 10^4 * 10^4 run-time of the calculation: 3 sec "-> 20^4 * 10^4 " : more than 100 sec There is an error, I apologize. The second calculation is obviously on a matrix of dimensions : dim1(= 2* 10^4 ) * dim2 (=10^4) Thanks Cristian [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Otis Bricker Guest Posted: Wed May 17, 2006 2:21 am    Post subject: Re: Segmentation faultwhy??? "kanze" <kanze@gabi-soft.fr> wrote in news:1147446737.039642.306680 @j33g2000cwa.googlegroups.com: Quote: for example: m--> 104 * 104 run-time of the calculation: 3 sec "-> 204 * 104 " : more than 100 sec Why? Well, you've doubled the number of allocations. In all likely hood, you've stumbled across some undocumented internal boundary which has led to the initialisations not being more or less contiguous, have ended up with bad locality, and have started paging. Unless I am missing some of the math here, 2*104 * 104 - 2*108. So for doubles(8 bytes) that would require 1.6*109 Unless your machine has >2G I expct you will be have started swapping to the pagefile on disk. Otis [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ Language (Moderated) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.