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• Why do debug symbols so adversely affect the performance of threaded applications on Linux?

  - by fluffels

  Hi. I'm writing a ray tracer. Recently, I added threading to the program to exploit the additional cores on my i5 Quad Core. In a weird turn of events the debug version of the application is now running slower, but the optimized build is running faster than before I added threading. I'm passing the "-g -pg" flags to gcc for the debug build and the "-O3" flag for the optimized build. Host system: Ubuntu Linux 10.4 AMD64. I know that debug symbols add significant overhead to the program, but the relative performance has always been maintained. I.e. a faster algorithm will always run faster in both debug and optimization builds. Any idea why I'm seeing this behavior?

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• Windows based development for ARM processors

  - by user367231

  I am a complete newbie to the ARM world. I need to be able to write C code, compile it, and then download into an ARM emulator, and execute. I need to use the GCC 4.1.2 compiler for the C code compilation. Can anybody point me in the correct directions for the following issues? What tool chain to use? What emulator to use? Are there tutorials or guides on setting up the tool chain?

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• 'SHA1' is deprecated: first deprecated in OS X 10.7?

  - by sukhvir

  So I was trying to compile a code which has a SHA1 function .. I included following header: #include <openssl/sha.h> And I got the following error while compiling: test.c:9:5: error: 'SHA1' is deprecated: first deprecated in OS X 10.7 [-Werror,-Wdeprecated-declarations] SHA1(msg, strlen(msg), hs); ^ But man pages still have the descriptions for that function. Can anyone suggest any other header for a similar function ( MD5 or SHA1 )? PS - also do I need to link any libraries while compiling using gcc?

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• Constructing a function call in C

  - by 0x6adb015

  Given that I have a pointer to a function (provided by dlsym() for example) and a linked list of typed arguments, how can I construct a C function call with those arguments? Example: struct param { enum type { INT32, INT64, STRING, BOOL } type; union { int i32; long long i64; char *str; bool b; } value; struct param *next; }; int call_this(int (*function)(), struct param *args) { int result; /* magic here that calls function(), which has a prototype of f(int, long long, char *, bool); , when args consist of a linked list of INT32, INT64, STRING, BOOL types. */ return result; } The OS is Linux. I would like the solution to be portable across MIPS, PPC and x86 (all 32 bits) architecture, using GCC as the compiler. Thanks!

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• Libxml2: undefined reference to xmlTextReaderConstName

  - by Dmitry

  I have installed the latest libxml2-2.8.0, as usual: $ ./configure, $ make, $ make install. The $ xml2-config --cflags --libs gives this output: -I/usr/local/include/libxml2 -L/usr/local/lib -lxml2 -lm But trying to compile any example... $ gcc `xml2-config --cflags --libs` xmltest.c The linker says: /tmp/cc8ezrPl.o: In function `processNode': xmltest.c:(.text+0x19): undefined reference to `xmlTextReaderConstName' xmltest.c:(.text+0x38): undefined reference to `xmlTextReaderConstValue' ...etc. Anything I've googled can be solved by xml2-config --cflags --libs flags, or upgrading to the latest version of libxml2, or something. Unfortunately, neither works for me. What can be the steps to identify the problem? Using Ubuntu 12.04 64-bit.

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• Makefile automatic link dependency ?

  - by Kuang Chen

  It's easy to let program figure out the dependency at compile time, (with gcc -MM). Nevertheless, link dependency (deciding which libraries should be linked to) seems to be difficult to figure out. This issue become emergent when multiple targets with individual libraries to link to are needed. For instance, three dynamic library targets t1.so, t2.so and t3.so needs to be built. t1.so needs math library (-lm), while t2 and t3 don't. It would be tedious to write separate rules. A single rule requiring the three targets linked with math library saves the trouble. However, it causes inflation of target size since math library is unused for t2.so and t3.so. Any ideas?

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• C/C++: Scanning a TIFF file using LIBTIFF

  - by Matt07

  My problem is to scan a tiff image in C and get all the pixel value (let's say for saving them in a txt file) in C/C++. I scanned the web and i found a library named "TIFFLIB" that should do what i was looking for. I downloaded it using the ubuntu package manager, but the gcc doesn't recognize the library. How do i link the library to the compiler? Have I installed it correctly? Is there any better/easier way to do that?

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• how to use static function in header and compare with float array

  - by ed k

  I wrote this function: static bool colorIsEmpty(const Color col) { return (col[0] == 0 && col[1] == 0 && col[2] == 0 ); } where Color is simply a float[3]; the function doesn't work if col[3] are all 0; but this works: if(col[0] == col[1] == col[2] == 0) { //gets called } however gcc gives me warning: cColorTest.c:212:5: warning: suggest parentheses around comparison in operand of ‘==’ [-Wparentheses] so it would be nice if that function works,why it doesn't work?

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• Why doesn't a 32bit .deb package install on 64bit Ubuntu?

  - by codebox_rob

  My .deb package, built on 32-bit Ubuntu and containing executables compiled with gcc, won't install on the 64-bit version of the OS (the error message says 'Wrong architecture i386'). This is confusing to me because I thought that in general 32-bit software worked on 64-bit hardware, but not vice-versa. Will it be possible for me to produce a .deb file that I can install on a 64-bit OS, using my 32-bit machine? Is it just a matter of using the appropriate compiler flags to produce the executables (and if so what are they), or is the .deb file itself somehow specific to one processor architecture?

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• Bug in malloc or mine?

  - by Martin

  Is this my bug or a bug/assertion fail in malloc itself? alloc.c:2451: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)-bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) = (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)-size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed. libstdc++6:amd64 4.7.2-2ubuntu1 gcc 4.7.2 ubuntu 12.10/64bit

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• Same memory space being allocated again & again while using malloc()

  - by shadyabhi

  In each loop iteration, variable j is declared again and again. Then why is its address remaining same? Shouldn't it be given some random address each time? Is this compiler dependent? #include<stdio.h> #include<malloc.h> int main() { int i=3; while (i--) { int j; printf("%p\n", &j); } return 0; } Testrun:- shadyabhi@shadyabhi-desktop:~/c$ gcc test.c shadyabhi@shadyabhi-desktop:~/c$ ./a.out 0x7fffc0b8e138 0x7fffc0b8e138 0x7fffc0b8e138 shadyabhi@shadyabhi-desktop:~/c$

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• Referencing invalid memory locations with C++ Iterators

  - by themoondothshine

  I am a big fan of GCC, but recently I noticed a vague anomaly. Using __gnu_cxx::__normal_iterator (ie, the most common iterator type used in libstdc++, the C++ STL) it is possible to refer to an arbitrary memory location and even change its value without causing an exception! Is this expected behavior? If so, isn't a security loophole? Here's an example: #include <iostream> using namespace std; int main() { basic_string<char> str("Hello world!"); basic_string<char>::iterator iter = str.end(); iter += str.capacity() + 99999; *iter = 'x'; cout << "Value: " << *iter << endl; }

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• Does a c/c++ compiler optimize constant divisions by power-of-two value into shifts?

  - by porgarmingduod

  Question says it all. Does anyone know if the following... size_t div(size_t value) { const size_t x = 64; return value / x; } ...is optimized into? size_t div(size_t value) { return value >> 6; } Do compilers do this? (My interest lies in GCC). Are there situations where it does and others where it doesn't? I would really like to know, because every time I write a division that could be optimized like this I spend some mental energy wondering about whether precious nothings of a second is wasted doing a division where a shift would suffice.

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• Same memory space being allocated again & again

  - by shadyabhi

  In each loop iteration, variable j is declared again and again. Then why is its address remaining same? Shouldn't it be given some random address each time? Is this compiler dependent? #include<stdio.h> #include<malloc.h> int main() { int i=3; while (i--) { int j; printf("%p\n", &j); } return 0; } Testrun:- shadyabhi@shadyabhi-desktop:~/c$ gcc test.c shadyabhi@shadyabhi-desktop:~/c$ ./a.out 0x7fffc0b8e138 0x7fffc0b8e138 0x7fffc0b8e138 shadyabhi@shadyabhi-desktop:~/c$

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• Best environment to port C/C++ code from Linux to Windows.

  - by Simone Margaritelli

  I'd like to make a big project of mine buildable on Windows platforms. The project itself it's written in C/C++ following POSIX standards, with some library dependencies such as libxml2, libcurl and so on. I'm more a Linux developer rather than a Windows developer, so i have no idea of which compiler suite i should use to port the code. Which one offers more compatibility with gcc 4.4.3 i'm using right now? My project needs flex and bison, is there any "ready to use" environment to port such projects to windows platforms? Thanks.

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• How to list library dependencies of a non-native binary?

  - by lvella

  When developing for native platform, I can use ldd to list all the shared libraries (.so files) a binary executable I build will try to load upon start-up. But when cross-compiling, I don't know how to get the same information. The ldd is not a normal binutils utility, like strip or ar, that can be built alongside gcc for cross compiling, but instead, it is a cryptic shell script that apparently can only run on native platform. So, using the cross-target binutils tools, is there any way to get a list of the dynamically linked dependency for of a foreign binary?

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• Eliminating inherited overlong MACRO

  - by ExpatEgghead

  I have inherited a very long set of macros from some C algorithm code.They basically call free on a number of structures as the function exits either abnormally or normally. I would like to replace these with something more debuggable and readable. A snippet is shown below #define FREE_ALL_VECS {FREE_VEC_COND(kernel);FREE_VEC_COND(cirradCS); FREE_VEC_COND(pixAccum)..... #define FREE_ALL_2D_MATS {FREE_2D_MAT_COND(circenCS); FREE_2D_MAT_COND(cirradCS_2); } #define FREE_ALL_IMAGES {immFreeImg(&imgC); immFreeImg(&smal..... #define COND_FREE_ALLOC_VARS {FREE_ALL_VECS FREE_ALL_2D_MATS FREE_ALL_IMAGES} What approach would be best? Should I just leave well alone if it works? This macro set is called twelve times in one function. I'm on Linux with gcc.

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• Is this a valid C statement ?

  - by Philando Gullible

  Lets say I write char c[99] = {'Stack Overflow'}; in C or C++ it does compiles fine but does this valid? By valid I meant not invoking any kind of undefined or unspecified behavior. Again if I write char c[99] = 'Stack Overflow'; gcc complains about multicharacter constant which is obvious but in the above when I am enclosing within curly brackets compiler is happy! why is it so ? I also notice that puts(c); after the first statement will output 'w' precisely the last character of a general string in-place of Stack Overflow. why so ? Could somebody explain this behavior may be separately.

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• warning: '0' flag ignored with precision and ‘%i’ gnu_printf format

  - by morpheous

  I am getting the following warning when compiling some legacy C code on Ubuntu Karmic, using gcc 4.4.1 The warning is: src/filename.c:385: warning: '0' flag ignored with precision and ‘%i’ gnu_printf format The snippet which causes the warning to be emitted is: char buffer[256] ; long fnum ; /* some initialization code here ... */ sprintf(buffer, "F%03.3i.DTA", (int)fnum); /* <- warning emitted here */ I think I understand the warning, but I would like to check in here to see if I am right, and also the (definite) correct way of resolving this.

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• Linking to MSVC DLL from MinGW

  - by IndigoFire

  I'm trying to link the LizardTech GeoExpress DSDK into my own application. I use gcc so that we can compile on for platforms. On Linux and Mac this works easily: they provide a static library (libltidsdk.a) and headers and all that we have to do is use them. Compiling for windows isn't so easy. They've built the library using Microsoft Visual Studio, and we use MinGW. I've read the MinGW FAQ, and I'm running into the problems below. The library is all C++, so my first question: is this even possible? Just linking against the dll as provided yields "undefined reference" errors for all of the C++ calls (constructors, desctructors, methods, etc). Based on the MinGW Wiki: http://www.mingw.org/wiki/MSVC%5Fand%5FMinGW%5FDLLs I should be able to use the utility reimp to convert a .lib into something useable. I've tried all of the .lib files provided by LizardTech, and they all give "invalid or corrupt import library". I've tried both version 0.4 and 0.3 of the reimp utility. Using the second method described in the wiki, I've run pexport and dlltool over the dll to get a .a archive, but that produces the same undefined references. BTW: I have read the discussion below. It left some ambiguity as to whether this is possible, and given the MinGW Wiki page it seems like this should be doable. If it is impossible, that's all I need to know. If it can be done, I'd like to know how I can get this to happen. stackoverflow.com/questions/1796209/how-to-link-to-vs2008-generated-libs-from-g Thanks!

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• What is weird about wrapping setjmp and longjmp?

  - by Max

  Hello. I am using setjmp and longjmp for the first time, and I ran across an issue that comes about when I wrap setjmp and longjmp. I boiled the code down to the following example: #include <stdio.h> #include <setjmp.h> jmp_buf jb; int mywrap_save() { int i = setjmp(jb); return i; } int mywrap_call() { longjmp(jb, 1); printf("this shouldn't appear\n"); } void example_wrap() { if (mywrap_save() == 0){ printf("wrap: try block\n"); mywrap_call(); } else { printf("wrap: catch block\n"); } } void example_non_wrap() { if (setjmp(jb) == 0){ printf("non_wrap: try block\n"); longjmp(jb, 1); } else { printf("non_wrap: catch block\n"); } } int main() { example_wrap(); example_non_wrap(); } Initially I thought example_wrap() and example_non_wrap() would behave the same. However, the result of running the program (GCC 4.4, Linux): wrap: try block non_wrap: try block non_wrap: catch block If I trace the program in gdb, I see that even though mywrap_save() returns 1, the else branch after returning is oddly ignored. Can anyone explain what is going on?

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• How does one properly redefine self while avoiding the "Multiple methods named ..." warning?

  - by Elise van Looij

  In Apple's The Objective-C Programming Language: Defining a Class the section named "Redifining self" recommends that that class methods allocate and return instances use 'self' only to allocate an instance and then refer only to that instance. Thus, I have a number of subclasses, that have class methods along the lines of: + (id)scrollViewWithFrame: (NSRect)rectFrame { id newInstance = [[[self alloc] initWithFrame:rectFrame] autorelease]; [newInstance setHasHorizontalScroller: YES]; [newInstance setHasVerticalScroller: YES]; [newInstance setBorderType: NSNoBorder]; [newInstance setAutoresizingMask: (NSViewWidthSizable | NSViewHeightSizable)]; return newInstance; } The above is, of course, a subclass of NSScrollView. Unfortunately, Xcode 3.x all these NSView subclasses now raise warnings: "Warning: Multiple methods named '-setAutoresizingMask' found". I believe it has something to do with GCC 4.2, the settings of which I have not changed. The warning is correct, of course, since NSView and its various subclasses all implement setAutoresizingMask, but it is also unnecessary. Since they're only warnings, I ignore them but there is a risk that in between the thirty or so unnecessary ones, a really useful warning lurks which I simply don't see. So, what to do? I do want to adhere to good coding practices and I want to build warning-free apps -- how can I do both?

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• Trying to compile x264 and ffmpeg for iPhone - "missing required architecture arm in file"

  - by jtrim

  I'm trying to compile x264 for use in an iPhone application. I see there are instructions on how to compile ffmpeg for use on the platform here: http://lists.mplayerhq.hu/pipermail/ffmpeg-devel/2009-October/076618.html , but I can't seem to find anything this complete for compiling x264 on the iPhone. I've found this source tree: http://gitorious.org/x264-arm that seems to have support for the ARM platform. Here is my config line: ./configure --cross-prefix=/usr/bin/ --host=arm-apple-darwin10 --extra-cflags="-B /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS3.2.sdk/usr/lib/ -I /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS3.2.sdk/usr/lib/" ...and inside configure I'm using the gas-preprocessor script (first link above) as my assembler: gas-preprocessor.pl gcc When I start compiling, it chunks away for a little while, then it spits out these warnings and a huge list of undefined symbols: ld: warning: option -s is obsolete and being ignored ld: warning: -force_cpusubtype_ALL will become unsupported for ARM architectures ld: warning: in /usr/lib/crt1.o, missing required architecture arm in file ld: warning: in /usr/X11R6/lib/libX11.dylib, missing required architecture arm in file ld: warning: in /usr/lib/libm.dylib, missing required architecture arm in file ld: warning: in /usr/lib/libpthread.dylib, missing required architecture arm in file ld: warning: in /usr/lib/libgcc_s.1.dylib, missing required architecture arm in file ld: warning: in /usr/lib/libSystem.dylib, missing required architecture arm in file Undefined symbols: My guess would be that the problem has to do with the "missing required architecture arm in file" warning...any ideas?

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• C malloc assertion help

  - by Chris

  I am implementing a divide and conquer polynomial algorithm so i can bench it against an opencl implementation, but i can't seem to get malloc to work. When I run the program it allocates a bunch of stuff, checks some things, then sends the size/2 to the algorithm. Then when I hit the malloc line again it spits out this: malloc.c:3096: sYSMALLOc: Assertion `(old_top == (((mbinptr) (((char *) &((av)-bins[((1) - 1) * 2])) - __builtin_offsetof (struct malloc_chunk, fd)))) && old_size == 0) || ((unsigned long) (old_size) = (unsigned long)((((__builtin_offsetof (struct malloc_chunk, fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 * (sizeof(size_t))) - 1))) && ((old_top)-size & 0x1) && ((unsigned long)old_end & pagemask) == 0)' failed. Aborted The line in question is: int *out, .....other vars....; out = (int *)malloc(sizeof(int) * size * 2); I have checked size with fprintf and it is a positive int (usually 50 at that point). I have tried calling malloc with a plain number as well and i still get the error. I'm just stumped at what's going on, and nothing from google that I have found so far has been too helpful. Any ideas what's going on? I'm trying to figure out how to compile a newer GCC in case it's a compiler error, but i really doubt it.

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• Compile and optimize for different target architectures

  - by Peter Smit

  Summary: I want to take advantage of compiler optimizations and processor instruction sets, but still have a portable application (running on different processors). Normally I could indeed compile 5 times and let the user choose the right one to run. My question is: how can I can automate this, so that the processor is detected at runtime and the right executable is executed without the user having to chose it? I have an application with a lot of low level math calculations. These calculations will typically run for a long time. I would like to take advantage of as much optimization as possible, preferably also of (not always supported) instruction sets. On the other hand I would like my application to be portable and easy to use (so I would not like to compile 5 different versions and let the user choose). Is there a possibility to compile 5 different versions of my code and run dynamically the most optimized version that's possible at execution time? With 5 different versions I mean with different instruction sets and different optimizations for processors. I don't care about the size of the application. At this moment I'm using gcc on Linux (my code is in C++), but I'm also interested in this for the Intel compiler and for the MinGW compiler for compilation to Windows. The executable doesn't have to be able to run on different OS'es, but ideally there would be something possible with automatically selecting 32 bit and 64 bit as well. Edit: Please give clear pointers how to do it, preferably with small code examples or links to explanations. From my point of view I need a super generic solution, which is applicable on any random C++ project I have later. Edit I assigned the bounty to ShuggyCoUk, he had a great number of pointers to look out for. I would have liked to split it between multiple answers but that is not possible. I'm not having this implemented yet, so the question is still 'open'! Please, still add and/or improve answers, even though there is no bounty to be given anymore. Thanks everybody!

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