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  • Pointer Implementation Details in C

    - by Will Bickford
    I would like to know architectures which violate the assumptions I've listed below. Also I would like to know if any of the assumptions are false for all architectures (i.e. if any of them are just completely wrong). sizeof(int *) == sizeof(char *) == sizeof(void *) == sizeof(func_ptr *) The in-memory representation of all pointers for a given architecture is the same regardless of the data type pointed to. The in-memory representation of a pointer is the same as an integer of the same bit length as the architecture. Multiplication and division of pointer data types are only forbidden by the compiler. NOTE: Yes I know this is nonsensical. What I mean is - is there hardware support to forbid this incorrect usage? All pointer values can be casted to a single integer. In other words, what architectures still make use of segments and offsets? Incrementing a pointer is equivalent to adding sizeof(the pointed data type) to the memory address stored by the pointer. If p is an int32* then p+1 is equal to the memory address 4 bytes after p. I'm most used to pointers being used in a contiguous, virtual memory space. For that usage, I can generally get by thinking of them as addresses on a number line. See (http://stackoverflow.com/questions/1350471/pointer-comparison/1350488#1350488).

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  • Why does the output look like this?

    - by cjk
    I have a c program below, I would like to send out a 32 bit message in a particular order Eg.0x00000001. #include <unistd.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/types.h> #include <stdint.h> struct test { uint16_t a; uint16_t b; }; int main(int argc, char const *argv[]) { char buf[4]; struct test* ptr=(struct test*)buf; ptr->a=0x0000; ptr->b=0x0001; printf("%x %x\n",buf[0],buf[1]); //output is 0 0 printf("%x %x\n",buf[2],buf[3]); //output is 1 0 return 0; } Then I test it by print out the values in char array. I got output in the above comments. Shouldn't the output be 0 0 and 0 1? since but[3] is the last byte? Is there anything I missed? Thanks!

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  • Why are 32-bit application pools more efficient in IIS? [closed]

    - by mhenry1384
    I've been running load tests with two different ASP.NET web applications in IIS. The tests are run with 5,10,25, and 250 user agents. Tested on a box with 8 GB RAM, Windows 7 Ultimate x64. The same box running both IIS and the load test project. I did many runs, and the data is very consistent. For every load, I see a lower "Avg. Page Time (sec)" and a lower "Avg. Response Time (sec)" if I have "Enable 32-bit Applications" set to True in the Application Pools. The difference gets more pronounced the higher the load. At very high loads, the web applications start to throw errors (503) if the application pools are 64-bit, but they can can keep up if set to 32-bit. Why are 32-bit app pools so much more efficient? Why isn't the default for application pools 32-bit?

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  • How do you return a string from a function correctly in Dynamic C?

    - by aquanar
    I have a program I am trying to debug, but Dynamic C apparently treats strings differently than normal C does (well, character arrays, anyway). I have a function that I made to make an 8 character long (well, 10 to include the \0 ) string of 0s and 1s to show me the contents of an 8-bit char variable. (IE, I give it the number 13, it returns the string "0001101\0" ) When I use the code below, it prints out !{happy face] 6 times (well, the second one is the happy face alone for some reason), each return comes back as 0xDEAE or "!\x02. I thought it would dereference it and return the appropriate string, but it appears to just be sending the pointer and attempting to parse it. This may seem silly, but my experience was actually in C++ and Java, so going back to C brings up a few issues that were dealt with in later programming languages that I'm not entirely sure how to deal with (like the lack of string variables). How could I fix this code, or how would be a better way to do what I am trying to do (I thought maybe sending in a pointer to a character array and working on it from the function might work, but I thought I should ask to see if maybe I'm just trying to reinvent the wheel). Currently I have it set up like this: this is an excerpt from the main() display[0] = '\0'; for(i=0;i<6;i++) { sprintf(s, "%s ", *char_to_bits(buffer[i])); strcat(display, s); } DispStr(8,5, display); and this is the offending function: char *char_to_bits(char x) { char bits[16]; strcpy(bits,"00000000\0"); if (x & 0x01) bits[7]='1'; if (x & 0x02) bits[6]='1'; if (x & 0x04) bits[5]='1'; if (x & 0x08) bits[4]='1'; if (x & 0x10) bits[3]='1'; if (x & 0x20) bits[2]='1'; if (x & 0x40) bits[1]='1'; if (x & 0x80) bits[0]='1'; return bits; } and just for the sake of completion, the other function is used to output to the stdio window at a specific location: void DispStr(int x, int y, char *s) { x += 0x20; y += 0x20; printf ("\x1B=%c%c%s", x, y, s); }

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  • How can I get the palette of an 8-bit surface in SDL.NET/Tao.SDL?

    - by lolmaster
    I'm looking to get the palette of an 8-bit surface in SDL.NET if possible, or (more than likely) using Tao.SDL. This is because I want to do palette swapping with the palette directly, instead of blitting surfaces together to replace colours like how you would do it with a 32-bit surface. I've gotten the SDL_Surface and the SDL_PixelFormat, however when I go to get the palette in the same way, I get a System.ExecutionEngineException: private Tao.Sdl.Sdl.SDL_Palette GetPalette(Surface surf) { // Get surface. Tao.Sdl.Sdl.SDL_Surface sdlSurface = (Tao.Sdl.Sdl.SDL_Surface)System.Runtime.InteropServices.Marshal.PtrToStructure(surf.Handle, typeof(Tao.Sdl.Sdl.SDL_Surface)); // Get pixel format. Tao.Sdl.Sdl.SDL_PixelFormat pixelFormat = (Tao.Sdl.Sdl.SDL_PixelFormat)System.Runtime.InteropServices.Marshal.PtrToStructure(sdlSurface.format, typeof(Tao.Sdl.Sdl.SDL_PixelFormat)); // Execution exception here. Tao.Sdl.Sdl.SDL_Palette palette = (Tao.Sdl.Sdl.SDL_Palette)System.Runtime.InteropServices.Marshal.PtrToStructure(pixelFormat.palette, typeof(Tao.Sdl.Sdl.SDL_Palette)); return palette; } When I used unsafe code to get the palette, I got a compile time error: "Cannot take the address of, get the size of, or declare a pointer to a managed type ('Tao.Sdl.Sdl.SDL_Palette')". My unsafe code to get the palette was this: unsafe { Tao.Sdl.Sdl.SDL_Palette* pal = (Tao.Sdl.Sdl.SDL_Palette*)pixelFormat.palette; } From what I've read, a managed type in this case is when a structure has some sort of reference inside it as a field. The SDL_Palette structure happens to have an array of SDL_Color's, so I'm assuming that's the reference type that is causing issues. However I'm still not sure how to work around that to get the underlying palette. So if anyone knows how to get the palette from an 8-bit surface, whether it's through safe or unsafe code, the help would be greatly appreciated.

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  • When to use Shift operators << >> in C# ?

    - by Junior Mayhé
    I was studying shift operators in C#, trying to find out when to use them in my code. I found an answer but for Java, you could: a) Make faster integer multiplication and division operations: *4839534 * 4* can be done like this: 4839534 << 2 or 543894 / 2 can be done like this: 543894 1 Shift operations much more faster than multiplication for most of processors. b) Reassembling byte streams to int values c) For accelerating operations with graphics since Red, Green and Blue colors coded by separate bytes. d) Packing small numbers into one single long... For b, c and d I can't imagine here a real sample. Does anyone know if we can accomplish all these items in C#? Is there more practical use for shift operators in C#?

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  • Saturated addition of two signed Java 'long' values

    - by finnw
    How can one add two long values (call them x and y) in Java so that if the result overflows then it is clamped to the range Long.MIN_VALUE..Long.MAX_VALUE? For adding ints one can perform the arithmetic in long precision and cast the result back to an int, e.g.: int saturatedAdd(int x, int y) { long sum = (long) x + (long) y; long clampedSum = Math.max((long) Integer.MIN_VALUE, Math.min(sum, (long) Integer.MAX_VALUE)); return (int) clampedSum; } or import com.google.common.primitives.Ints; int saturatedAdd(int x, int y) { long sum = (long) x + (long) y; return Ints.saturatedCast(sum); } but in the case of long there is no larger primitive type that can hold the intermediate (unclamped) sum. Since this is Java, I cannot use inline assembly (in particular SSE's saturated add instructions.) It can be implemented using BigInteger, e.g. static final BigInteger bigMin = BigInteger.valueOf(Long.MIN_VALUE); static final BigInteger bigMax = BigInteger.valueOf(Long.MAX_VALUE); long saturatedAdd(long x, long y) { BigInteger sum = BigInteger.valueOf(x).add(BigInteger.valueOf(y)); return bigMin.max(sum).min(bigMax).longValue(); } however performance is important so this method is not ideal (though useful for testing.) I don't know whether avoiding branching can significantly affect performance in Java. I assume it can, but I would like to benchmark methods both with and without branching. Related: http://stackoverflow.com/questions/121240/saturating-addition-in-c

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  • Shift count negative or too big error - correct solution?

    - by PeterK
    I have the following function for reading a big-endian quadword (in a abstract base file I/O class): unsigned long long CGenFile::readBEq(){ unsigned long long qT = 0; qT |= readb() << 56; qT |= readb() << 48; qT |= readb() << 40; qT |= readb() << 32; qT |= readb() << 24; qT |= readb() << 16; qT |= readb() << 8; qT |= readb() << 0; return qT; } The readb() functions reads a BYTE. Here are the typedefs used: typedef unsigned char BYTE; typedef unsigned short WORD; typedef unsigned long DWORD; The thing is that i get 4 compiler warnings on the first four lines with the shift operation: warning C4293: '<<' : shift count negative or too big, undefined behavior I understand why this warning occurs, but i can't seem to figure out how to get rid of it correctly. I could do something like: qT |= (unsigned long long)readb() << 56; This removes the warning, but isn't there any other problem, will the BYTE be correctly extended all the time? Maybe i'm just thinking about it too much and the solution is that simple. Can you guys help me out here? Thanks.

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  • Bitshift in javascript

    - by pingvinus
    I've got a really big number: 5799218898. And want to shift it right to 13 bits. So, windows-calculator or python gives me: 5799218898 13 | 100010100100001110011111100001 13 70791 | 10001010010000111 As expected. But Javascript: 5799218898 13 | 100010100100001110011111100001 13 183624 | 101100110101001000 I think it because of internal integer representation in javascript, but cannot find anything about that.

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  • Interface Builder error: IBXMLDecoder: The value for key is too large to fit into a 32 bit integer

    - by stdout
    I'm working with Robert Payne's fork of PSMTabBarControl that works with IB 3.2 (thanks BTW Robert!): http://codaset.com/robertjpayne/psmtabbarcontrol/. The demo application works fine on 64-bit systems, but when I try to open the XIB file in Interface Builder on a 32-bit system I get: IBXMLDecoder: The value (4654500848) for key (myTrackingRectTag) is too large to fit into a 32 bit integer Building the app as 32 bit works, but then running it gives: PSMTabBarControlDemo[9073:80f] * -[NSKeyedUnarchiver decodeInt32ForKey:]: value (4654500848) for key (myTrackingRectTag) too large to fit in 32-bit integer Not sure if this is a generic IB issue that can occur when moving between 64 and 32 bit systems, or if this is a more specific issue with this code. Has anyone else run into this?

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  • Negative logical shift

    - by user320862
    In Java, why does -32 -1 = 1 ? It's not specific to just -32. It works for all negative numbers as long as they're not too big. I've found that x -1 = 1 x -2 = 3 x -3 = 7 x -4 = 15 given 0 x some large negative number Isn't -1 the same as << 1? But -32 << 1 = -64. I've read up on two's complements, but still don't understand the reasoning.

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  • Better name for CHAR_BIT?

    - by Potatoswatter
    I was just checking an answer and realized that CHAR_BIT isn't defined by headers as I'd expect, not even by #include <bitset>, on newer GCC. Do I really have to #include <climits> just to get the "functionality" of CHAR_BIT?

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  • ARM assembly puzzle

    - by ivant
    First of all, I'm not sure if solution even exists. I spent more than a couple of hours trying to come up with one, so beware. The problem: r1 contains an arbitrary integer, flags are not set according to its value. Set r0 to 1 if r1 is 0x80000000, to 0 otherwise, using only two instructions. It's easy to do that in 3 instructions (there are many ways), however doing it in 2 seems very hard, and may very well be impossible.

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  • How to calculate 2^n-1 efficiently without overflow?

    - by Ludwig Weinzierl
    I want to calculate 2^n-1 for a 64bit integer value. What I currently do is this for(i=0; i<n; i++) r|=1<<i; and I wonder if there is more elegant way to do it. The line is in an inner loop, so I need it to be fast. I thought of r=(1ULL<<n)-1; but it doesn't work for n=64, because << is only defined for values of n up to 63.

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  • Macros to set and clear bits

    - by volting
    Im trying to write a few simple macros to simplify the task of setting and clearing bits which should be a simple task however I cant seem to get them to work correctly. #define SET_BIT(p,n) ((p) |= (1 << (n))) #define CLR_BIT(p,n) ((p) &= (~(1) << (n)))

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