next_permutation seg fault

This is a discussion on next_permutation seg fault within the C++ Programming forums, part of the General Programming Boards category; Hello. Im writing a program that will test *a lot* of permutations, only while using the STL's next_permutation function I ...

  1. #1
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    next_permutation seg fault

    Hello. Im writing a program that will test *a lot* of permutations, only while using the STL's next_permutation function I seem to seg fault.

    Heres the code, where I left the while that does the permutations and forced one if to never run. This seems to be the only way to get it to not seg fault. I'm fairly sure its not because of my other code, but ill attach it incase some one feels nice enough to look threw it quick

    Thanks

    Code:
    #include <iostream>
    #include <algorithm>
    #include <cstdio>
    #include <cstring>
    using namespace std;
    
    #include <fcntl.h>
    
    extern "C" {
    	#include "../../lib/present_conds/present_conds.h"
    }
    
    int createBoxes(int (*func)(unsigned char *), int buff_size, int device_fd) {
    	int retval;
    	int index = 0;
    	unsigned char sbox[16];
    	unsigned char buffer[buff_size][16];
    	
    	// Create initial sbox
    	for(int i = 0; i < 16; ++i)
    		sbox[i] = i;
    	
    	// Test all permutations
    	do {
    		if(false) {
    		//if(func(sbox) == 0) {
    			memcpy(buffer[index], sbox, sizeof(unsigned char[16]));
    			++index;
    
    			if(index + 1 == buff_size) {
    				retval = write(device_fd, buffer, 
    					       sizeof(char[buff_size][16]));
    				if(retval == -1) {
    					perror("Error while writing buffer");
    					return 1;
    				}
    
    				index = 0;
    			}
    		}
    	} while(true);
    //	} while(next_permutation(sbox, sbox + 16) == true);
    
    	return 0;
    
    	// Write whats left of buffer
    	retval = write(device_fd, buffer, sizeof(char[index][16]));
    	if(retval == -1) {
    		perror("Error while writing final buffer");
    		return 1;
    	}
    
    	return 0;
    }
    
    int main() {
    	int device_fd;
    	mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH;
    
    	device_fd = open("output", O_WRONLY | O_CREAT | O_TRUNC, mode);
    	if(device_fd == -1) {
    		perror("Error opening output");
    		return 1;	
    	}
    	createBoxes(testAllConds, 1 << 21, device_fd); 
    
    	return 0;
    }
    Attached Files Attached Files

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    There are 16! = 20922789888000 permutations of 16 objects. You have room to store 2^21 = 2097152 of them.

  3. #3
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    Im not storing them, only a small percentage of them.

    Plus I'm really just worried about the seg fault now, space I can worry about after.

  4. #4
    and the Hat of Guessing tabstop's Avatar
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    I didn't see func anywhere, so I have no idea how many you're actually storing. But since seg fault means "you are writing to memory you don't have", I would start worrying about space now, if I were you.

  5. #5
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    func is a function pointer.

    Im not actually storing anything yet, that branch never runs....

  6. #6
    and the Hat of Guessing tabstop's Avatar
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    I thought you said that it only segfaulted if you did run the branch. Or at least that's what I took this sentence to mean:
    I left the while that does the permutations and forced one if to never run. This seems to be the only way to get it to not seg fault.
    So if that's not true, when does it seg fault and when does it not?

  7. #7
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    After being utterly disgusted by it for awhile I tried converting all my c libraries into c++ ones. That did the trick it seems.

    Very very weird...

  8. #8
    Cat without Hat CornedBee's Avatar
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    You can't have VLAs in C++.

    int buffer[buff_size][16];

    is invalid.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  9. #9
    and the hat of wrongness Salem's Avatar
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    Not to mention, that it's also 32MB in size, given the current code.
    That's a lot, when your default stack size is what - 1MB?

    Sure you just didn't blow the stack with the segfault?
    Does it work with smaller sizes?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  10. #10
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    Quote Originally Posted by zxcv View Post
    Im not storing them, only a small percentage of them.

    Plus I'm really just worried about the seg fault now, space I can worry about after.
    You're not worried about the fact running through all permutations will take several days at the very least?

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