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Printf acting weird


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#1 HelloSkitty   Members   -  Reputation: 152

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Posted 09 April 2013 - 01:38 PM

I am trying to debug a program in order to find the cause of a segmentation fault.  So I commented all of the code and slowly uncommented from the start to find the location of the segmentation fault.  This was successful and I have discovered the function that is causing it.  Then, I decided to add a printf to the start of that function, and oddly enough nothing printed.  I then added printf to the calling function, and nothing printed there either.  So in the same manner of finding the location of the segmentation fault, I set about finding the place my program stopped printing.  And I did.  It was the line:

 

retVal->maxSize=10;

 

And I have no idea why.  All of the relevant code is here:

 

circular.h

typedef struct Quote {
	unsigned int time;
	double rate;
} quote;

typedef struct CBuf {
	unsigned int maxSize;
	unsigned int size;
	unsigned int first;
	unsigned int last;
	quote* arrayPointer;
} cbuf;

cbuf* cbuf_init();

void cbuf_delete(cbuf* cb_ptr);

void cbuf_update(cbuf* cb_ptr,unsigned int time,double rate);

double cbuf_average(cbuf* cb_ptr);

quote* cbuf_start(cbuf* cb_ptr);

quote* cbuf_end(cbuf* cb_ptr);

void cbuf_dump(cbuf* cb_ptr);

void cbuf_stats(cbuf* cb_ptr);

 

circular.c

#include "circular.h"
#include <stdlib.h>
#include <stdio.h>

cbuf* cbuf_init() {
	printf("cbufinit\n");
	cbuf* retVal;
	printf("makingretval\n");
	printf("%d\n",(*retVal).maxSize);
	retVal->maxSize=10;
	printf("9\n");
	retVal->size=0;
	retVal->first=0;
	retVal->last=0;
	quote* quotes=malloc(10*sizeof(quote));
	retVal->arrayPointer=quotes;
	return retVal;
}

// More code below, but this is the only function that is called
// makingretval and -72537468 are printed
// 9 is not printed

 

main6a.c

#include <stdio.h>
#include <stdlib.h>
#include "circular.h"

int main() { 
	printf("start\n");
	printf("1\n");
   cbuf *cb1 ;
	printf("2\n");
   cb1 = cbuf_init() ;
	printf("3\n");
   return 0 ;
}

// 1 and 2 are printed
// 3 is not printed

 

 

Everything compiles fine, it is simply that printf does not seem to work at all following the retVal->maxSize=10; line.  In a separate run, I added a getchar call after the printf("3\n") which ran fine.  So the program is still getting to the end with no issue.  I just want to know why it is not printing.

 

Thanks!


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#2 Grimshaw   Members   -  Reputation: 619

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Posted 09 April 2013 - 01:48 PM

Please allocate retVal before using it!

 

= malloc(sizeof(cbuf));

 

You re accessing a random memory position, which is undefined behavior. The printf not working is simply one of the many possible effects, thats why it is called undefined! :)


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#3 ApochPiQ   Moderators   -  Reputation: 15072

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Posted 09 April 2013 - 01:49 PM

printf() is a red herring. Your problem can be simplified to this:
int main()
{
    int* retval;
    *retval = 42;

    return 0;
}
You don't initialize retval, so it's pointing at random memory. Reading from it or writing to it is undefined behaviour, and may or may not cause catastrophic weirdness in your program.

You should malloc() retval or something similar so it has an actual address and not just random stack garbage :-)

#4 Dave Hunt   Crossbones+   -  Reputation: 2417

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Posted 09 April 2013 - 01:49 PM

The problem is that you declare retVal as a pointer to cbuf, but you never assign it a value. Thus, retVal is an invalid pointer with a random value. When you start assigning to is cbuf properties, you start overwriting memory that does not belong to it.

 

You need to do something like:

 

    retVal = new cbuf(); // (c++)

 

or

 

    retVal = (cbuf*)malloc(sizeof(cbuf)); // c



#5 HelloSkitty   Members   -  Reputation: 152

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Posted 09 April 2013 - 02:07 PM

Thanks for all the fast responses!  Fixed it and it works now - I should probably be thankful my compiler/runner is in safe mode :D


A penny for my thoughts? Do you think I have only half a brain?

Not-so-proud owner of blog: http://agathokakologicalartolater.wordpress.com/

#6 KnolanCross   Members   -  Reputation: 1271

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Posted 09 April 2013 - 11:18 PM

In time: don't forget to free that memory after you don't use it anymore.


Currently working on a scene editor for ORX (http://orx-project.org), using kivy (http://kivy.org).


#7 Khatharr   Crossbones+   -  Reputation: 2962

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Posted 10 April 2013 - 05:26 AM

Also, please learn to use a debugger. Commenting out lines in order to hunt for bugs is so extremely slow and painful. The debugger can take you step by step through the code and give you all sorts of tools to help look at what's going on while the program runs.


Edited by Khatharr, 10 April 2013 - 05:27 AM.

void hurrrrrrrr() {__asm sub [ebp+4],5;}

There are ten kinds of people in this world: those who understand binary and those who don't.




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