Segmentation fault when using pointers in between functions

Question

I'm getting a segmentation fault when assigning the value stored at a pointer to a local variable in a function. I have a hunch that I'm handling pointers poorly, but I can't seem to figure out what the issue is. Any ideas? Thanks!

I've tried adding breakpoints, and I've determine that the actual seg fault happens at "int i = *AlignCounter;"

When looking at the variables window, I see this:

Here's the function where the seg fault happens:

void align_S_T(int *StringCounterS, int 
*StringCounterT, int *AlignCounter, char *StringS, char 
*StringT, char *AlignmentS, char *AlignmentT)
{
//Local Variables
int i = *AlignCounter; //<--Seg Fault Line
int s = *StringCounterS;
int t = *StringCounterT;
//Set Strings
AlignmentS[i] = StringS[s];
AlignmentT[i] = StringT[t];

//Increment Counters
*AlignCounter++;
*StringCounterS++;
*StringCounterT++;

}

Here's how the pointers are created and allocated in main:

    int* StringCounterT; //T String Counter
    int* AlignCounter; //Align Counter

    StringCounterS=0;
    StringCounterT=0;
    AlignCounter=0;

What are the values of giving the parameter AlignCounter when you use the align_S_T function? — Miguel
– Miguel, Commented Mar 31, 2019 at 2:39
Do you think you are allowed to access the memory location at 0x0 (at the very base of the System Reserved memory range)? What do you think is stored there? int i = *AlignCounter; //<--Seg Fault Line is a good indication of the answer to the first question. In other words AlignCounter=0; stores the Address 0 as the value for AlignCounter and when you attempt to access the memory location you have no right to access, BAM SegFault. — David C. Rankin
– David C. Rankin, Commented Mar 31, 2019 at 2:40
before use the align_S_T function did you allocated dynamic memory to AlignCounter? — Miguel
– Miguel, Commented Mar 31, 2019 at 2:43
What do you think happens when you try to dereference a pointer pointing to nowhere then? — user9254539
– user9254539, Commented Mar 31, 2019 at 3:10

André A. G. Scotá · Accepted Answer · 2019-03-31 03:47:37Z

You are indicating that AlignCounter points to address 0 (AlignCounter=0;), then, inside align_S_T() you are trying to get the value stored inside this adress with "*" operator(int i = *AlignCounter;). You cannt do this because it is not a valid address (try to point AlignCounter to a declared integer before calling align_S_T(), the segFault goes to next line, rss, same thing).

void align_S_T(int *StringCounterS, int 
*StringCounterT, int *AlignCounter, char *StringS, char 
*StringT, char *AlignmentS, char *AlignmentT)
{
//Local Variables
int i = *AlignCounter;
int s = *StringCounterS;  //<--Seg Fault Line!!
int t = *StringCounterT;
//Set Strings
AlignmentS[i] = StringS[s];
AlignmentT[i] = StringT[t];

//Increment Counters
*AlignCounter++;
*StringCounterS++;
*StringCounterT++;

}

int main(int argc, char *argv[]){
     int* StringCounterT; //T String Counter
     int* StringCounterS; //T String Counter
     int* AlignCounter; //Align Counter
     char *a, *b, *c, *d;
     int x;

     a = b = c = d = "";
     x = 10;

     StringCounterS=0;
     StringCounterT=0;
     AlignCounter=&x;  

     align_S_T(StringCounterS, StringCounterT, AlignCounter, a, b, c, d);
     return(0);
}

Miguel · Accepted Answer · 2019-03-31 03:01:07Z

0

I have isolated your problem, you need to assign dynamic memory or static to AlignCounter variable before calling the function align_S_T as you can see in the following code.

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

void  align_S_T(int *AlignCounter)
{
   int i = *AlignCounter;
   *AlignCounter++;
}

int main()
{
   // Dynamic memory
   int *AlignCounter=NULL;
   AlignCounter = (int*) malloc(10 * sizeof(int)); 
   *AlignCounter=0;
   // static memory
   // int AlignCounter[buffer_needed];
   // AlignCounter[0]=0;
   function(AlignCounter);
   free(AlignCounter);

   return 0;
}

I hope it help you.

answered Mar 31, 2019 at 3:01

Miguel

4175 silver badges17 bronze badges

Comments

T W Bennet · Accepted Answer · 2019-03-31 03:01:56Z

0

Well, int* AlignCounter creates a pointer but doesn't allocate anything for it to point to. Then AlignCounter=0; makes it NULL. If you're then sending it to align_S_T, you're just referencing a NULL pointer. Boom.

You probably want to declare int AlignCounter = 0, then send &AlignCounter to your function.

answered Mar 31, 2019 at 3:01

T W Bennet

3831 silver badge7 bronze badges

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