Thread: Segmentation Fault argc and argv[] become zero after function call.

I'm using gdb in an attempt to debug my program. gdb has made me aware that line 17 is where the fault occurs. For some reason my input parameters for main become zero.

Breakpoint 2, main (argc=0, argv=0x401c0000) at polygon.c:17
17 fclose(fp);



Consequently Line 18 can't access them. I tried to print the variables, and received the warning "Cannot access memory at address 0x401c0008C" I'm stumped. I have no idea why it's not working. Please help

Code:

  
    # include<stdio.h># define MAXIMUM_SIZE 50


int get_corners(FILE *fp,
     double matrix_x[MAXIMUM_SIZE], double
     matrix_y[MAXIMUM_SIZE]);
void output_corners(FILE *fp, double array1[], double array2[], int actual_size);


int main( int argc, char *argv[])


{
  FILE* fp;
  int actual_size;
  double matrix_x[MAXIMUM_SIZE], matrix_y[MAXIMUM_SIZE];
  fp = fopen(argv[1], "r");
  actual_size = get_corners( fp, &matrix_x[MAXIMUM_SIZE], &matrix_y[MAXIMUM_SIZE]);
  fclose(fp);
  fp = fopen(argv[2], "w");
  output_corners( fp, &matrix_x[MAXIMUM_SIZE], &matrix_y[MAXIMUM_SIZE], actual_size);
  return(0);
}


int get_corners(FILE *fp,
      double matrix_x[MAXIMUM_SIZE], double matrix_y[MAXIMUM_SIZE])


{
  double x_cord, y_cord;
  int i = 0, input_status, matrix_size;
  fscanf(fp, "%lf", &x_cord);
  input_status = fscanf(fp, "%lf", &y_cord);
  matrix_size = 0;
  while (input_status != EOF ) {
       printf("%lf\n", x_cord);
    matrix_x[i] = x_cord;
    matrix_y[i] = y_cord;
    i = i + 1;
    input_status = fscanf(fp, "%lf", &x_cord);
    fscanf(fp, "%lf", &y_cord);
    matrix_size += 1;
    };
return (matrix_size);
}


void output_corners(FILE *fp, double matrix_x[], double matrix_y[], int actual_size)


{


  int i;


  for (i = 0 ; i < ( actual_size - 1 ) ;
    ++i){
    fprintf(fp, "%.1lf %.1lf\n", matrix_x[i], matrix_y[i]);
    }
  printf("%lf", matrix_x[1]);
}

Did you check argc? Is it greater than one?
Did you check fp?Is it different than NULL?

Code:

int main( int argc, char *argv[])
 
 
{
  FILE* fp;
  int actual_size;
  double matrix_x[MAXIMUM_SIZE], matrix_y[MAXIMUM_SIZE];
  fp = fopen(argv[1], "r");

  // WRONG!
  actual_size = get_corners( fp, &matrix_x[MAXIMUM_SIZE], &matrix_y[MAXIMUM_SIZE]);
  fclose(fp);
  fp = fopen(argv[2], "w");

  // WRONG!!
  output_corners( fp, &matrix_x[MAXIMUM_SIZE], &matrix_y[MAXIMUM_SIZE], actual_size);
  return(0);
}

Try this

Code:

int main( int argc, char *argv[])
 
 
{
  FILE* fp;
  int actual_size;
  double matrix_x[MAXIMUM_SIZE], matrix_y[MAXIMUM_SIZE];
  fp = fopen(argv[1], "r");

  // no &, no indexing
  actual_size = get_corners( fp, matrix_x, matrix_y);
  fclose(fp);
  fp = fopen(argv[2], "w");

  // no &, no indexing
  output_corners( fp, matrix_x, matrix_y, actual_size);
  return(0);
}

As an explanation to qny's suggestion, this line of code

Originally Posted by Kurtanius21

Code:

  actual_size = get_corners( fp, &matrix_x[MAXIMUM_SIZE], &matrix_y[MAXIMUM_SIZE]);

means

Code:

  actual_size = get_corners(fp, after the final element of matrix_x array, after the final element of matrix_y array);

In other words, you're not supplying the arrays as parameters, but pointers to the data that follows the arrays.