This c program gives a segmentation fault

Projects Programming
1

I am write a c program to test structure pass into a function

but it give segmentation fault ,why?

please give me the solution to solve this issue?

The code i am tried is given below,

#include <stdio.h>

#include <string.h>

#define GRAYSCALE
void set_pixel_data(int depth, void   *p);
   
struct pixel_gray { char m; };
struct pixel_color { char r; char g; char b; };


int main()
{
struct pixel_gray  pg;
struct pixel_color pc;
#ifdef GRAYSCALE
	set_pixel_data( 1, &pg);
#else
	set_pixel_data( 3, &pc);
#endif

	return(0);
}
void set_pixel_data(int depth, void   *p)
{

switch(depth)
{
struct pixel_gray* p1;
case 1:
p1=(struct pixel_gray*)p;
strcpy(p1->m,"s");                                   //here is the problem
//printf("%s",p1->m);


    break;
case 3:
    printf("33");
    break;
default:
    printf("nothing he he ");
    break;
}

}
2

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3 
      strcpy(p1->m, "s");                                  //here is the problem

m is a char. why are you trying to copy a full cString into it. just do

     p1->m = 's';                  //and this problem goes away

if you want a local temporary pointer just to provide the type in each case, you'll need to put {} around your case.

  switch (depth) {
    case 1:
      {
        struct pixel_gray* p1 = (struct pixel_gray*)p;
        p1->m = 's';
      }
      break;
      ....
  }

but you don't need really the struct pixel_gray* p1, just cast your void* to the right type depending on the case value.

#include <stdio.h>
#include <string.h>
//#define GRAYSCALE

void set_pixel_data(int depth, void   *p);

struct pixel_gray {
  char m;
};

struct pixel_color {
  char r;
  char g;
  char b;
};


int main()
{
#ifdef GRAYSCALE
  struct pixel_gray  pg;
  set_pixel_data( 1, &pg);
#else
  struct pixel_color pc;
  set_pixel_data( 3, &pc);
#endif
  return (0);
}

void set_pixel_data(int depth, void* p)
{
  switch (depth) {
    case 1:
      ((struct pixel_gray*)p)->m = 's';
      break;
    case 3:
      ((struct pixel_color*)p)->r = 'r';
      ((struct pixel_color*)p)->g = 'g';
      ((struct pixel_color*)p)->b = 'b';
      break;
  }
}

and you don't need the switch at all, put conditional code based on the value of #ifdef GRAYSCALE

#include <stdio.h>
#include <string.h>
//#define GRAYSCALE

void set_pixel_data(void   *p);

struct pixel_gray {
  char m;
};

struct pixel_color {
  char r;
  char g;
  char b;
};


int main()
{
#ifdef GRAYSCALE
  struct pixel_gray  pg;
  set_pixel_data(&pg);
#else
  struct pixel_color pc;
  set_pixel_data(&pc);
#endif
  return (0);
}

void set_pixel_data(void* p)
{
#ifdef GRAYSCALE
  ((struct pixel_gray*)p)->m = 's';
#else
  ((struct pixel_color*)p)->r = 'r';
  ((struct pixel_color*)p)->g = 'g';
  ((struct pixel_color*)p)->b = 'b';
#endif
}

and last but not least, are you sure you want to hold characters in the structure? shouldn't a uint8_t be better? (a number fitting on a byte - value between 0 and 255)

4

thanks alot

5

wounder1:
thanks alot

you are welcome; I added a few comments above whilst you were responding

6

You should turn your warning level up to 'All' (in Preferences) so you get useful feedback when your code does something that even the compiler thinks is weird.

Arduino: 1.8.10 (Mac OS X), Board: "Arduino/Genuino Uno"


/Users/john/Documents/Arduino/sketch_nov08a/sketch_nov08a.ino: In function 'int main()'
/Users/john/Documents/Arduino/sketch_nov08a/sketch_nov08a.ino: In function 'void set_pixel_data(int, void*)':
sketch_nov08a:41:18: error: invalid conversion from 'char' to 'char*' [-fpermissive]
       strcpy(p1->m, "s");                                  //here is the problem
              ~~~~^
In file included from /Applications/Arduino1.8.10.app/Contents/Java/hardware/arduino/avr/cores/arduino/Arduino.h:25:0,
                 from sketch/sketch_nov08a.ino.cpp:1:
/Applications/Arduino1.8.10.app/Contents/Java/hardware/tools/avr/avr/include/string.h:305:14: note:   initializing argument 1 of 'char* strcpy(char*, const char*)'
 extern char *strcpy(char *, const char *);
              ^~~~~~
exit status 1
invalid conversion from 'char' to 'char*' [-fpermissive]