Thread: printing the contents the pointer points to

Code:

  

sprintf((char *)magval, "P%c", ch);
  printf("magic no: %s\n", *magval);

What am I doing wrong here?

magval is declared as char *magval (in sprintf()my compiler seems to want the cast for some reason)

But ultimately I want to print the contents that magval points to. How do I do this?

This is really ticking me off right now.

> sprintf((char *)magval, "P%c", ch);
My guess is, you didn't allocate any space
>I have, magval is a member in a structure

> (in sprintf()my compiler seems to want the cast for some reason)
Which one would that be?
>gcc (via MinGW) - some settings in my compiler I need to check

> printf("magic no: %s\n", *magval);
To print a string, use
printf("magic no: %s\n", magval);
>This works, but need to check it's in the member variable.

Originally Posted by cblix

> sprintf((char *)magval, "P%c", ch);
My guess is, you didn't allocate any space
>I have, magval is a member in a structure

What Salem is saying is that magval has to point to something. If not then you are using an uninitialized pointer. magval should be set to the address of some character buffer/array or the return value from some malloc call prior to you writing/reading from it.

Originally Posted by John_

you are trying to access magval as if it is a regular variable, if it is a member of a structure, you need to access it like

structurename.magval

Yeah that is the point at the moment, so I can later assign to the member of the structure.

Hopefully this helps:

Code:

typedef struct Image {
	char magval;
	int width;
	int height;
	int maxval;
	char *data;
} Image;

void Q_readPPMHeader(FILE *fp, char magval, int *width, int *height, int *maxval)
{
  char ch;
 

  if ((fscanf(fp, "P%c\n", &ch) != 1) && (ch != '6' || ch != '5' || ch !='3' || ch != '2')) 
	Q_error("Cannot read either .PPM or .PGM file");
  
  sprintf(magval, "P%c", ch); /**ISSUE IS HERE**/
  printf("magic no: %s\n", magval);/**ISSUE IS HERE**/


  /* skip comments */
  ch = getc(fp);
  while (ch == '#') {
	do {
		ch = getc(fp);
	} while (ch != '\n');	/* read to the end of the line */
		  ch = getc(fp);            
    }

  if (!isdigit(ch)) Q_error("cannot read header information from ppm file");

  ungetc(ch, fp);		/* put that digit back */

  /* read the width, height, and maximum value for a pixel */
  fscanf(fp, "%d%d%d\n", width, height, maxval);

  if (*maxval != 255) Q_error("image is not true-color (24 bit); read failed");

 
  printf("width:    %d\n", *width);
  printf("height:   %d\n", *height);
  printf("maxval:   %d\n", *maxval);
}

Image *Q_ImageRead(char *filename)
{
  int width, height, maxval, size;
  char magval; /**ISSUE IS HERE**/


  Image *image = (Image *) malloc(sizeof(Image));
  FILE  *fp    = fopen(filename, "r");

  if (!image) Q_error("cannot allocate memory for new image");
  if (!fp)    Q_error("cannot open file for reading");

  Q_readPPMHeader(fp, &magval, &width, &height, &maxval);

  size = width * height;
 
  image->magval = magval;
  image->width  = width;
  image->height = height;
  image->maxval = maxval;
  image->data   = (char *) malloc(size);
  
  if (!image->data) Q_error("cannot allocate memory for new image");

  fread((char *)image->data, 1, (size_t) size, fp);
  
  fclose(fp);

  return image;
}

You can't write a multi-character string into a single character. You don't print a single character with a string specifier.