Question

This is got me pretty stuck, how do I fix this? I know I haven’t got error checking, but they aren’t required i’d guess since it’s restricted to my desktop. It obveously can’t be EOF. It’s for the infoheader struct, fileheader works fine. Do i need to take a new line or something?

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

typedef struct
{
    unsigned char fileMarker1;       /* 'B' */                       
    unsigned char fileMarker2;       /* 'M' */ 
    unsigned int   bfSize;             
    unsigned short unused1;           
    unsigned short unused2;           
    unsigned int   imageDataOffset;  /* Offset to the start of image data */
}FILEHEADER; 

typedef struct                       
{ 
    unsigned int   biSize;            
    int            width;            /* Width of the image */ 
    int            height;           /* Height of the image */ 
    unsigned short planes;             
    unsigned short bitPix;             
    unsigned int   biCompression;      
    unsigned int   biSizeImage;        
    int            biXPelsPerMeter;    
    int            biYPelsPerMeter;    
    unsigned int   biClrUsed;          
    unsigned int   biClrImportant;     
}INFOHEADER; 

typedef struct                        
{ 
    unsigned char  b;         /* Blue value */ 
    unsigned char  g;         /* Green value */ 
    unsigned char  r;         /* Red value */ 
 }IMAGECOMPONENT; 

 int fileheadfunc(FILE *image);
 int infoheadfunc(FILE *image);

 int main( int argc, char *argv[] )
 {
    char *filename; /* *threshholdInput = argv[2]; */
    FILE *image;
    int filehead, infohead;
    filename = argv[1];
    /* int threshhold = atoi(threshholdInput); */

    if (argc != 2) 
    {
              printf(" Incorrect Number Of Command Line Arguments\n");
              return(0);
    }

    image = fopen( filename, "r");

        if (image == NULL)
    {
    fprintf(stderr, "Error, cannot find file %s\n", filename);
    exit(1);
    }

    filehead = fileheadfunc(image);
    infohead = infoheadfunc(image);
    fclose(image);

   return(0);             
}

int fileheadfunc(FILE *image)
{
    FILEHEADER *header;
    long pos;

    fseek (image , 0 , SEEK_SET);

    fread( (unsigned char*)header, sizeof(FILEHEADER), 1, image );


    if ( (*header).fileMarker1 != 'B'  || (*header).fileMarker2 != 'M' )
    {
    fprintf(stderr, "Incorrect file format");
    exit(1);
    }

    printf("This is a bitmap!\n");
    pos = ftell(image);
printf("%ld\n", pos);
printf("%zu\n", sizeof(FILEHEADER));

return(0);
}

int infoheadfunc(FILE *image)
{
    INFOHEADER *iheader;

    fseek (image, 0, SEEK_CUR ); 
    fread( (unsigned int*)iheader, sizeof(INFOHEADER), 1, image );

    printf("Width: %i\n", (*iheader).width);
    printf("Height: %i\n", (*iheader).height);

    return(0);
}

Answer 1

There are two problems with the code:

Alignment

For performance reasons the compiler will, unless instructed to do otherwise, arrange struct fields on its "natural boundaries", effectively leaving uninitialised gaps between byte-size fields. Add

#pragma pack(1) 

before the struct definitions and you should be fine. It’s also easy to test: just print out the struct size without and with pragma pack in place, and you’ll see the difference.

Allocation

As Paul R already said, you should allocate space for the headers, not just provide a pointer to the structures. The fact that fileheadfunc works is a coincidence, there just wasn’t anything in the way that got smashed when data got written outside of the allocated space.

A last one, just for prevention sake: should you ever want to return the read structures to the calling program, do not just return a pointer to the structure allocated in the function as that will cause problems similat to the unallocated variables you have now. Allocate them in the calling function, and pass a pointer to that variable to the header read functions.

EDIT clarification regarding the last point:

DON’T

FILEHEADER * fileheadfunc(FILE *image)
{
    FILEHEADER header;
    ...
    return &header; // returns an address on the function stack that will 
                    // disappear once you return
}

DO

int fileheadfunc(FILE *image, FILEHEADER *header)
{
    ...
}

which will be called like this

...
FILEHEADER header;
returnvalue = fileheaderfunc(imagefile,&header);

EDIT2: just noticed that the way you read the DIB header is not correct. There are several variations of that header, with different sizes. So after reading the file header you first need to read 4 bytes into an unsigned int and based on the value read select the correct DIB header structure to use (don’t forget you already read its first field!) or tell the user you encountered an unsupported file format.