Struct size incorrect

I've created the following struct...

Code:

---

typedef struct {
        short        type;
        int        fileSize;
        short        res0;
        short        res1;
        int        dataOffset;
} BMPFILEHEADER;

---

By my calculations, it should equal 14 bytes. Two bytes for each short and four bytes for each int. The thing is, if I do sizeof(BMPFILEHEADER) it equals 16 and I have no clue why. Especially when if I do sizeof(int) and sizeof(short) they come out at four bytes and two bytes respectively :)

I'm using gcc in Cygwin as my compiler, if that makes a difference. I'm downloading the express edition of Visual C++ 2008 as I write this to see if that makes a difference.

Any ideas?

The compiler is free to add "padding" bytes between members of the structure to preserve alignment requirements of the various data types.

Aligned data is often more efficiently accessed than unaligned data.

A search for "padding and alignment" should find more information.

Ah, right. I seem to remember reading something about this a while ago. Isn't there some kind of pragma directive I can add to stop it from doing this?

You usually don't want to do that since it optimizes for the processor to access data.
Otherwise look in your compiler's settings, because it's different for each compiler.

There may be, but the syntax varies from compiler to compiler.

#pragma pack(1)
For example.

Even if that solves the size problem, you may have to solve the endian problem as well.

The only truly portable answer is to read the file a byte at a time and move that byte to the correct location in the struct.

hehe, i just came here to ask absolutely the same question...
I have a structure (a string of 50 bytes, a double of 4 bytes and an int of 4 bytes)...

Code:

---

typedef struct product {
        char name[50];
        float price;
        int quantity;
};

---

I kept getting 60 bytes when i checked its size instead of 58. Now i know why...

P.S. sorry to hijack your thread redneon, but what if i wanted to read the second piece of the structure (i.e. price), what should the offset be?

[name][padding][price][padding][quantity]
__50__+___1___+__4__+___1___+___4___ = 60 bytes

Depends on the padding. Again, to be safe, just let the compiler do its work. Access the member by its name, and it will work fine.

Code:

---

struct product prod;
prod.price = 1.50;

---

Sorry, i shoudl've been clearer... If I had written the structure into a file and then wanted to access only the price (to edit it for example)... Would I use something of this sort?

Code:

---

fseek(file_pointer, sizeof(product.name) + 1, SEEK_SET)

---

Once again, depends on padding. You can do trial & error, but the safest way is to write the entire struct and read the entire struct from the file, unchanged. This also takes into account that you did not change padding settings and that you did not change endianness - ie trying to read an existing file on another platform.
I'm not sure how much or where the padding is, but you could find it by doing like this:

Code:

---

struct product prod;
int off = &prod.price - ∏ // Calculate offset of price inside the struct of prod.

---

Quote:

---

Originally Posted by Karpaty

[name][padding][price][padding][quantity]
__50__+___1___+__4__+___1___+___4___ = 60 bytes

---

NO, it wont be like that. It will be like this:
[name][padding][price][quantity]
__50__+___2___+__4__+___4___ = 60 bytes
The compiler doesn't just artibrarily stick extra bytes between fields. It puts them in specific places to align fields to word or dword boundaries etc.

Oh, thanks for clearing that up...
btw, Elysia, I have this function to add a new product:

Code:

---

void add_product(product *p) {
        printf("Enter product description : ");
        gets(p->name);
        printf("Enter price per unit (cent) : ");
        scanf("%f", &p->price);
        printf("Enter stock level: ");
        scanf("%d", &p->quantity);
}

---

if I wanted to modify just the price, would the following function work? (Thats assuming I moved the offset to the start of the product structure)...

Code:

---

void edit_price(product *p) {
        printf("Enter price per unit (cent) : ");
        scanf("%f", &p->price);
}

---

The code is pretty much the same apart the left out name and quantity parts...

Yes, it would work, even if you don't move the price to start of the structure.
Why? Because it reads the address of &price, and not the address of the structure start.

Cheers... I've been struggling with this problem all weekend... Will now try and put the theory in practice ;)

Good luck.

I know for a fact that the Mingw port of GCC, and GCC support the following format:

Code:

---

struct foobar {

    int yo;

    char homie;

}__attribute__ ((packed));

---

Im still unable to make this program work...

Ok, here's what I did:

In the menu I gave the user a choice to add new product, modify the name of a specific product or exit. In the structure i removed all numeral values and left just 3 chars set to 5 bytes each so that it would be easier to read it from the file...

I added 3 new items to the "database":
Value1: Value2: Value3:
11111 aaaaa zzzzz
22222 bbbbb yyyyy
33333 cccccc xxxxx

So, what i get written into the file are 3 separate entries, all follow each other:

Code:

---

11111aaaaazzzzz22222bbbbbyyyyy33333cccccxxxxx

---

Now i run the program and go to part b (to modify the name - 11111, 22222 or 33333)...
The program asks me what product i want to change (1-3). I say 2. Write the new description: 55555 and get the following in the updated file:

Code:

---

11111aaaaazzzzz55555 A      33333cccccxxxxx

---

The above happens when I exit the program and reenter to edit the name... If I dont exit, and modify the description as soon as i add the 3 products i get the following:

Code:

---

11111aaaaazzzzz55555 ccccxxxxx33333cccccxxxxx

---

How do I go about making this work?


P.S.

Here's the code:

Code:

---

#include <stdio.h>

struct product {
        char name[5];
        char price[5];
        char quantity[5];
};

int main() {

        int menu_stock(char*);
        char choice_stock;
        void add_product(product*);
        void change_description(product*);       
        void seek_product_number(int, FILE*);
        int total_products(FILE*);
        int get_product_number(int);
        void seek_end_db(FILE*);
        void write_to_db(FILE*, product*);

        product p;
        FILE *file_sales;

        file_sales = fopen("./sales.txt", "r+");

        while(menu_stock(&choice_stock) == 1) {
                switch(choice_stock) {
                        case 'A':
                        case 'a':
                                add_product(&p);
                                seek_end_db(file_sales);
                                write_to_db(file_sales, &p);
                                break;
                        case 'B':
                        case 'b':
                                seek_product_number(get_product_number(total_products(file_sales)), file_sales);
                                change_description(&p);
                                write_to_db(file_sales, &p);
                                break;
                }
        }
}

int menu_stock(char *c) {
        void clear_input_buffer();
        do {
                printf("*****************************************\n");
                printf("* Add new product                  (A) *\n");
                printf("* Change product description        (B) *\n");
                printf("* Exit                              (X) *\n");
                printf("*****************************************\n");
                printf("                              --> ");
                *c = (char)getchar();
                clear_input_buffer();
        } while(*c != 'A' && *c != 'a' && *c != 'B' && *c != 'b' && *c != 'X' && *c != 'x');
        if(*c == 'X' || *c == 'x') return 0;
        else return 1;
}

void clear_input_buffer() {
        int ch;
        while ((ch = getchar()) != '\n' && ch != EOF);
        clearerr(stdin);
}

void add_product(product *p) {
        printf("Enter product description : ");
        gets(p->name);
        printf("Enter price per unit (cent) : ");
        gets(p->price);
        printf("Enter stock level: ");
        gets(p->quantity);
}
void change_description(product *p) {
        printf("Enter product description : ");
        gets(p->name);
}
void seek_end_db(FILE *file_sales) {
        fseek(file_sales, 0, SEEK_END);
}
void write_to_db(FILE *file_sales, product *p) {
        fwrite(p, sizeof(product), 1, file_sales);
}


void seek_product_number(int product_number, FILE *file_sales) {
        fseek(file_sales, (product_number - 1) * sizeof(product), SEEK_SET);
}
int total_products(FILE *file_sales) {
        int file_size;
        fseek(file_sales, 0, SEEK_END);
        file_size = ftell(file_sales);
        return file_size / sizeof(product);
}
int get_product_number(int totalproducts) {
        int product_number;
        void clear_input_buffer();
        if(totalproducts <= 1) {
                return 1;
        }
        else {
                do {
                        printf("Enter product number (1 - &#37;d): ", totalproducts);
                        scanf("%d", &product_number);
                        clear_input_buffer();
                } while(product_number <= 0 || product_number > totalproducts);
                return product_number;
        }
}

---

The first thing I see is that your declarations are put inside your functions - this is wrong. They're supposed to be in your header or at the start of your file.
I'll fire up the debugger and take a look at it for you.
Just one question: is this windows or linux, or some such? Windows use "\" while linux use "/".
The second thing is, using a struct name directly without "struct" first is illegal in C. Use something li,e:

Code:

---

typedef struct product product;

---

I'm not surprised you get that bug. That's beacuse you put 55555 into name, but the rest of the struct is not initialized. Further, you write the ENTIRE struct to the file.
There are two options: read previous data for the product into the struct or seek to the offset of the product + name and then write only name.

Its Windows... thank you :)

Yay, the good old "I want to read a chunk of formatted binary data directly into a struct" discussion. Yes, you can pack the struct using compiler settings. No, you shouldn't do that. You should never, ever care what the precise value of a sizeof() is.

Quote:

---

Originally Posted by brewbuck

Yay, the good old "I want to read a chunk of formatted binary data directly into a struct" discussion. Yes, you can pack the struct using compiler settings. No, you shouldn't do that. You should never, ever care what the precise value of a sizeof() is.

---

If this has been discussed before, can you please point me to where I can read about it... I searched this forum for hours and couldnt find much help. perhaps i was searching for the wrong keywords?

A few finds:
http://cboard.cprogramming.com/showt...ry+file+create
http://cboard.cprogramming.com/showt...ry+file+create

It is a commonly discussed subject - creating binary files is not a good idea for portability between compilers, architectures or OS's - and one or two of those may be useful to consider - what if you need to migrate to Windows 64-bit because your major customer(s) are doing that? What if your major customer is moving to Linux? What if you find that the current compiler can't do what you need, and you want to upgrade to the next version, which just so happens to not have the same rules on data-layout in structs?

[It is fine to break the rules if you know them - and the consequences of doing so :)]

--
Mats

I finally got it working... It was quite simple actually...
The way I got it to work is, as suggest by Elysia, by reading the previous data for the product into the structure and only then changing the name... Also, one pitfall that I got on my way: I had a char array of [5] and was adding a new name 5 characters long. It messed up my price part, since my string array could only store 4 chars due to the fact that the last (5th) byte was reserved for "\0" character...

Big 'THANK YOU' to everyone who helped me... :)