How to write a struct to a file using fwrite?

Question

I'm very new to C, and I am having trouble with fwrite.

I'm looking to use a struct that holds two values:

struct keyEncode{
    unsigned short key[2];
    unsigned short encoded[2];
};

I then declare my struct and a pointer to that struct in my main:

struct keyEncode keynEncode;
struct keyEncode *storedVal = &keynEncode;

I then assign values to the struct and want to write the struct to a file using fwrite:

keynEncode.key[0] = k1[0];
keynEncode.key[1] = k1[1];
keynEncode.encoded[0] = p[0];
keynEncode.encoded[1] = p[1];
// i tried to use storedVal.key[0] = k1[0]; but i was getting compile errors

fwrite(storedVal, sizeof(storedVal), 0xffff, fp);

Now my problem is that fwrite writes nothing to the file.

Where am I going wrong?

Solution 1

You are using sizeof on a pointer, this won't calculate the size of the effective struct but the one of the pointer (that could be 4 or 8 bytes). Try with sizeof(struct keyEncode) (sizeof(keyEncode) is enough if you are using C++).

Then I don't get why you have 0xFFFF as count, shouldn't it be just 1?

Solution 2

Assuming you only have one such struct, then you need to change:

fwrite(storedVal, sizeof(storedVal), 0xffff, fp);

to

fwrite(storedVal, sizeof(*storedVal), 1, fp);

Solution 3

The arguments to fwrite() are the data to be printed, the size of one data item, the number of data items, and the file pointer.

You have two problems with the sizes:

1. You specify 'sizeof(storedVal)', which is the size of a pointer - not the size of the structure.
2. You specify that 65,535 of them need to be written.

So, you need to use:

 if (fwrite(storedVal, sizeof(*storedVal), 1, fp) != 1)
    ...error handling...

Note that fwrite() returns the number of items written. In the general case, you have n items to write, and you should check:

 if (fwrite(storedVal, sizeof(*storedVal), n, fp) != n)
    ...error handling...

Comments

[Rabeel]

sizeof(storedVal) will give the size of the pointer, not the type pointed to. Should be sizeof(*storedVal)

[molleman]

ok thank you very much, i dont really understand the third parameter in the fwrite, if you could give some insight that would be great???

[Paul R]

@Worrier, @Paul: I think the above two comments were probably written while I was still editing.

[Tom Hennen]

Also, in the first two lines, 'file' should be 'fp'.

[molleman]

also i would like to write the value of the struct to a binary file

[Paul R]

you haven't installed Telepathy 1.0 ? ;-)

[Jack]

the third parameter is used in combination with size if you want to write multiple items with just a fwrite. Basically the total number of bytes written should be (second parameter * third parameter). With second you define the size of every item while third decides how many of them you are gonna write, like if you need to write out an array.

[Alexander Aleksandrovič Klimov]

The second param is the size of each of the items you are writing. The third is the number of those you are writing. It returns the number actually written, so you should check the return value to ensure everything happened as expected

[Jack]

Yes, what you are gonna write with fwrite is binary if the fields of the struct are binary, ASCII if they are char *. Of course they'll be mixed together.

[Nisse Engström]

You should always check the return values of fwrite() and fclose().