Thread: Question about file I/O from a newbie

Hey all!
Got some problems with basic file I/O on binary file. It seems like i can write to the file bacause its beeing created. But when i type in information, write it to file, and then reading from file, it seems OK. But if i close the program and try to open the file, all that its printed are just jibberish.

// My defines

Code:

#define TY 20
#define FY 50
#define HD 100
#define TI sizeof(struct TestInfo)
#define CO sizeof(struct Course)

My struct:

Code:

struct Course
{
        char kurskod[TY];
        char kursnamn[TY];
        char inst[TY];
        char larare[TY];
        char kurskommentar[FY];
};

My write to file:

Code:

void write_to_file(struct Course *co)
{
    FILE *fp;
    fp=fopen("courseinfo.bin", "wb");
    fwrite(&co,CO,1, fp);
    fclose(fp);
}

My read from file:

Code:

void read_from_file(struct Course *co)
{
    FILE *inFile;

    if(!(inFile = fopen("courseinfo.bin", "r")))
    fread(&co,CO,1,inFile);

    fclose(inFile);

}

The way i call them:

Code:

struct Course new_course;
read_from_file(&new_course);
write_to_file(&new_course);

Can anyone see whats from with my functions (or something else for that matter)?

Regards
/Henrik!

Code:

if(!(inFile = fopen("courseinfo.bin", "r")))
fread(&co,CO,1,inFile);

This doesn't do what you think it does. The following code is equivalent:

Code:

inFile = fopen("courseinfo.bin", "r");
if(inFile == NULL)
{
  fread(&co,CO,1,inFile);
}

You're reading if the file couldn't be opened. It should be the other way around.

Next, this function (read_from_file()) is being passed a pointer to a struct; you then use the & operator on it, giving you a pointer to a pointer to a struct. Since it's already a pointer, don't use the &. It's also probably a good idea to use sizeof directly with your identifier (variable name), so there's less of a chance for a size mismatch:

Code:

fread(co, sizeof *co, 1, inFile);

The use of & applies to your write_to_file() function, too. As does properly checking fopen()'s return value against NULL.

Originally Posted by matsp

These I don't like the least bit - or in Swedish "Om detta tycker jag inte". I'll continue in English tho', as it may get others to understand what we're talking about.

Or rather "detta tycker jag inte om"
Anyway, I also agree that you should a sizeof the actual variable and not use defines because it's more confusing your way. sizeof *p usually gives you gauranteed correct size of your buffer.

Thanks for all the quick replys. I'll shall try to do the things you suggested and learn from that.

Thanks again!

/Henrik!