fwrite() doe not move pointer, why?

realos · 12-07-2005, 10:28 AM

I am using C++BuilderX and am facing some problems while working with files.
My understanding was that fwrite() moves the pointer when writing something in a file.

Code:

 
char person[30];
char read_person[30];
FILE *fp;
fp = fopen("myfile.dat","wb+");

...(I read something in string person from stdin)

 fwrite(&person,sizeof(person),1,fp);
  fread(&read_person,sizeof(read_person),1,fp);
fclose(fp);

Don't I need to go back to the beginning of string, i.e. with fseek(),in file befor I can read it with fread()?

Any idea or hint is why fread() is reading the string I entered in file withoug use of fseek()?

Thanks in advance.

regards,

doraiashok · 12-08-2005, 03:37 AM

you are passing &person and &read_person to fwrite and fread respectively which are (char **). you should have passed person and read_person.

I claim the behaivour to be undefined in this case because all you are reading is some garbage characters.

realos · 12-08-2005, 06:08 AM

Well, I was trying to read a whole struct with fread() and fwrite(). It does not seem to work.

Code:

typedef struct  	{
                char 	vorname[30];
                char 	name[30];
                char 	strasse[50];
                char 	plz[10];
                char 	ort[30];
}ADRESSE_t;


ADRESSE_t person;

fwrite(&person,sizeof(person),1,fp);
fread(&read_person,sizeof(read_person),1,fp);

In order to cut short the code for forum questions, I did not mention that person is a struct.
Actually, sizeof() works fine with struct. Maybe fread() and fwrite() can't?? I also tried without the & sign but to no avail.

Definition of fread() and fwrite() suggests buffer could be anything:

Code:

  int fread( void *buffer, size_t size, size_t num, FILE *stream );
  int fwrite( const void *buffer, size_t size, size_t count, FILE *stream );

What kind of buffer is possible in fread()/fwrite()? Only standard variables like int, char, char[] or also made-up structs??

Can anybody clarify on this?

Thanks in anticipation.

doraiashok · 12-08-2005, 07:08 AM

yes, you can read/write structures into file using fread/fwrite.

In this case even, you would need to use fseek to read back the data written.

realos · 12-08-2005, 12:48 PM

Thanks doraiashok.
A little more reading and experimenting revealed that any type of structured data can also be dealt with by fread()/fwrite().