Help for file processing

[EvenDevil]

The problem is :there are two files, a.bin of 1024B in size and b.bin of 512B in size,I need copy the b.bin to the top 512B of a.bin and don't change the back 512B.
it seems easy,but i failed,even tried several times.
I open file like this:

 FILE *fa=fopen("a.bin",wb);
 FILE *fb=fopen("b.bin","rb");

the files can be opened successfully,but i finded the a.bin resized to 512B when i read binary from b.bin and writed to a.bin.
so i used another way to open a.bin :FILE *fa=fopen("a.bin",ab);
but this time the a.bin changed to 1.5K in size.

what should I do to solve the problems?
thank you

[quzah]

w - truncate the file and start over (start with the file pointer at the beginning)
a - append on the end of the file (start with the file pointer at the end)

You can use a, but you need to rewind so you can start at the beginning. Alternately, you could use something like r+ which would open for reading and writing, and start at the beginning of the file.


Quzah.

[EvenDevil]

w - truncate the file and start over (start with the file pointer at the beginning)
a - append on the end of the file (start with the file pointer at the end)

You can use a, but you need to rewind so you can start at the beginning. Alternately, you could use something like r+ which would open for reading and writing, and start at the beginning of the file.


Quzah.

Thanks for your reply.
I have used rewind() function before,but it failed still.The b.bin is appended to the tail of a.bin.

FILE* fa,*fb;
	if((fb=fopen("b.bin","rb")) == NULL )
      return false;
	if((fa=fopen("a.bin","ab"))== NULL )
      return false;
	unsigned char buff[512];
	memset(buff,0,512);
	fread(buff,512,1,fb);
	rewind(fa);
	fwrite(buff,512,1,fa);
	fclose(fa);
	fclose(fb);

according to your tip,I open a.bin using "r+b",and it works this time.
So thank u again.But i am still confused why the rewind function dose not work.

[Bayint Naung]

``r+'' Open for reading and writing. The stream is positioned at the
beginning of the file.

``a'' Open for writing. The file is created if it does not exist. The
stream is positioned at the end of the file. Subsequent writes
to the file will always end up at the then current end of file,
irrespective of any intervening fseek(3) or similar

man page fopen section 3

[CommonTater]

Most likely you need to open your A file in binary update mode...

FILE * fa = fopen("Filea.bin","rb+");
FILE * fb = fopen("Fileb.bin","r");

then just load the entire file B into a buffer and write it over top of file A.

[quzah]

Subsequent writes
to the file will always end up at the then current end of file,
irrespective of any intervening fseek(3) or similar

man page fopen section 3

Ah. I forgot about that. Good catch. That's why rewind doesn't do anything useful on just "a".


Quzah.