Stream buffer size of open file in GNU c library

rajesh_b · 01-11-2007, 12:51 AM

Hi,

I have a doubt regarding buffer size provided by file stream. I know that what ever read or write we are doing on a open file ( FILE pointer of fopen()) will act on the buffer provided by GNU c library. I want to know the exact size of the buffer provided by GNU c library. Is there any way to configure this buffer size ?

Thanks in advance.

Regards,
Rajesh

Guttorm · 01-11-2007, 02:06 AM

Hi

Not really sure what the size is. But you can call setvbuf right after fopen so you can set it yourself. See "man setvbuf".

Edit:
Just looked at stdio.h and it has there is a:
#define BUFSIZ _IO_BUFSIZ

grep "#define _IO_BUFSIZ" /usr/include/*
/usr/include/libio.h:#define _IO_BUFSIZ _G_BUFSIZ

grep "#define _G_BUFSIZ" /usr/include/*
/usr/include/_G_config.h:#define _G_BUFSIZ 8192

So it's 8k. So increasing it with setvbuf is probably a good idea to save kernel calls to speed up things, if you're working with big files.