On Mon, Jun 06, 2011 at 08:05:31PM +0530, Prabhu wrote:

Hi all,

I am writing user- space driver. My intention is to print whatever string i am enter into terminal.

Here i am using setbuf function to sent the data to the print after some fixed string length.

I wrote below program for understand setbuf functionality. But i am getting unpredictable result for some output . can any one explain the why i am getting these result.

root@desktop:/home/prabhu# cat a.c #include <stdio.h> main() { char buff2[]= "hello world"; char buf1[6]="";

From the manpage:

You must make sure that the space that buf points to still exists by the time stream is closed, which also happens at program termination. For example, the following is invalid: #include <stdio.h> int main(void) { char buf[BUFSIZ]; setbuf(stdin, buf); printf("Hello, world!\n"); return 0; } In addition setbuf assumes the buffer to be BUFSIZE big.

setbuf(stdin, buf1); getchar(); printf ("%s",buf1);

Do you really want to print buf1 here?

}

root@desktop:/home/prabhu# cc a.c [ don't shoot yourself in the foot, root ] root@desktop:/home/prabhu# ./a.out a a root@desktop:/home/prabhu# ./a.out aa aa root@desktop:/home/prabhu# ./a.out aaa aaa

Here you're just lucky you found some zeros on the stack. You generally _cannot_ predict what you will find on the stack, if you didn't write to it in the same function (or a function called by it). (buf1 now contains {'a', 'a', 'a', '\n', zero, something_likely_zero})

root@desktop:/home/prabhu# ./a.out aaaa aaaa root@desktop:/home/prabhu# ./a.out aaaaa aaaaa hello worldroot@desktop:/home/prabhu# ./a.out

You seem to have a stack layout like this: [ buf1 ] [ buf2 ] stdio now tries to write "aaaaa\n" (without null-termination) into buf1. printf sees this: "aaaaa\nhello world\0", and, of course, prints everything up to the \0. Btw, your program has severe security vulnerabilities; try feeding it long (> 18 chars) strings or strings containing printf format specifiers :-).

aaaaaa aaaaaa ello worldroot@desktop:/home/prabhu# ./a.out aaaaaaa aaaaaaa llo worldroot@desktop:/home/prabhu# ./a.out aaaaaaaa aaaaaaaa lo worldroot@desktop:/home/prabhu# ./a.out aaaaaaaaa aaaaaaaaa o worldroot@desktop:/home/prabhu# ./a.out aaaaaaaaaa aaaaaaaaaa worldroot@desktop:/home/prabhu# ./a.out aaaaaaaaaaa aaaaaaaaaaa

Thanks / HTH, Jonathan Neuschäfer