[lustre-discuss] more on lustre striping

John Bauer bauerj at iodoctors.com
Fri Jun 10 09:04:52 PDT 2016

To confirm the point that you can not intercept the open called by fopen 
by using LD_PRELOAD, I have written a simple test case. Note that the 
runtime linker never looks for open().  Only fopen()

*$ cat a.c*
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <stdio.h>

main(int argc, char ** argv ){
    FILE *f = fopen("a", "r" ) ;
*$ file a*
a: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically 
linked (uses shared libs), for GNU/Linux 2.6.32, 
BuildID[sha1]=dfe043b4ec8cf19d5fd3fab524d7c72ed1453574, not stripped
*$ cat a.csh*
setenv LD_DEBUG all
./a >&! a.cpr
*$ ./a.csh*
*$ grep -i open a.cpr*
     120584:     symbol=fopen;  lookup in file=./a [0]
     120584:     symbol=fopen;  lookup in file=/lib64/libc.so.6 [0]
     120584:     binding file ./a [0] to /lib64/libc.so.6 [0]: normal 
symbol `fopen' [GLIBC_2.2.5]

On 6/10/2016 7:29 AM, Ashley Pittman wrote:
> On 22/05/16 02:56, John Bauer wrote:
>> Oleg
>> I can intercept the fopen(), but that does me no good as I can't set 
>> the O_LOV_DELAY_CREATE bit.  What I can not intercept is the open() 
>> downstream of fopen().  If one examines the symbols in libc you will 
>> see there are no unsatisfied externals relating to open, which means 
>> there is nothing for the runtime linker to find concerning open's.  I 
>> will have a look at the Lustre 1.8 source, but I seriously doubt that 
>> the open beneath fopen() was intercepted with LD_PRELOAD.  I would 
>> love to find a way to do that.  I could throw away a lot of code. 
>> Thanks,  John
> Could you not intercept fopen() and implement it with calls to open() 
> and fdopen() yourself which would give you full control over what 
> you're looking for here?
> Ashley.

I/O Doctors, LLC
bauerj at iodoctors.com

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