ld --wrap and zcat

ld --wrap and zcat

[Shaun Jackman]

fstream::open in libstdc++ calls fopen in libc. I'd like to use ld 
--wrap to hook the fopen call to call popen to decompress a .gz file 
using zcat. Is this possible with ld --wrap? From my experimentation, 
it seems not. Is this possible using LD_PRELOAD and dlsym?

Thanks,
Shaun

Re: ld --wrap and zcat

[Nick Clifton]

Hi Shaun,

> fstream::open in libstdc++ calls fopen in libc. I'd like to use ld 
> --wrap to hook the fopen call to call popen to decompress a .gz file 
> using zcat. Is this possible with ld --wrap? From my experimentation, it 
> seems not. Is this possible using LD_PRELOAD and dlsym?

Do you have access to the linker command line used to build your 
executable ?  If so you can just create your own custom library that 
contains its own implementation of fopen (which calls popen as you 
desire) and then insert the name of this library in between the 
libstdc++-v3 library and the libc library on the linker command line.

Cheers
   Nick

Re: ld --wrap and zcat

[Shaun Jackman]

Neat, I didn't think of that. Thanks, Nick.

The solution I settled didn't require LD_PRELOAD, which I'm happy 
about, but did use -ldl and dlsym. I created a function `fopen' in my 
application, which -- and I don't entirely understand why this is -- 
libstdc++ does call. Within my fopen, I used dlsym(RTLD_NEXT, "fopen") 
to call the fopen of libc.

Cheers,
Shaun

Nick Clifton wrote:
> Hi Shaun,
> 
>> fstream::open in libstdc++ calls fopen in libc. I'd like to use ld 
>> --wrap to hook the fopen call to call popen to decompress a .gz file 
>> using zcat. Is this possible with ld --wrap? From my experimentation, it 
>> seems not. Is this possible using LD_PRELOAD and dlsym?
> 
> Do you have access to the linker command line used to build your 
> executable ?  If so you can just create your own custom library that 
> contains its own implementation of fopen (which calls popen as you 
> desire) and then insert the name of this library in between the 
> libstdc++-v3 library and the libc library on the linker command line.
> 
> Cheers
>    Nick