Issues with LD when using GCC in a C program

Issues with LD when using GCC in a C program

[Marcus Clyne]

Hi,

I'm writing a C program which calls gcc using execl().

I have successfully called gcc to create an object file, but when I try 
to create the shared object file, I get problems with ld.

The error I'm getting is:
------------------
/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../lib/crt1.o: In function 
`_start':
(.text+0x18): undefined reference to `main'
collect2: ld returned 1 exit status
------------------

I have added a PATH and LD_LIBRARY_PATH in the evironment, but this 
doesn't solve the problem.

The relevant code is:
-------------------
setenv ("PATH", 
"/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games", 
1);
setenv ("LD_LIBRARY_PATH", 
"/usr/local/lib:/usr/lib:/lib:/lib/tls/i686/cmov", 1);


// compile the shared object

execl ("/usr/bin/gcc",
           "-shared",
           "-lmylib",
           "-o",
           so_file,
           object_file,
           NULL);
-------------------

Where so_file and object_file are NULL-terminated char*'s, and the 
object_file has been created.

I can successfully compile the files using gcc from my shell, and ld is 
located at /usr/bin/ld.

I'm guessing that I need to add something to my env, or perhaps to the 
gcc options, but I'm not quite sure what.


Can anyone help?

Thanks,

Marcus Clyne.

Re: Issues with LD when using GCC in a C program

[Ian Lance Taylor]

Marcus Clyne <eugaia@gmail.com> writes:

> // compile the shared object
>
> execl ("/usr/bin/gcc",
>           "-shared",
>           "-lmylib",
>           "-o",
>           so_file,
>           object_file,
>           NULL);

Note that -lmylib is in the wrong place.  Libraries must follow the
objects which refer to them.

It does not make sense that you would get an undefined symbol error when
using -shared, so I don't know what is going on.  It is unlikely that
you are missing anything from the environment.

I recommend adding the -v option to your execl, to verify that the
expected options are being passed to the subprocesses.  Compare to the
output using -v on the command line.

Ian

Re: Issues with LD when using GCC in a C program

[Philip Herron]

Hey

Instead of using execl you could also use: popen

http://linux.die.net/man/3/popen

Brings you back the outputs like stdout stderr etc...

-Phil

2009/3/24 Ian Lance Taylor <iant@google.com>:
> Marcus Clyne <eugaia@gmail.com> writes:
>
>> // compile the shared object
>>
>> execl ("/usr/bin/gcc",
>>           "-shared",
>>           "-lmylib",
>>           "-o",
>>           so_file,
>>           object_file,
>>           NULL);
>
> Note that -lmylib is in the wrong place.  Libraries must follow the
> objects which refer to them.
>
> It does not make sense that you would get an undefined symbol error when
> using -shared, so I don't know what is going on.  It is unlikely that
> you are missing anything from the environment.
>
> I recommend adding the -v option to your execl, to verify that the
> expected options are being passed to the subprocesses.  Compare to the
> output using -v on the command line.
>
> Ian
>

Re: Issues with LD when using GCC in a C program

[Ian Lance Taylor]

Philip Herron <herron.philip@googlemail.com> writes:

> Instead of using execl you could also use: popen
>
> http://linux.die.net/man/3/popen
>
> Brings you back the outputs like stdout stderr etc...

Note that popen, unlike execl, runs the shell, which leads to various
security issues.  execl is definitely to be preferred when there is any
user control over arguments or the environment.  That may not be an
issue in this particular case, of course.

Ian



> 2009/3/24 Ian Lance Taylor <iant@google.com>:
>> Marcus Clyne <eugaia@gmail.com> writes:
>>
>>> // compile the shared object
>>>
>>> execl ("/usr/bin/gcc",
>>>           "-shared",
>>>           "-lmylib",
>>>           "-o",
>>>           so_file,
>>>           object_file,
>>>           NULL);
>>
>> Note that -lmylib is in the wrong place.  Libraries must follow the
>> objects which refer to them.
>>
>> It does not make sense that you would get an undefined symbol error when
>> using -shared, so I don't know what is going on.  It is unlikely that
>> you are missing anything from the environment.
>>
>> I recommend adding the -v option to your execl, to verify that the
>> expected options are being passed to the subprocesses.  Compare to the
>> output using -v on the command line.
>>
>> Ian
>>