stupid gcc & ld problem

[odradek]

jimihex@b0x:~/devel/C$ ls
hello.c
jimihex@b0x:~/devel/C$ cat hello.c
#include <stdio.h>
int main () {
        printf("Hello world\n");
        getchar();
        return 0;
}
jimihex@b0x:~/devel/C$ gcc -o hello hello.c
jimihex@b0x:~/devel/C$ ls
hello  hello.c
jimihex@b0x:~/devel/C$ ./hello
Hello world

jimihex@b0x:~/devel/C$ gcc -c -o hello2.o hello.c
jimihex@b0x:~/devel/C$ ld -o hello2 hello2.o
ld: warning: cannot find entry symbol _start; defaulting to 0000000008048094
hello2.o: In function `main':hello.c:(.text+0x24): undefined reference to `puts'
:hello.c:(.text+0x29): undefined reference to `getchar'
jimihex@b0x:~/devel/C$

Help, please...

[WW]

Use gcc again, instead of ld:

$ gcc -o hello2 hello2.o

[phossal]

GCC is doing much more than most people think on an Ubuntu System. For example, it typically links with ld using something like this:

/usr/lib/gcc/i486-linux-gnu/4.1.2/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -oS /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crt1.o /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crti.o /usr/lib/gcc/i486-linux-gnu/4.1.2/crtbegin.o -L/usr/lib/gcc/i486-linux-gnu/4.1.2 -L/usr/lib/gcc/i486-linux-gnu/4.1.2 -L/usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib -L/lib/../lib -L/usr/lib/../lib S.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/i486-linux-gnu/4.1.2/crtend.o /usr/lib/gcc/i486-linux-gnu/4.1.2/../../../../lib/crtn.o

You can always compile using the -v (verbose) flag to see everything gcc is doing to your files.

Regards.

[odradek]

Use gcc again, instead of ld:

$ gcc -o hello2 hello2.o

Yes, I know that. I need ld for some other purposes, so I want to know what is going on during the linking.

phossal, thank you a lot, this helped, but I still didn't find with what arguments I should use ld for right linking. Any sugestions ?

[DoubleQuadword]

The following command line should link your program without warnings and/or unresolved references:

ld hello2.o -o hello2 -lc --entry main

Note that -lc tells the compiler to use the standard C library and --entry main tells the compiler to use the address of 'main' as the application's entry point.

Regards.

[phossal]

Yes, I know that. I need ld for some other purposes, so I want to know what is going on during the linking.

phossal, thank you a lot, this helped, but I still didn't find with what arguments I should use ld for right linking. Any sugestions ?

In the example I posted earlier, I highlighted my object file - it's S.o - in blue. Everything else was included by gcc. I had prepared S.o manually using GAS. It was simply a "Hello, World!" program.

DoubleQuadWord may have correctly identified the minimal things necessary for linking, but if that fails, you should be able to substitute your object file in place of S.o and get it to work.

I'm stuck playing in Windows, so I can't see what does/doesn't work like I normally would. Sorry.

[Wybiral]

I'm curious what you need ld for... ld is just a linker, it doesn't detect C's "main" as the starting point.

As DoubleQuadword showed, you can link to the C library... But if you're linking to the C library, why use ld and not GCC?

[odradek]

The following command line should link your program without warnings and/or unresolved references:

ld hello2.o -o hello2 -lc --entry main

Note that -lc tells the compiler to use the standard C library and --entry main tells the compiler to use the address of 'main' as the application's entry point.

Regards.

Thank you. This works, but, something strange is going on :

jimihex@b0x:~/devel/C$ ld -lc --entry main -o hello2 hello2.o
jimihex@b0x:~/devel/C$ ls
hello2  hello2.o  hello.c
jimihex@b0x:~/devel/C$ ./hello2
bash: ./hello2: No such file or directory
jimihex@b0x:~/devel/C$

phossal, I saw that too with "-v" (Sorry because of my poor english), but, are all those arguments necessary ? On Windows, with DJGPP this works fine, without problems. Btw, thanks a lot for helping me.

[odradek]

I'm curious what you need ld for... ld is just a linker, it doesn't detect C's "main" as the starting point.

As DoubleQuadword showed, you can link to the C library... But if you're linking to the C library, why use ld and not GCC?

I am working on some small OS development project and I need to link several object files into one flat binary. But, I am using Ubuntu only for a few months, so, it's little hard to adjust.

[rplantz]

I am working on some small OS development project and I need to link several object files into one flat binary. But, I am using Ubuntu only for a few months, so, it's little hard to adjust.

You can include your object files, assuming they are ELF format, on the same line with your C/C++ files. For example,

gcc myProg.c sub1.c sub2.o -o myProg

I do this often when I write functions in assembly language. I first assemble them, giving their respective object files. Then I use gcc to compile the main function (which is written in C) and link it with the already assembled object files. gcc is smart enough to know whether it has to compile the function or not. Then it goes on to the ld phase, automatically linking in the required C libraries.

(I do not show various options you may wish to use in my command above.)