I got a problem with link objective files on a Mac OS X. Tracing back the problem is,
here is my C hello world program
#include <stdio.h>
int main(){
printf("Hello, world!\n");
return 0;
}
//Compile with gcc (clang LLVM compiler on mac)
$ gcc -c hello.c
The output file is hello.o
link with gcc and run the executable is
$ gcc hello.o -o hello
$ ./hello
Now, I have to use the mac linker program ld or Ld to link the the objective files instead of gcc. In this case, what arguments should I pass into the ld program in order to get the program run? A simple pass in the object file name, i.e.
$ ld hello.o
resulting in
ld: warning: -macosx_version_min not specified, assuming 10.6
Undefined symbols for architecture x86_64:
"_printf", referenced from:
_main in hello.o
"start", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for inferred architecture x86_64
So what other files that i need to include to link or architecture information that I need to specify? Thanks.
For a reference, my complete linker options are
ld -demangle -dynamic -arch x86_64
-macosx_version_min 10.9.0
-o hello
-lSystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/5.0/lib/darwin/libclang_rt.osx.a
Okay, I had this question before too. Yes, the reason for the linker errors is because you need to feed it all the magic arguments that gcc does. And the easy way to discover those is to invoke the -v option on gcc to reveal all the commands executed in the compilation stages. In your case, run:
gcc hello.o -o hello -v
...the output of which, on my system, ends with the line:
/usr/libexec/gcc/i686-apple-darwin9/4.2.1/collect2 -dynamic -arch i386 -macosx_version_min 10.5.8 -weak_reference_mismatches non-weak -o test -lcrt1.10.5.o -L/usr/lib/i686-apple-darwin9/4.2.1 -L/usr/lib/gcc/i686-apple-darwin9/4.2.1 -L/usr/lib/gcc/i686-apple-darwin9/4.2.1 -L/usr/lib/gcc/i686-apple-darwin9/4.2.1/../../../i686-apple-darwin9/4.2.1 -L/usr/lib/gcc/i686-apple-darwin9/4.2.1/../../.. test.o -lgcc_s.10.5 -lgcc -lSystem
I don't know what the collect2 program is, but if you feed all those arguments to ld it should work just the same (at least it does on my system).
Related
I'm trying to get my head around how the linking process works when producing an executable. To do that I'm reading Ian Taylor's blog series about it, but a lot of it is beyond me at the moment - so I'd like to see how it works in practice.
At the moment I produce some object files and link them via gcc with:
gcc -m32 -o test.o -c test.c
gcc -m32 -o main.o -c main.c
gcc -m32 -o test main.o test.o
How do I replicate the gcc -m32 -o test main.o test.o stage using ld?
I've tried a very naive: ld -A i386 ./test.o ./main.o
But that returns me these errors:
ld: i386 architecture of input file `./test.o' is incompatible with i386:x86-64 output
ld: i386 architecture of input file `./main.o' is incompatible with i386:x86-64 output
ld: warning: cannot find entry symbol _start; defaulting to 00000000004000b0
./test.o: In function `print_hello':
test.c:(.text+0xd): undefined reference to `_GLOBAL_OFFSET_TABLE_'
test.c:(.text+0x1e): undefined reference to `puts'
./main.o: In function `main':
main.c:(.text+0x15): undefined reference to `_GLOBAL_OFFSET_TABLE_
I'm most confused by _start and _GLOBAL_OFFSET_TABLE_ being missing - what additional info does gcc give to ld to add them?
Here are the files:
main.c
#include "test.h"
void main()
{
print_hello();
}
test.h
void print_hello();
test.c
#include <stdio.h>
void print_hello()
{
puts("Hello, world");
}
#sam : I am not the best people to answer your question because I am a beginner in compilation. I know how to compile programs but I do not really understand all the details (https://en.wikipedia.org/wiki/Compilers:_Principles,_Techniques,_and_Tools)
So, I decided this year to try to understand how compilation works and I tried to do, more or less, the same things as you tried a few days ago. As nobody has answered, I am going to expose what I have done but I hope an expert will supplement my answer.
Short answer : It is recommended to not use ld directly but to use gcc directly instead. Nevertheless, it is, as you write, interesting to know how the linking process works. This command works on my computer :
ld -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o test test.o main.o /usr/lib/crt1.o /usr/lib/libc.so /usr/lib/crti.o /usr/lib/crtn.o
Very Long answer :
How did I find the command above ?
As n.m suggested, run gcc with -v option.
gcc -v -m32 -o test main.o test.o
... /usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2 ... (many
options and parameters)....
If you run ld with these options and parameters (copy and paste), it should work.
Try your command with -m elf_i386 (cf. collect2 parameters)
ld -m elf_i386 test.o main.o
ld: warning: cannot find entry symbol _start; ....
Look for symbol _start in object files used in the full ld command.
readelf -s /usr/lib/crt1.o (or objdump -t)
Symbol table '.symtab' contains 18 entries: Num: Value Size
Type Bind Vis Ndx Name... 11: 00000000 0 FUNC
GLOBAL DEFAULT 2 _start
Add this object to your ld command :ld -m elf_i386 test.o main.o /usr/lib/crt1.o
... undefined reference to `__libc_csu_fini'...
Look for this new reference in object files. It is not so obvious to know which library/object files are used because of -L, -l options and some .so include other libraries. For example, cat /usr/lib/libc.so. But, ld with --trace option helps. Try this commandld --trace ... (collect2 parameters)At the end, you should findld -m elf_i386 -o test test.o main.o /usr/lib/crt1.o /usr/lib/libc_nonshared.a /lib/libc.so.6 /usr/lib/crti.oor shorter (cf. cat /usr/lib/libc.so) ld -m elf_i386 -o test test.o main.o /usr/lib/crt1.o /usr/lib/libc.so /usr/lib/crti.o
It compiles but it does not run (Try to run ./test). It needs the right -dynamic-linker option because it is a dynamically linked ELF executable. (cf collect2 parameters to find it) ld -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o test test.o main.o /usr/lib/crt1.o /usr/lib/libc.so /usr/lib/crti.o But, it does not run (Segmentation fault (core dumped)) because you need the epilogue of the _init and _fini functions (https://gcc.gnu.org/onlinedocs/gccint/Initialization.html). Add the ctrn.o object. ld -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o test test.o main.o /usr/lib/crt1.o /usr/lib/libc.so /usr/lib/crti.o /usr/lib/crtn.o./test
Hello, world
I just want to test -pg, the source file is very simple, my environment is
cygwin,
$ uname -a
CYGWIN_NT-10.0 SHA-LPLATOW 2.8.2(0.313/5/3) 2017-07-12 10:58 x86_64 Cygwin
$ vi pgtest.c
#include <stdio.h>
void main(void){
printf("hello, world\n");
}
no -pg compiling is OK.
$ gcc -c pgtest.c
$ gcc -o pgtest.exe pgtest.o
but -pg report error
$ gcc -pg -c pgtest.c
$ gcc -o pgtest.exe pgtest.o
pgtest.o:pgtest.c:(.text+0x1): undefined reference to `__fentry__'
pgtest.o:pgtest.c:(.text+0x1): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `__fentry__'
pgtest.o:pgtest.c:(.text+0xe): undefined reference to `_monstartup'
pgtest.o:pgtest.c:(.text+0xe): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `_monstartup'
collect2: error: ld returned 1 exit status
I have tried LDFLAGS, it is the same.
export LDFLAGS="-pg" ; gcc -o pgtest.exe pgtest.o
from the gcc info page
'-pg'
Generate extra code to write profile information suitable for the
analysis program 'gprof'. You must use this option when compiling
the source files you want data about, and you must also use it when
linking.
so if you want to do a separate compilation and linking you need to repeate the -pg
$ gcc -c pgtest.c -pg
$ gcc -o pgtest.exe pgtest.o -pg
My base assembler file foidlrt.asm started getting a bit too large so I broke it up into two. Here is the entirety of the second file folder_stdio.asm:
; foidl_stdio.asm
%include "foidlstnd.inc"
section .text
DEFAULT REL
global foidl_fclose ; Raw file close
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; foidl_close
; Raw file close
; REGISTERS (1):
; RDI file handle
; CALLS:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
foidl_fclose:
mov rax,SYSCALL_FILE_CLOSE ; 0x2000006
syscall
ret
However, now when I build I am now getting this error from make despite the global declaration in the new file:
nasm src/foidlrt.asm -f macho64 --prefix _ -g -O0 -Iincludes/ -o asmobjs/foildrt.o
nasm src/foidlrt.asm -f macho64 --prefix _ -g -O0 -Iincludes/ -o asmobjs/foidl_stdio.o
libtool -static -s -o libs/libfoidlrt.a asmobjs/foildrt.o asmobjs/foidl_stdio.o
gcc src/testlink.c -L libs -l foidlrt -Wall -g -L. -Wl,-pie -I. -o bin/testlink
Undefined symbols for architecture x86_64:
"_foidl_fclose", referenced from:
_main in testlink-4b5ad3.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Version information:
XCode - 7.2.1 (7C1002)
nasm - NASM version 2.12 compiled on Feb 28 2016
gcc - Apple LLVM version 7.0.2 (clang-700.1.81)
RESOLVED
Error was all mine, makefile rule was bad. Working as expected now.
So, i write some instruction on asm, and compile them.
nasm -f macho test.asm
Now, nasm generate obj file, test.o
gcc test.o
Returned next error:
ld: warning: ignoring file test.o, file was built for unsupported file
format which is not the architecture being linked (x86_64)
Undefined
symbols for architecture x86_64: "_main", referenced from:
start in crt1.10.6.o ld: symbol(s) not found for architecture x86_64 collect2: ld returned 1 exit status
in gcc line, i used -arch i386 (x86_64), returned same error.
Can anybody help? :)
These commands work for me (OS X 10.12 x64):
nasm -f macho test.asm -DDARWIN
ld -o test test.o -arch i386 -lc -no_pie -macosx_version_min 10.12 -lSystem
macho is a 32 bit format.
Try to use macho64 instead of macho. Complete line:
nasm -f macho64 test.asm
Try to update your nasm version and use the following command:
/usr/local/bin/nasm -f macho64 ${file}
ld -o ${file_path}/${file_base_name} -e _main ${file_path}/${file_base_name}.o
${file_path}/${file_base_name}
I am observing a difference when trying to do the same operation on GCC 4.4 and GCC 4.5. Because the code I am doing this with is proprietary, I am unable to provide it, but I am observing a similar failure with this simple test case.
What I am basically trying to do is have one shared library (libb) depend on another shared library (liba). When loading libb, I assume that liba should be loaded as well - even though libb is not necessarily using the symbols in liba.
What I am observing is when I compile with GCC 4.4, I observe that the liba is loaded, but if I compile with GCC 4.5, libb is not loaded.
I have a small test case that consists of two files, a.c and b.c . The contents of the files:
//a.c
int a(){
return 0;
}
//b.c
int b(){
return 0;
}
//c.c
#include <stdio.h>
int a();
int b();
int main()
{
printf("%d\n", a()+b());
return 0;
}
//test.sh
$CC -o liba.so a.c -shared
$CC -o libb.so b.c -shared -L. -la -Wl,-rpath-link .
$CC c.c -L. -lb -Wl,-rpath-link .
LD_LIBRARY_PATH=. ./a.out
This is my output with different versions of GCC
$ CC=gcc-4.4 ./test.sh
1
$ CC=gcc-4.5 ./test.sh
/tmp/cceJhAqy.o: In function `main':
c.c:(.text+0xf): undefined reference to `a'
collect2: ld returned 1 exit status
./test.sh: line 4: ./a.out: No such file or directory
$ CC=gcc-4.6 ./test.sh
/tmp/ccoovR0x.o: In function `main':
c.c:(.text+0xf): undefined reference to `a'
collect2: ld returned 1 exit status
./test.sh: line 4: ./a.out: No such file or directory
$
Can anyone explain what is happening? Another extra bit of information is that ldd on libb.so does show liba.so on GCC 4.4 but not on GCC 4.5.
EDIT
I changed test.sh to the following:
$CC -shared -o liba.so a.c
$CC -L. -Wl,--no-as-needed -Wl,--copy-dt-needed-entries -la -shared -o libb.so b.c -Wl,-rpath-link .
$CC -L. c.c -lb -Wl,-rpath-link .
LD_LIBRARY_PATH=. ./a.out
This gave the following output with GCC 4.5:
/usr/bin/ld: /tmp/cc5IJ8Ks.o: undefined reference to symbol 'a'
/usr/bin/ld: note: 'a' is defined in DSO ./liba.so so try adding it to the linker command line
./liba.so: could not read symbols: Invalid operation
collect2: ld returned 1 exit status
./test.sh: line 4: ./a.out: No such file or directory
There seems to have been changes in how DT_NEEDED libraries are treated during linking by ld. Here's the relevant part of current man ld:
With --copy-dt-needed-entries dynamic libraries mentioned on the command
line will be recursively searched, following their DT_NEEDED tags to other libraries, in order to resolve symbols required by the output binary. With the
default setting however the searching of dynamic libraries that follow it will stop with the dynamic library itself. No DT_NEEDED links will be traversed
to resolve symbols.
(part of the --copy-dt-needed-entries section).
Some time between GCC 4.4 and GCC 4.5 (apparently, see some reference here - can't find anything really authoritative), the default was changed from the recursive search, to no recursive search (as you are seeing with the newer GCCs).
In any case, you can (and should) fix it by specifying liba in your final link step:
$CC c.c -L. -lb -la -Wl,-rpath-link .
You can check that this linker setting is indeed (at least part of) the issue by running with your newer compilers and this command line:
$CC c.c -L. -Wl,--copy-dt-needed-entries -lb -Wl,--no-copy-dt-needed-entries \
-Wl,-rpath-link .