I am porting NewLib for my own OS by following a tutorial.
It says that once I finished my crt0, I have to "link it as the first object". How can I do that?
A:
One way, for testing at least, is to place crt0.o as the first file on the compiler or linker command line.
For production, you'd put it in the linker command file.
Richard Pennington
2010-08-01 11:38:59
+2
A:
It says that once I finished my crt0, I have to "link it as the first object".
That means precisely what is says. When an application for your OS is being linked, the crt0 must be the very first object file on linker's command line: before any other object file.
Traditionally, UNIX linkers resolve a symbol by making a look up starting from first object/library looping over to the last object/library. Putting crt0 as the first object file makes sure that system symbols (functions, variables) are picked from the crt0 file, not some other file.
Additionally, as pointed by R.., traditional linkers assume that application's entry point is at the beginning of the code segment. That also fits to the source code found on the linked page: first symbol in the code is _start, the usual name of the program's entry point.
E.g. running gcc -v a.c -o a on my Debian (notice the -v), one sees following linking command (I have added new lines for readability):
/usr/lib/gcc/i486-linux-gnu/4.4.4/collect2
--build-id
--eh-frame-hdr
-m elf_i386
--hash-style=both
-dynamic-linker /lib/ld-linux.so.2
-o a
/usr/lib/gcc/i486-linux-gnu/4.4.4/../../../../lib/crt1.o
/usr/lib/gcc/i486-linux-gnu/4.4.4/../../../../lib/crti.o
/usr/lib/gcc/i486-linux-gnu/4.4.4/crtbegin.o
-L/usr/lib/gcc/i486-linux-gnu/4.4.4
-L/usr/lib/gcc/i486-linux-gnu/4.4.4
-L/usr/lib/gcc/i486-linux-gnu/4.4.4/../../../../lib
-L/lib/../lib
-L/usr/lib/../lib
-L/usr/lib/gcc/i486-linux-gnu/4.4.4/../../..
/tmp/ccndJ4YV.o
-lgcc
--as-needed
-lgcc_s
--no-as-needed
-lc
-lgcc
--as-needed
-lgcc_s
--no-as-needed
/usr/lib/gcc/i486-linux-gnu/4.4.4/crtend.o
/usr/lib/gcc/i486-linux-gnu/4.4.4/../../../../lib/crtn.o
One can see that crt1 is first object on the command line. Looking at the linker script (-m elf_i386 -> find /usr -name '*elf_i386*' -> on my system /usr/lib/ldscripts/elf_i386.x) one can verify that on Linux there is no crt0, but there are: crt1, crti, crtbegin, crtend, crtn. And the application object files (/tmp/ccndJ4YV.o in the sample above) are put between the crtbegin and crtend.
Dummy00001
2010-08-01 11:45:20
Actually the traditional reason `crt0.o` needs to be the first object is that traditional linkers didn't look for a particular symbol name to use for the entry point, but instead simply put the entry point at the beginning of `.text`.
R..
2010-08-01 17:08:26
@R..: you might be right esp for the case. that would explain why the linked wiki article was directing to read about the linker script. would correct my response.
Dummy00001
2010-08-01 17:20:13