> From owner-freebsd-questi...@freebsd.org Wed Sep 29 05:50:13 2010 > Date: Wed, 29 Sep 2010 11:51:09 +0100 (BST) > From: Anton Shterenlikht <me...@bristol.ac.uk> > To: Michel Talon <ta...@lpthe.jussieu.fr> > Cc: freebsd-questions@freebsd.org > Subject: Re: ld(1) cannot find entry symbol _start; > > > > On Tue, 28 Sep 2010, Michel Talon wrote: > > > Paul B Mahol said: > > On 9/28/10, Anton Shterenlikht <me...@bristol.ac.uk> wrote: > >>> I'm trying to learn the very basics of the > >>> compile - assemble - link process on FreeBSD. > >>> Please don't shoot me. > >> .... > >>> Then I try to link the object file into > >>> an executable: > >>> > >>> % ld tmp.o > >> > >> You are missing something in above command. > >> > > > > More precisely, if you run gcc -v on a C file you get someting like: > > /usr/bin/ld --eh-frame-hdr -V -dynamic-linker /libexec/ld-elf.so.1 > > /usr/lib/crt1.o /usr/lib/crti.o /usr/lib/crtbegin.o -L/usr/lib > > -L/usr/lib /var/tmp//cco5EINk.o -lgcc --as-needed -lgcc_s --no-as-needed > > -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/crtend.o > > /usr/lib/crtn.o > > > > > > where the object file produced by compilation and assembling is > > /var/tmp//cco5EINk.o > > > > That is adds several other object files to your own in order to get > > an executable. > > > > In particular the start symbol, at which execution begins is in > > /usr/lib/crt1.o > > > > as you can see from > > niobe% nm /usr/lib/crt1.o > > w _DYNAMIC > > 00000000 D __progname > > U _fini > > U _init > > U _init_tls > > 00000000 T _start > > 00000020 t _start1 > > 00000000 r abitag > > U atexit > > 00000004 C environ > > U exit > > U main > > which shows that _start is defined here, (but not e.g. _init). On the > > other hand the function main() which is defined in your program is > > referred to but undefined here. > > thank you. Where can I read more on what each file is for: > > % ls -al /usr/lib/crt* > -r--r--r-- 1 root wheel 2552 Sep 15 13:52 /usr/lib/crt1.o > -r--r--r-- 1 root wheel 4656 Sep 15 13:53 /usr/lib/crtbegin.o > -r--r--r-- 1 root wheel 4936 Sep 15 13:53 /usr/lib/crtbeginS.o > -r--r--r-- 1 root wheel 4656 Sep 15 13:53 /usr/lib/crtbeginT.o > -r--r--r-- 1 root wheel 3648 Sep 15 13:53 /usr/lib/crtend.o > -r--r--r-- 1 root wheel 3648 Sep 15 13:53 /usr/lib/crtendS.o > -r--r--r-- 1 root wheel 1928 Sep 15 13:52 /usr/lib/crti.o > -r--r--r-- 1 root wheel 1087 Sep 15 13:52 /usr/lib/crtn.o > > The sources for these files are in asm, so would be good > to read a more accessible introduction.
Those routines set up the "run-time envrionment" that any UNIX c-language evironment expects. that's that the name 'crt' prefix means '<c>-language <r>un-<t>ime' this involves a bunch of 'gory mechanical details' that *DON'T* really matter how they get accomplished, jus that they _do_ get done. This includes things like setting up the 'stack', and the 'heap', initializing the tables for the dynamic-memory management routines (malloc and friends), setting up 'stdin/stdout/stderr', and getting the command-line arguments processed so that they can be passed as 'argc', and 'argv' (also the 'environment', in 'envp') to your 'main()' program. > > Also, it seems only crt1, crti and crtn are provided > by FreeBSD itself (/usr/src/lib/csu/ia64), crtbegin and > crtend are under /usr/src/contrib/gcc/config/ia64/, > and sources for *S.o and *T.o I can't find at all. > So which of these are specific to GCC on FreeBSD, and > which aren't? "who cares?" applies. it is all simmply "required housekeeping" go bring the state of the current running process (i.e, the task/ address-space/etc) to what is defined as the 'initial state' for a c-language 'main()' program. > > For example if I use g95 compiler instead of gfortran45, > will the linker still need all above object files? If you use 'the compiler' to manage the linking process -- e.g. {compilername} -o {executable} {one-or-more-'object' .o file} the right files will be automatically included in the executable. 'crt0.o' is the classical program entry point -- it may or may not rely on other routines to get 'the environment' set up -- especially depending on what 'advanced' features the program uses.. If you use light-weight-process 'threads' (-lpthread) this may call for different crt0 code. Unless you are engaged in porting the compiler and O/S to a completely new hardware architecture, or trying to generate 'stand-alone' code, e.g. for an embedded processor tht runs witout _anuting called a 'cpu', you don't have any reason to worry about this housekeeping code. Its there, it works, and it does what it's supposed to. <grin> Oh yeah, if you tamper with it, you make it incompatible with the use of _any_ existing object files. _______________________________________________ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to "freebsd-questions-unsubscr...@freebsd.org"