How to assemble the output of gcc -S ?

[manasij7479]

I am reading a book on Assembly Language (Programming from the ground up)... and suddenly had the urge to learn by examining gcc's assembly output, because some of the computer architecture stuff boring to me now and seems not really necessary(I know the general picture pretty well) for getting the basics of asm.
SO, I compiled

int main(void)
{
        return 0;
}

to get

        .file   "a.c"
        .text
.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        movl    $0, %eax
        popl    %ebp
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.6.1 20110908 (Red Hat 4.6.1-9)"
        .section        .note.GNU-stack,"",@progbits

But this does not have a _start and the following fails to assemble and link it correctly.

as foo.s -o foo.o
ld foo.o -o foo

How should I modify the assembly and linking commands to correctly assemble the code without modification ?

P.S.
1. gcc's verbose option may show it .. but I am unable to decipher the gobbledygook.
2. Linking libc in the linker step leads to a segfault at execution .

[grumpy]

Assembler is machine dependent, so the techniques for assembling it are also machine dependent.

The back end of gcc is based on the gnu assembler (xommonly known as gas). Google for "gas gnu".

[GReaper]

GCC can assemble it by itself, using the the assembler mentioned by @grumpy, just by giving a (*.s) extension to the file.

[manasij7479]

...
It seems that 'gas' is same as 'as' (the only assembler in the binutils package is given as 'as' ).

GCC can assemble it by itself

Exactly what I'm looking for is the way it does so.

[Salem]

$ gcc -v foo.c
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.3-4ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i486 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) 
COLLECT_GCC_OPTIONS='-v' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.4.3/cc1 -quiet -v foo.c -D_FORTIFY_SOURCE=2 -quiet -dumpbase foo.c -mtune=generic -auxbase foo -version -fstack-protector -o /tmp/ccy9quou.s
GNU C (Ubuntu 4.4.3-4ubuntu5) version 4.4.3 (x86_64-linux-gnu)
	compiled by GNU C version 4.4.3, GMP version 4.3.2, MPFR version 2.4.2-p1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/usr/include/x86_64-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.4.3/include
 /usr/lib/gcc/x86_64-linux-gnu/4.4.3/include-fixed
 /usr/include
End of search list.
GNU C (Ubuntu 4.4.3-4ubuntu5) version 4.4.3 (x86_64-linux-gnu)
	compiled by GNU C version 4.4.3, GMP version 4.3.2, MPFR version 2.4.2-p1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 2129e1a56226bd6e8f7af5e0a3ff467d
COLLECT_GCC_OPTIONS='-v' '-mtune=generic'
 as -V -Qy -o /tmp/ccPaKnbN.o /tmp/ccy9quou.s
GNU assembler version 2.20.1 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.20.1-system.20100303
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.3/:/usr/lib/gcc/x86_64-linux-gnu/4.4.3/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.3/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.4.3/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.4.3/:/usr/lib/gcc/x86_64-linux-gnu/4.4.3/:/usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/
COLLECT_GCC_OPTIONS='-v' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.4.3/collect2 --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=both -dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro /usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../lib/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../lib/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.4.3/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.4.3 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.3 -L/usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../.. -L/usr/lib/x86_64-linux-gnu /tmp/ccPaKnbN.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.4.3/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.4.3/../../../../lib/crtn.o
$ gcc -S foo.c
$ cat foo.s
	.file	"foo.c"
	.section	.rodata
.LC0:
	.string	"hello world"
	.text
.globl main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	movq	%rsp, %rbp
	.cfi_offset 6, -16
	.cfi_def_cfa_register 6
	subq	$16, %rsp
	movl	%edi, -4(%rbp)
	movq	%rsi, -16(%rbp)
	movl	$.LC0, %edi
	call	puts
	movl	$0, %eax
	leave
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
	.section	.note.GNU-stack,"",@progbits
$ as -V -Qy -o foo.o foo.s
GNU assembler version 2.20.1 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.20.1-system.20100303

[manasij7479]

..Thanks..
Can't believe I was lazy enough not to look, though.

btw..I'm finally leaving the linking to gcc; that way, it works.
If I do it with ld(possibly..er obviously... incorrectly), it still shows a segfault.

[Fordy]

IIRC GAS (Gnu Assembler) likes a "_start" entrypoint. If you assemble is with GCC it starts at "main"

I prefer to work with NASM for this stuff, you can then output to ELF and link in with GCC and your C code