M K Saravanan | mksarav AT gmail DOT com
May 2007
gcc-exercise-1
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$ cat /etc/redhat-release
Red Hat Enterprise Linux ES release 4 (Nahant)

$ uname -a
Linux rdfwvoip 2.6.9-5.ELsmp #1 SMP Wed Jan 5 19:30:39 EST 2005 i686 i686 i386 G 
NU/Linux

man gcc

$ gcc --version
gcc (GCC) 3.4.3 20041212 (Red Hat 3.4.3-9.EL4)
Copyright (C) 2004 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

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1) using -S option

Stop after the stage of compilation proper; do not assemble.  The output is in 
the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the 
suffix .c, .i, etc., with .s.

Input files that don't require compilation are ignored. 

/* hello.c */
#include <stdio.h>

int main()
{
	printf("Hello, world\n");
	return 0;
}

/* end of hello.c */

gcc -S hello.c 

out: hello.s  (open in a text editor and have a look)

hello.s contents:


        .file   "hello.c"
        .section        .rodata
.LC0:
        .string "Hello, world\n"
        .text
.globl main
        .type   main, @function
main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        andl    $-16, %esp
        movl    $0, %eax
        addl    $15, %eax
        addl    $15, %eax
        shrl    $4, %eax
        sall    $4, %eax
        subl    %eax, %esp
        subl    $12, %esp
        pushl   $.LC0
        call    printf
        addl    $16, %esp
        movl    $0, %eax
        leave
        ret
        .size   main, .-main
        .section        .note.GNU-stack,"",@progbits
        .ident  "GCC: (GNU) 3.4.3 20041212 (Red Hat 3.4.3-9.EL4)"

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2) using -E option

Stop after the preprocessing stage; do not run the compiler proper. The output 
is in the form of preprocessed source code, which is sent to the standard 
output.

Input files which don't require preprocessing are ignored.

/* ex1.c */
#include "ex1.h"

int main()
{
	return 0;
}
/* end of ex1.c */

/* ex1.h */
#ifndef _EX1_H_
#define _EX1_H_


#define ONE 1
#define TWO 2

#endif /* _EX1_H_ */
/* end of ex1.h */


gcc -E ex1.c > ex1.i

out: ex1.i (open in a text editor and have a look)

ex1.i contents:
# 1 "ex1.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "ex1.c"

# 1 "ex1.h" 1
# 3 "ex1.c" 2

int main()
{
 return 0;
}

The same effect can also be produced by using the GNU C pre-processor, cpp.

cpp ex1.c > ex1.i

man cpp

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3) using -c option

Compile or assemble the source files, but do not link.  The linking stage simply 
is not done.  The ultimate output is in the form of an object file for each 
source file.

By default, the object file name for a source file is made by replacing the 
suffix .c, .i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are ignored.

gcc -c hello.c

out: hello.o

"file" utility will tell you about the content of a file.  Try this on hello.o.

$ file ./hello.o
./hello.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripp 
ed

The utility "objdump" will display info from object files.

man objdump

Try the following:

objdump -x ./hello.o
objdump -s ./hello.o
objdump -t ./hello.o
objdmp -T ./hello.o

Also experiment with other options in objdump.

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4) all in one step

-o is used to mention output file name.

gcc -o hello hello.c

$ file ./hello
./hello: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux
 2.2.5, dynamically linked (uses shared libs), not stripped

$ ls -l hello
-rwxr-xr-x  1 mksarav firmware 4743 May 17 14:23 hello

Note the size of "hello".  It is 4743 bytes. 

The "strip" utility discard symbols from object files. 

$ strip ./hello

$ file ./hello
./hello: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), for GNU/Linux
 2.2.5, dynamically linked (uses shared libs), stripped

 "file" now says "hello" is stripped.

$ ls -l hello
-rwxr-xr-x  1 mksarav firmware 2960 May 17 14:24 hello

Note down the size of the "hello" now--it is 2960 bytes!

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