Debugging the Code - c

Question

Hello Guys I am starting the voyage of debugging the code, and ran the following commands as per the book just for some analysis for the source code below
// hello_world-1.c
#include <stdio.h>
int main(void)
{
printf("hello world\n");
return 0;
gcc -Wall -Wextra -c hello_world-1.c // What is wall and wextra here ?
$ size hello_world-1 hello_world-1.o
text data bss dec hex filename
916 256 4 1176 498 hello_world-1
48 0 0 48 30 hello_world-1.o
$ objdump -h hello_world-1.o
hello_world-1.o: file format elf32-i386
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000023 00000000 00000000 00000034 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
1 .data 00000000 00000000 00000000 00000058 2**2
CONTENTS, ALLOC, LOAD, DATA
2 .bss 00000000 00000000 00000000 00000058 2**2
ALLOC
3 .rodata 0000000d 00000000 00000000 00000058 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .note.GNU-stack 00000000 00000000 00000000 00000065 2**0
CONTENTS, READONLY
5 .comment 0000001b 00000000 00000000 00000065 2**0
CONTENTS, READONLY
48 0 0 48 30 hello_world-1.o
I have some question here
1) There are no global variables in hello_world-1.c. Then why the size reports that the data and bss segments have zero length for the object file but non zero for the executable?
2) Why size and objdump report different sizes for the text segment?

Answer 1

Object file consists of .text (i.e. binary CPU instructions), .rodata (read-only data - "hello world"\10\0 - 13 bytes total) and .comment (additional linking information).
Executable file consists of the same minus .comment plus standard library stuff plus import dynamic library data, if any.
Standard library adds at least startup code, which makes executable bigger. So your difference is: executable .text = .object text + startup code + stdlibrary code (if static linking).

Answer 2

regarding your question:
"gcc -Wall -Wextra -c hello_world-1.c // What is wall and wextra here ?"
(note capitalization counts)
-Wall tell the compiler to enable most warnings
-Wextra tell the compiler to enable even more warnings
-c tells the compiler to only compile, not link.
because no '-o objfilename.o' parameter was included,
the compiler will ouput an object file with the same name as the input file, with a '.o' extension.
suggest always include the '-o objfilename.o' parameter explicitly
suggest performing some online googling for such things, wherein you would have found pages similar to :
https://gcc.gnu.org/onlinedocs/gcc-3.0.4/gcc_3.html

Answer 3

here is a copy of the man page for 'size'
SIZE(1) GNU Development Tools SIZE(1)
NAME
size - list section sizes and total size.
SYNOPSIS
size [-A|-B|--format=compatibility]
[--help]
[-d|-o|-x|--radix=number]
[--common]
[-t|--totals]
[--target=bfdname] [-V|--version]
[objfile...]
DESCRIPTION
The GNU size utility lists the section sizes---and the total size---for
each of the object or archive files objfile in its argument list. By
default, one line of output is generated for each object file or each
module in an archive.
objfile... are the object files to be examined. If none are specified,
the file "a.out" will be used.
OPTIONS
The command line options have the following meanings:
-A
-B
--format=compatibility
Using one of these options, you can choose whether the output from
GNU size resembles output from System V size (using -A, or
--format=sysv), or Berkeley size (using -B, or --format=berkeley).
The default is the one-line format similar to Berkeley's.
Here is an example of the Berkeley (default) format of output from
size:
$ size --format=Berkeley ranlib size
text data bss dec hex filename
294880 81920 11592 388392 5ed28 ranlib
294880 81920 11888 388688 5ee50 size
This is the same data, but displayed closer to System V
conventions:
$ size --format=SysV ranlib size
ranlib :
section size addr
.text 294880 8192
.data 81920 303104
.bss 11592 385024
Total 388392
size :
section size addr
.text 294880 8192
.data 81920 303104
.bss 11888 385024
Total 388688
--help
Show a summary of acceptable arguments and options.
-d
-o
-x
--radix=number
Using one of these options, you can control whether the size of
each section is given in decimal (-d, or --radix=10); octal (-o, or
--radix=8); or hexadecimal (-x, or --radix=16). In --radix=number,
only the three values (8, 10, 16) are supported. The total size is
always given in two radices; decimal and hexadecimal for -d or -x
output, or octal and hexadecimal if you're using -o.
--common
Print total size of common symbols in each file. When using
Berkeley format these are included in the bss size.
-t
--totals
Show totals of all objects listed (Berkeley format listing mode
only).
--target=bfdname
Specify that the object-code format for objfile is bfdname. This
option may not be necessary; size can automatically recognize many
formats.
-V
--version
Display the version number of size.
#file
Read command-line options from file. The options read are inserted
in place of the original #file option. If file does not exist, or
cannot be read, then the option will be treated literally, and not
removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character (including
a backslash) may be included by prefixing the character to be
included with a backslash. The file may itself contain additional
#file options; any such options will be processed recursively.
SEE ALSO
ar(1), objdump(1), readelf(1), and the Info entries for binutils.
COPYRIGHT
Copyright (c) 1991-2013 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is included in the section entitled "GNU
Free Documentation License".
binutils-2.23.91 2013-11-18 SIZE(1)