- gcc- c- compiler (32- bit with inline assembly)
- ld- linker/locator (32- bit)
- as- assembler (32- bit)
- as86- real- mode compiler (for example to create a boot sector- even Pentium starts in the real-mode)
- ld86- real- mode linker (for example to create a boot sector)
- GNU make
- gdb debugger
- ar library (archive) program for static libraries (dynamic libraries -DLLs are created with ld)
- nm, objcopy, objdump, ranlib, size, strings for (not so common) working with object files
The introductory lecture and the laboratory material is about basics to get the whole picture (not all details included). You can get a detailed and full information of all the tools using Linux man- and info command (info gcc, info make man ld )and /usr/doc/HOWTO.
The professional program development work needs (in addition to the software management and design process) understanding of few basic technologies not depending on the operating system:
- Compiling separately source modules to object files and linking object files to an executable.
- Using, creating and maintaining libraries of object files
- Using, creating and maintaining include files
- Using make- command to make the whole process automatic and efficient
- Understanding of data formats
- Procedure calling conventions (the usage of the stack to link objects compiler from different programming language sources, mainly C and assembly)
- Debugging skills
Experimenting to understand compiling, linking, libraries and make-command :
The following presentation is the principle used in all the operating systems, not only UNIX or Linux. The names of the commands and how to hide them from an user can be different. For example Borland C linker name is TLINK and UNIX name is ld. UNIX library program name is ar and Borland C name is TLIB.
- A simple c- program to start with:
_______________
#include <stdio.h>
short int i=0xe589;
main()
{
printf("helloi=%d\n",i)
}
______________________________________________
__________________________________________________
- gcc -c main.c
/* Input is the source file main.c, output is
the object file main.o */
gcc main.o -o executable_name/* Input main.o, output executable_name */
Compiling?
A compiler generates an object file from a source file
.
An object file?
Machine level instructions generated by a compiler
and information to a linker to solve unresolved symbols (for example calling
subprogram printf).
Example to understanding the output of the compiler at
binary level:
#include <sdtio.h>
int i = e589;
- main()
{
printf("Hello! i=%d \n",i);
};
________________________________
objdump --disassemble-all main.o__________________________
_________________________________main.o: file format elf32-i386Disassembly of section .text:
00000000 <main>:
0: 55 pushl %ebp
1: 89 e5 movl %esp,%ebp <-----a hex number same as the integer i in the example program BUT these are in the code segment meaning the processor interprets this as an instruction
3: 0f bf 05 00 00 movswl 0x0,%eax
8: 00 00
a: 50 pushl %eax
b: 68 00 00 00 00 pushl $0x0
10: e8 fc ff ff ff call 11 <main+0x11> <------here we call printf (address resolved by linker later)
15: 83 c4 08 addl $0x8,%esp
18: c9 leave
19: c3 ret
Disassembly of section .data:00000000 <i>:
0: 89 e5 movl %esp,%ebp <--------This is integer i not a program code
Disassembly of section .rodata:00000000 <.rodata>:
0: 68 65 6c 6c 6f pushl $0x6f6c6c65
5: 0a 20 orb (%eax),%ahobjdump --reloc main.o
main.o: file format elf32-i386
RELOCATION RECORDS FOR [.text]:
OFFSET TYPE VALUE
00000006 R_386_32 i
0000000c R_386_32 .rodata
00000011 R_386_PC32 printf
Run the program now:
- ./executable_name
gcc main.c
__________________________________
Linking plenty of object modules to an executable:
- gcc object1.o object2.o object3.o -o
executable_name /* One and
only one module named main */
Linking (static) with linker program ld:
Compiler starts the linker automatically in the previous
example.
To link directly with ld the first module
to be linked must be a so called start up library.
- ld /lib/crt0.o main.o sub.o -o exe
/* The start up library contains some initialization
code */
___________________________________
Libraries (archives):
How to create your own library and maintain and use it?
- ar cr mylib.a sub.o
There are plenty of flags supported by ar, consult the manual page of ar.
How to use our brand new library?
gcc main.c mylib.a -o exe
- int sub(int,int);
main()
{
int i;
i =sub(1,2);
}
We have a separate include file containing only the text int sub(int,int); and we have not the prototype definition in the main file. Now you have in the main program #include "myheader.h" ( < and > are for system wide headers in /usr/include).
______________________________
The make command:
make(1)
make(1)
NAME
make - maintain, update,
and regenerate groups of programs
SYN OPSIS
make [-f makefile] [-bBdeiknpPqrsStuw]
[macro_name=value] [names]
DESCRIPTION
Makefile Structure
A makefile can contain four different
kinds of lines: target lines,
shell command lines, macro definitions,
and include lines.
Lines of the form string1 = string2 are macro definitions.
The following makefile says that excecutable pgm depends
on two files: a.o and
b.o, and
that they in turn depend on their corresponding source files
(a.c and
b.c) and a common file incl.h:
OBJS = a.o b.o
pgm: $(OBJS)
cc $(OBJS) -o pgm
<----Always tab-characer or nothing in the beginning of
a line, no spaces!
a.o: incl.h a.c
cc -c a.c
b.o: incl.h b.c
cc -c b.c
So, always on the left we have a file to be created if
the files after semicolon are changed.
Next line tells how this is to be done.
Important: The make command is very sensitive, you
must have always tab- character (no spaces) in the beginning of the command
line.
The following example is from my real-time operaing systems
course material. This makefile executed by the main makefile creates the
debugger task:
- include ../make_defs
TARGET = _debugger.o
OBJS = debugger.o
_debugger.o:$(OBJS)
ld -r -o $(TARGET) $(OBJS)
dep:
$(CC)
-M *.c >.dep
clean:
rm *.o
include .dep
.c.o:
$(CC) $(CCFLAGS) -c $< -o $@
/* ld -r -o $(TARGET) $(OBJS), we link debugger object as "re-linkable" */
/* $(CC) -M *.c >.dep, GNU make creates all dependencies, which means for example that if debugger.c contains a include file xxxxx.h */
/* .c.o: short rule to the make command how to change sources (.c) to objects (.o) */
This was not all about the make. To learn more:
- Read Linux material (command info make)
- Read the makefiles of my kernel (link from my web page, this helps also the real- time operating system course)
- Read the makefiles of Linux (/usr/src/linux)
- Make exercises
