what gcc main.c means ?
The GNU Compiler Collection (GCC) is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain and the standard compiler for most projects related to GNU and Linux, including the Linux kernel. The Free Software Foundation (FSF) distributes GCC under the GNU General Public License (GNU GPL). GCC has played an important role in the growth of free software, as both a tool and an example.
The Four Stages of Compiling a C Program
Compiling a C program is a multi-stage process. At an overview level, the process can be split into four separate stages: Preprocessing, compilation, assembly, and linking.
In this post, I’ll walk through each of the four stages of compiling the following C program:
* "Hello, World!": A classic.
The first stage of compilation is called preprocessing. In this stage, lines starting with a
# character are interpreted by the preprocessor as preprocessor commands. These commands form a simple macro language with its own syntax and semantics. This language is used to reduce repetition in source code by providing functionality to inline files, define macros, and to conditionally omit code.
Before interpreting commands, the preprocessor does some initial processing. This includes joining continued lines (lines ending with a
\) and stripping comments.
To print the result of the preprocessing stage, pass the
-E option to
cc -E hello_world.c
Given the “Hello, World!” example above, the preprocessor will produce the contents of the
stdio.h header file joined with the contents of the
hello_world.c file, stripped free from its leading comment:
[lines omitted for brevity]extern int __vsnprintf_chk (char * restrict, size_t,
int, size_t, const char * restrict, va_list);
# 493 "/usr/include/stdio.h" 2 3 4
# 2 "hello_world.c" 2int
The second stage of compilation is confusingly enough called compilation. In this stage, the preprocessed code is translated to assembly instructions specific to the target processor architecture. These form an intermediate human readable language.
The existence of this step allows for C code to contain inline assembly instructions and for different assemblers to be used.
Some compilers also supports the use of an integrated assembler, in which the compilation stage generates machine code directly, avoiding the overhead of generating the intermediate assembly instructions and invoking the assembler.
To save the result of the compilation stage, pass the
-S option to
cc -S hello_world.c
This will create a file named
hello_world.s, containing the generated assembly instructions. On macOS 10.10.4, where
cc is an alias for
clang, the following output is generated:
.macosx_version_min 10, 10
.align 4, 0x90
_main: ## @main
.cfi_offset %rbp, -16
movq %rsp, %rbp
subq $16, %rsp
leaq L_.str(%rip), %rdi
movl $0, -4(%rbp)
xorl %ecx, %ecx
movl %eax, -8(%rbp) ## 4-byte Spill
movl %ecx, %eax
addq $16, %rsp
.cfi_endproc .section __TEXT,__cstring,cstring_literals
L_.str: ## @.str
.asciz "Hello, World!"
During this stage, an assembler is used to translate the assembly instructions to object code. The output consists of actual instructions to be run by the target processor.
To save the result of the assembly stage, pass the
-c option to
cc -c hello_world.c
Running the above command will create a file named
hello_world.o, containing the object code of the program. The contents of this file is in a binary format and can be inspected using
od by running either one of the following commands:
od -c hello_world.o
The object code generated in the assembly stage is composed of machine instructions that the processor understands but some pieces of the program are out of order or missing. To produce an executable program, the existing pieces have to be rearranged and the missing ones filled in. This process is called linking.
The linker will arrange the pieces of object code so that functions in some pieces can successfully call functions in other ones. It will also add pieces containing the instructions for library functions used by the program. In the case of the “Hello, World!” program, the linker will add the object code for the
The result of this stage is the final executable program. When run without options,
cc will name this file
a.out. To name the file something else, pass the
-o option to
cc -o hello_world hello_world.c
You can see how magnificent to write just one line, a whole operation happens within seconds and you don’t even notice that it happened.