[bash]$gcc -O0 hello.c -o hello
or
[bash]$gcc hello.c -o hello
[bash]$ gcc -fdefer-pop hello.c –o hello
[bash]$ gcc -O1 -fno-defer-pop -o test test.c
[bash]$gcc -O2 hello.c -o hello
[bash]$gcc -Os hello.c -o hello
[bash]$gcc -O3 hello.c -o hello
Compiling a C program - Compiling a simple C program using gcc.
Above article is a high level view of compliation in gcc. In present article let us see in depth view of the compilation order or flow in gcc which is a default C compiler in GCC.
gcc goes through a sequence of different intermediate steps before generating final executable. Those intermediate steps are the result of different tools which are invoked internally to complete the compilation of the source code.
The whole Compilation process is broken down into following phases:
As an example, we will examine these compilation stages individually using the same ‘hello.c’ program given below:
#include <stdio.h>
#define STRING "Hello World"
int main (void)
{
printf ("My First program - %s\n",STRING);
return 0;
}
NOTE: We dont need to manually go through all the intermediate stages to generate a executable using gcc. All these stages are directly taken care transparently by gcc internally, and can be seen using the -v option.
Although 'hello.c' program is very simple it uses external header files and libraries, and so exercises all the major steps of the compilation process. If we compile the 'hello.c' with a simple command namely gcc hello.c then we end-up creating an executable file called a.out.
Back in the days of the PDP computer, a.out stood for "assembler output". Today, it simply means an older executable file format. Modern versions of Unix and Linux use the ELF executable file format. The ELF format is much more sophisticated. So even though the default filename of the output of gcc is "a.out", its actually in ELF format.The Preprocessor
[bash]$ cpp hello.c > hello.iThe result is a file named hello.i which contains the source code with all macros expanded.
[bash]$ gcc -E hello.c -o hello.iNow we will try to check the contents of the preproccessed input file. Since the stdio.h file is fairly large the resultant output is cleaned up a bit.
# 1 "hello.c"Since our program has requested the stdio.h header be included into our source which in turn, requested a whole bunch of other header files. So, the preprocessor made a note of the file and line number where the request was made and made this information available to the next steps in the compilation process. Thus, the lines,
# 1 "/usr/include/stdio.h" 1 3
# 1 "/usr/include/_ansi.h" 1 3
# 1 "/usr/include/sys/config.h" 1 3
# 14 "/usr/include/sys/config.h" 3
# 25 "/usr/include/sys/config.h" 3
# 44 "/usr/include/sys/config.h" 3
# 40 "/usr/include/stdio.h" 2 3
# 1 "/usr/include/sys/reent.h" 1 3
int main(void){
printf ("My First Program - %s\n", "HELLO WORLD" );
return 0;
}
# 40 "/usr/include/stdio.h" 2 3indicates that the reent.h file was requested on line 40 of stdio.h. The preprocessor creates a line number and file name entry before what might be "interesting" to subsequent compilation steps, so that if there is an error, the compiler can report exactly where the error occurred.
# 1 "/usr/include/sys/reent.h" 1 3
[bash]$ gcc -Wall -S hello.i -o hello.sThe resulting assembly language is stored in the file ‘hello.s’.
[bash]$ as hello.s -o hello.oAs with gcc, the output file is specified with the -o option. The resulting file ‘hello.o’ contains the machine level code for 'hello.c' program.
[bash]$ gcc -c hello.cThe Linker
[bash]$ ld -dynamic-linker /lib/ld-linux.so.2/usr/lib/crt1.o /usr/lib/crti.o /usr/lib/gcc-lib/i686/3.3.1/crtbegin.o-L/usr/lib/gcc-lib/i686/3.3.1 hello.o -lgcc -lgcc_eh -lc -lgcc -lgcc_eh/usr/lib/gcc-lib/i686/3.3.1/crtend.o /usr/lib/crtn.oLuckily we are never asked to type the above command directly -- the entire linking process is handled transparently by gcc when invoked as follows:
[bash]$ gcc hello.oThis links the object file ‘hello.o’ to the C standard library, and produces an executable file ‘a.out’.
[bash]$ ./a.outAn object file for a C++ program can be linked to the C++ standard library in the same way with a single g++ command.
My First Program - Hello World
#include <stdio.h>
int main (void)
{
printf ("Hello, world!\n");
return 0;
}
[bash]$ gcc -Wall hello.c -o hello
[bash]$ ./hello
Hello, world!
Vmedia Research, Inc. unveiled its mobile entertainment media technology at the GSMA Mobile World Congress in Barcelona. Vmedia™ is an innovative miniature optical disc format designed to bring the quality of a big screen experience and the simplicity of home entertainment to mobile consumer electronics.