**Compiler Introduction**
In simple terms, a compiler is a program that translates "one language (typically a high-level language)" into "another language (usually a low-level or machine language)." The main process of a modern compiler includes: source code → preprocessor → compiler → object code → linker → executable files. This workflow ensures that human-readable code can be converted into instructions that a computer can execute directly.
**Types of Compilers**
Compilers can generate object code that runs on the same platform where the compiler is executed—these are known as "native" or "local" compilers. Alternatively, they can produce code for different platforms, which is referred to as a "cross compiler." Cross compilers are especially useful when developing software for new hardware. There are also "source-to-source compilers," which take a high-level language as input and output another high-level language. For example, automated parallelization tools may convert sequential code into parallel code using annotations like OpenMP or specific language constructs such as FORTRAN’s DOALL directive.
**How Compilers Work**
The primary function of a compiler is to translate high-level code into machine code that a computer can execute. However, not all compilers work in this direction—some can also convert low-level code back into high-level code, which are called decompilers. Additionally, there are compilers that generate intermediate code or even translate between different high-level languages.
A typical compiler output is an object file containing the entry point name, addresses, and external calls. These object files, regardless of their origin, can be linked together if they follow the same format, resulting in an executable that users can run directly.
**Compiler Working Process**
To run any source code, it must first be compiled into binary machine code. For instance, consider the following C code:
```c
#include
int main(void) {
fputs("Hello, world!", stdout);
return 0;
}
```
This code needs to be processed by a compiler before it can be executed. Using `gcc`, you can compile and run it like this:
```bash
$ gcc test.c
$ ./a.out
Hello, world!
```
For larger projects, the compilation process usually involves three steps:
```bash
$ ./configure
$ make
$ make install
```
This article explains how each of these steps contributes to the overall compiler process, focusing mainly on GCC for C and C++. It draws from Alex Smith's article “Building C Projects†and highlights the key stages involved in building software from source.
**First Step: Configuration (Configure)**
Before the actual compilation begins, the system environment must be checked. This includes knowing where libraries are located, what components are needed, and other relevant settings. This step, called "configure," allows the compiler to adapt to different environments and generate code that works across various systems.
The configuration information is stored in a script called `configure`, typically generated using the `autoconf` tool. This script checks the system and sets default values for compilation parameters. If your system has special requirements, you can manually adjust these parameters when running the configure script.
For example, the following command configures PHP with specific installation paths and enables MySQL support:
```bash
$ ./configure --prefix=/usr/local/php --with-mysql
```
This ensures that the final compiled software will be installed correctly and include necessary features based on user preferences.
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