A completely free operating system has this secret

The GNU Project, also known as the Gnu Project, was publicly launched by Richard Stallman on September 27, 1983. Its main goal is to create a completely free operating system. Over time, it helped establish the GNU C standard, which became fundamental when developing Linux. However, one of its most powerful features, the __attribute__ mechanism, remains unknown to many developers. In this article, we’ll explore the magic behind __attribute__ and how it can be used to enhance your code.

What makes the __attribute__ mechanism so special in GNU C? Well, it allows you to modify both variable and function attributes. The syntax is simple: __attribute__((parameter));. We'll go through various use cases and see how this feature can help improve your code structure and performance.

When learning a new programming language, writing a "Hello World" program is often the first step. Today, it's easy for us to write such a program. But have you ever seen a "Hello World" that runs before the main function or even after it? Let’s take a look at an example:

#include <stdio.h>
#include <stdlib.h>

__attribute__((constructor)) void pre_proc_1(void) {
    printf("hello world");
}

__attribute__((destructor)) void end_proc_1(void) {
    printf("Hello World, Line %d", __LINE__);
}

int main(int argc, char **argv) {
    return 0;
}

Can you guess what this program outputs? It prints "hello world" before the main function starts and "Hello World, Line X" after the program ends. That's because the functions decorated with __attribute__((constructor)) and __attribute__((destructor)) are executed automatically before and after the main function, respectively. You can also assign a priority number to these attributes, like __attribute__((constructor(101))), to control the order of execution. This is especially useful when managing multiple initialization or cleanup functions.

Now that we've had a quick and fun example, let's dive deeper into the __attribute__ mechanism. As mentioned earlier, it can be used to modify both variables and functions. Here are some of the most commonly used function attributes:

Function Attributes

(1) __attribute__((format(archtype, string-index, first-to-check))): This tells the compiler to check the parameters of the function according to the format rules of functions like printf, scanf, strftime, etc. For example:

__attribute__((format(printf, 1, 2)));
__attribute__((format(scanf, 1, 2))));

Here, m represents the index of the format string, and n refers to the index of the first checked parameter.

(2) __attribute__((noreturn)): This indicates that a function never returns. It is commonly used for functions like exit() or abort() to avoid compiler warnings about unreachable code.

(3) __attribute__((const)): This is used for functions that only take numeric arguments and always return the same result. The compiler can optimize such functions by reusing the result of the first call.

(4) __attribute__((noinline)) and __attribute__((always_inline)): These tell the compiler whether to inline a function or not. While inline functions are usually optimized, the compiler may choose not to inline them depending on the situation. These attributes give you more control over the optimization process.

(5) Multiple attributes can be applied to the same function. This is very common and useful in real-world applications.

Variable Attributes

(1) __attribute__((aligned(n))): This aligns a variable to a specific memory boundary. For example:

int a __attribute__((aligned(16))) = 0;

This ensures that the variable a is aligned to a 16-byte boundary. You can also use __attribute__((aligned)) without specifying a value, allowing the compiler to choose the best alignment based on the target architecture.

(2) __attribute__((packed)): This removes any padding between members of a structure or union. For example:

int a[10] __attribute__((packed));

This helps reduce memory usage but may affect performance due to misalignment.

(3) __attribute__((at(address))): This places a variable at a specific memory address. For example:

int a __attribute__((at(0x00))));

This is useful in embedded systems where you need to access hardware registers directly.

(4) __attribute__((section("section_name"))): This places a function or variable into a specific section of the binary file. For example:

void my_func() __attribute__((section("my_section")));

This is often used in low-level programming or when working with custom linker scripts.

(5) __attribute__((cleanup(function))): This calls a specified function when a variable goes out of scope. For example:

void print() { printf("end"); }

void test() {
    int a __attribute__((cleanup(print))) = 10;
}

In this case, the print() function will be called automatically when the variable a goes out of scope.

As you can see, the __attribute__ mechanism offers a wide range of powerful features that can significantly improve your code. Whether you're optimizing performance, managing memory, or working with low-level systems, understanding how to use these attributes can make a big difference. Do you now have a better understanding of the power of __attribute__?