How to Properly Compile and Run Your C Code in Assembly with GCC

Описание: Discover how to resolve common errors when compiling C code into assembly using GCC and NASM, ensuring your program runs smoothly.
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This video is based on the question https://stackoverflow.com/q/70673971/ asked by the user 'Saish Shankar' ( https://stackoverflow.com/u/13528269/ ) and on the answer https://stackoverflow.com/a/70674033/ provided by the user 'DYZ' ( https://stackoverflow.com/u/4492932/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Building a Simple OS with C and Assembly: How to Resolve Compilation Errors

When venturing into the depths of programming, especially in systems programming and operating system development, you might encounter some hurdles that can get discouraging. One such problem comes when you attempt to compile your C code into assembly. Today, we’re going to explore one instance where a simple C program fails to compile properly and guide you through resolving those issues so you can successfully run your code.

The Problem: C Code Compilation Fails in Assembly

You’ve started the exciting journey of building a simple operating system and decided to write a basic C program. The goal? To print "Hi!!!" to the console. You compiled your C code using the following command:

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However, upon trying to run the generated assembly file (foo.s), you encountered numerous errors indicating issues with the assembly syntax and instructions. Here's a snapshot of some of the errors you faced:

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Clearly, something is amiss. The generated foo.s file, although appearing to be correct at first glance, is not in the proper format expected for NASM (the Netwide Assembler), which is required to create an executable file.

Understanding the Solution: Compiling Correctly

The primary takeaway here is that the output of the gcc -S command does not produce assembly code that is compatible with NASM. Instead, it produces assembly code compatible with the GNU assembler (as). Here’s how to compile without running into those frustrating parsing errors.

Step-by-Step Solution

Assemble the Output with as: The first step is to use as, the assembler, to create an object file from your assembly code.

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Link the Object File Using gcc: Once you have the object file (foo.o), the next step is to link it, which will create an executable file:

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Running the Executable: Lastly, run your newly created executable:

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You should see the expected output:

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Summary of Key Points

The command gcc -S foo.c produces assembly code meant for the GNU assembler, not for NASM.

Use as to create an object file from the generated assembly (foo.s).

Link the object file with gcc, and finally run the executable to see your output.

Conclusion: Overcoming Challenges in Assembly Programming

Building an OS or delving into any low-level programming can present significant learning curves. However, resolving issues like these not only enhances your knowledge of the programming languages involved but also builds resilience.

The next time you encounter errors after compiling C code to assembly, remember this process — it's a step towards mastering the intricate relationship between C and assembly language programming. Keep experimenting and learning, and don't hesitate to reach out for help when things get complicated!