How to Generate Assembly Code from C/C using GCC and objdump?

How to Output Assembler Code from C/C Source in GCC

When analyzing compiled code, accessing the generated assembly code is essential. In GCC, this can be achieved using the "-S" option.

Using the -S Option

By default, "-S" runs the preprocessor on the source file, performs initial compilation but stops before invoking the assembler. Combined with "-fverbose-asm," this option associates C code with the assembly code as a comment, especially useful at optimization level -O0.

gcc -S helloworld.c

Customizing Output File

The output file is typically named after the source file with a .s extension. To customize the output file's location, use the "-o" option:

gcc -S -o my_asm_output.s helloworld.c

Alternatively, you can output to the console by using "-o -":

gcc -S -o - helloworld.c | less

Accessing Assembly for Executable Objects

If you do not have the original source code but have the object file, use objdump with the "--disassemble" option (-d):

objdump -S --disassemble helloworld > helloworld.dump

Enable debugging in the original compilation (-g) for enhanced disassembly output with source line references.

Additional Objdump Options

Consider using these objdump options for more detailed analysis:

• -rwC: Symbol relocations, disable line-wrapping, and demangle C names
• -Mintel: Intel syntax for x86 disassembly

For example, the following command provides rich disassembly output:

objdump -drwC -Mintel -S foo.o | less

Remember, "-r" is crucial to display symbol references in a .o file with placeholders.

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