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linuxassemblyx86gnu-assembler

Does GNU Assembler add its own entry point?

Say I have the following Assembly code:

.section .text

.globl _start
_start:

If I created an executable file using the following commands:

as 1.s -o 1.o
ld 1.o -o 1

Will the GNU Assembler add its own entry point to my executable which calls _start or will _start be the actual entry point?

See this question for more details.

Solution

  • The file crt0.o (or crt1.o or however this file is called) that contains the startup code mentioned in the other question has also been written in assembler.

    So what the Linker ("ld") does is to search all object files (which are in fact all created using "as") for a symbol named "_start" which becomes the entry point.

    You are of course free to add crt0.o to your assembler-written program when using "ld". In this case however you MUST NOT name your symbol "_start" but "main" in your assembler file:

    .globl main
.text
main:
    ...

    Otherwise "ld" will print an error message because it will find two symbols named "_start" and it does not know which one is the entry point!