CODEWITHSUNDEEP
gccassemblylinkerriscv
adrianmcmenamin
adrianmcmenamin

Reputation: 1129

Where is _start symbol likely to be defined

I have some startup assembly for RISCV which defines the .text section as beginning at .globl _start.

I know what this is - as a disassembly shows me the address, but I cannot see where it is defined. It's not in the linker script and a grep in the build directories shows it is in various binary files, but I cannot find a definition.

I am guessing this appears in a file somewhere as a function of the architecture, but can anyone tell me where? (This is all being built using RISCV GNU cross compilers on Linux)

Upvotes: 1

Views: 3187

Answers (2)

old_timer
old_timer

Reputation: 71536

Unless you control it yourself there is usually at least in the gnu tools world a file called crt0.s. Or perhaps some other name. Should be one per architecture since it is in assembly language. It is the default bootstrap, zeros .bss copies .data as needed, etc.

I dont remember if it is part of the C library (glibc, newlib, etc), or if it is added on later by folks that build a toolchain targeting some specific platform.

Not required certainly but it is not uncommon to see _start be the label of the beginning of the binary, it is supposed to be the entry point certainly. So if you have an operating system/loader that uses a binary with labels present (elf, etc), then it can load the binary and instead of branching to the first address it branches to the entry point.

Upvotes: 1

adrianmcmenamin
adrianmcmenamin

Reputation: 1129

So the _start is merely defined as being at the start of the .text section, and the address of the .text section is defined in the linker script.

Upvotes: 1

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