Program Start

The sequence of steps happening from the moment a Go program is launched until main.main function starts executing:

OS loader → _rt0_go (asm) → rt0_go (Go) → args + osinit + schedinit → runtime.main → run package inits → start main.main goroutine

The OS loader loads the binary into memory.
ELF/Mach-O/PE headers are parsed (depending on platform).
OS sets up the process image: stack, heap, environment, argc/argv, and entry point.

The entry point is not main.main — it's the Go runtime bootstrap symbol, usually _rt0_<arch>_<os>.

Defined in assembly (runtime/rt0_*.s) for each platform.
Responsibilities:
- Initialize stack and registers.
- Collect argc, argv, and envp from the OS.
- Call into runtime.rt0_go (written in Go).

Sets up command-line arguments and environment variables.
Calls runtime.args() → stores argc and argv.
Calls runtime.osinit() → initializes OS-specific parts (thread-local storage, signal stack, etc).
Calls runtime.schedinit() → initializes the scheduler and global runtime state.

Sets up:
- G0: the scheduler goroutine (special stack, runs runtime code).
- M0: the initial machine (thread) bound to G0.
- P's: processors (number = GOMAXPROCS, default = # of CPUs).

At this point, the runtime can schedule goroutines, but none exist yet except G0.

Runtime calls runtime.main().
Inside runtime.main:
- It creates the first real goroutine, which runs main.main (the program).
- Before main.main, it executes:
  - All init functions in the imported packages (topological order of imports).
  - The init function in main package (if present).

init functions run sequentially, not concurrently.

After all init functions complete, the scheduler runs the goroutine for main.main.
The program begins.

At this point:

The scheduler is live.
The garbage collector is initialized.
The network poller and timer system are ready.
Runtime services like panic handling, defer stack, type system, etc. are fully operational.