The Journey Before main()
Ever wondered what magic happens between clicking an icon and main() running? This deep dive meticulously unpacks the kernel's role in loading programs, from interpreting ELF binaries to setting up the stack and dynamic linker.
Hacker News, ever the stickler for low-level precision, quickly chimed in with crucial corrections and additional insights, turning a fascinating technical post into an even more robust explanation of systems internals.
It's a prime example of the community's penchant for dissecting foundational computing concepts, confirming and refining the intricate details of how our code actually comes to life.
The Lowdown
This article takes a deep dive into the often-overlooked journey a program makes from being invoked to its main() function executing. It meticulously explains the various stages and components involved, starting with the kernel's initial steps and progressing through the complexities of binary execution on Linux.
- The
execveSystem Call: The journey begins withexecve, the system call that signals the kernel to load and run a program, passing the executable path, arguments, and environment variables. - ELF File Format: On Linux, executables are typically in the ELF (Executable and Linkable Format). The article outlines key ELF header components like magic bytes, entry point address, and program/section headers, which act as a 'map' for the kernel.
- ELF Sections & Dynamic Linking: Beyond the header, ELF files contain various sections for code (
.text), data (.data), and symbols. Dynamic linking, using the Procedure Linkage Table (PLT) and shared libraries likelibc, is introduced, explaining how external functions are loaded. - The Stack's Setup: Before
main(), the kernel meticulously sets up the process's stack, populating it with command-line arguments (argv), environment variables (envp), and the ELF auxiliary vector (auxv), which provides crucial system information. - The Entrypoint (
_start): The kernel eventually transfers control to a program's designated entry point, typically_start. This initial function (often provided by the C standard library) handles language-specific runtime initialization, global constructors, and eventually calls the user-definedmain()function.
The author acknowledges that this is a simplified view, omitting complexities like kernel address space management and process tables, but aims to provide a solid primer on the foundational steps before user code begins execution.
The Gossip
Dynamic Linking Discrepancies
A central point of discussion revolved around the exact responsibilities of the kernel versus the dynamic linker (`ld.so`) in loading shared libraries and performing relocations. Commenters, particularly `fweimer` and `turbert`, clarified that the kernel's role is primarily to map the main program's segments and transfer control to the dynamic linker. It is the dynamic linker, not the kernel, that then handles self-relocation, loading other shared objects, and relocating them, correcting an initial simplification in the article. The author graciously acknowledged and committed to correcting this detail.
Memory Mapping Misconceptions
The way memory maps are visually represented in textbooks often leads to confusion, especially regarding stack growth. `bignerd_95` articulately argued that drawing higher addresses at the bottom (like scrolling through code) would be more intuitive than the traditional 'high addresses at top' convention. This alternative visualization would make the concept of the stack growing 'down' (towards lower addresses) clearer, as it would be moving 'up' the page. The author agreed, noting this was a common pedagogical challenge.
Runtime Runt & Standard Library Scrutiny
Several commenters debated the utility and 'bloat' of the C standard library. While the article highlighted the surprisingly large number of symbols for a 'Hello World' program using `musl`, a commenter noted that `glibc` produced far fewer symbols. This led to a broader discussion on the practice of avoiding standard libraries by directly calling Linux syscalls or Win32 APIs for smaller, more resource-efficient binaries. The nuances of Windows' ABI stability and C runtime requirements were also explored, contrasting them with the 'ABI hell' often attributed to POSIX systems.
Shebang Shenanigans & `execve` Errors
A common point of frustration for developers, the shebang line (`#!`) in scripts, came up in discussion. One user recounted a painful debugging experience where a Java application reported a misleading 'No such file or directory' error when the actual problem was an incorrect shebang path on a remote host. This highlighted how `execve` can produce generic errors even when the underlying issue is specific to interpreter resolution, with other users providing links to deeper dives on shebang functionality and debugging tips.