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98 lines
2.6 KiB
Markdown
98 lines
2.6 KiB
Markdown
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[← Home](../README.md) · [Exec Kernel](README.md)
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# Library System — OpenLibrary Lifecycle
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## Overview
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The AmigaOS library system provides **versioned, shared code** via a standardised interface. Libraries are identified by name, opened with a version check, and reference-counted for safe unloading.
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---
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## Library Node
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Every library is an `NT_LIBRARY` node on `SysBase->LibList`:
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```c
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struct Library {
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struct Node lib_Node; /* ln_Name = "dos.library" */
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UBYTE lib_Flags; /* LIBF_SUMUSED | LIBF_DELEXP */
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UBYTE lib_Pad;
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UWORD lib_NegSize; /* size of JMP table in bytes */
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UWORD lib_PosSize; /* size of library base struct */
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UWORD lib_Version; /* major version */
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UWORD lib_Revision; /* minor revision */
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APTR lib_IdString; /* "dos.library 40.1 (16.7.93)" */
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ULONG lib_Sum; /* JMP table checksum */
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UWORD lib_OpenCnt; /* reference count */
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};
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```
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---
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## OpenLibrary / CloseLibrary
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```c
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/* Open — get a reference: */
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struct DosLibrary *DOSBase =
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(struct DosLibrary *)OpenLibrary("dos.library", 40);
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/* Use the library ... */
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/* Close — release reference: */
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CloseLibrary((struct Library *)DOSBase);
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```
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Internally:
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1. `exec` scans `LibList` for `ln_Name == "dos.library"`
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2. If not found, searches resident list and `LIBS:` path
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3. If found on disk: `LoadSeg` + call `InitLib`
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4. Check `lib_Version >= requested_version`
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5. Call library's `Open()` vector → `lib_OpenCnt++`
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6. Return library base
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---
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## Library Flags
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| Flag | Value | Meaning |
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|---|---|---|
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| `LIBF_SUMUSED` | 0x01 | Checksum is maintained |
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| `LIBF_CHANGED` | 0x02 | Checksum needs recalculation |
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| `LIBF_DELEXP` | 0x04 | Expunge deferred (opened while expunge pending) |
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---
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## Version Numbering Convention
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`lib_Version.lib_Revision`:
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- `40.1` = OS 3.1 release
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- `40.x` = OS 3.1 (various revisions)
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- `44.x` = OS 3.2
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Increment rules:
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- `lib_Revision` — minor bugfix, compatible
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- `lib_Version` — API change or major update (requestors check this)
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---
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## Finding a Library Without Opening
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```c
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/* Read-only peek — no open count increment */
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Forbid();
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struct Library *lib = FindName(&SysBase->LibList, "graphics.library");
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Permit();
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if (lib) printf("Found v%d\n", lib->lib_Version);
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```
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> [!CAUTION]
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> Using `FindName` without `Forbid()` is a race condition — the library could be expunged between finding it and using it.
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---
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## References
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- NDK39: `exec/libraries.h`
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- ADCD 2.1: `OpenLibrary`, `CloseLibrary`, `FindName`
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- `04_linking_and_libraries/shared_libraries_runtime.md` — expunge lifecycle
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