amiga-bootcamp/06_exec_os/library_system.md

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Library System — OpenLibrary Lifecycle

Overview

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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Library Node

Every library is an NT_LIBRARY node on SysBase->LibList:

struct Library {
    struct Node  lib_Node;       /* ln_Name = "dos.library" */
    UBYTE        lib_Flags;      /* LIBF_SUMUSED | LIBF_DELEXP */
    UBYTE        lib_Pad;
    UWORD        lib_NegSize;    /* size of JMP table in bytes */
    UWORD        lib_PosSize;    /* size of library base struct */
    UWORD        lib_Version;    /* major version */
    UWORD        lib_Revision;   /* minor revision */
    APTR         lib_IdString;   /* "dos.library 40.1 (16.7.93)" */
    ULONG        lib_Sum;        /* JMP table checksum */
    UWORD        lib_OpenCnt;    /* reference count */
};

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OpenLibrary / CloseLibrary

/* Open — get a reference: */
struct DosLibrary *DOSBase =
    (struct DosLibrary *)OpenLibrary("dos.library", 40);

/* Use the library ... */

/* Close — release reference: */
CloseLibrary((struct Library *)DOSBase);

Internally:

1. exec scans LibList for ln_Name == "dos.library"
2. If not found, searches resident list and LIBS: path
3. If found on disk: LoadSeg + call InitLib
4. Check lib_Version >= requested_version
5. Call library's Open() vector → lib_OpenCnt++
6. Return library base

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Library Flags

Flag	Value	Meaning
LIBF_SUMUSED	0x01	Checksum is maintained
LIBF_CHANGED	0x02	Checksum needs recalculation
LIBF_DELEXP	0x04	Expunge deferred (opened while expunge pending)

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Version Numbering Convention

lib_Version.lib_Revision:

• 40.1 = OS 3.1 release
• 40.x = OS 3.1 (various revisions)
• 44.x = OS 3.2

Increment rules:

• lib_Revision — minor bugfix, compatible
• lib_Version — API change or major update (requestors check this)

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Finding a Library Without Opening

/* Read-only peek — no open count increment */
Forbid();
struct Library *lib = FindName(&SysBase->LibList, "graphics.library");
Permit();
if (lib) printf("Found v%d\n", lib->lib_Version);

Caution

Using FindName without Forbid() is a race condition — the library could be expunged between finding it and using it.

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References

• NDK39: exec/libraries.h
• ADCD 2.1: OpenLibrary, CloseLibrary, FindName
• shared_libraries_runtime.md — expunge lifecycle