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108 lines
2.7 KiB
Markdown
108 lines
2.7 KiB
Markdown
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[← Home](../README.md) · [AmigaDOS](README.md)
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# File I/O — Open, Close, Read, Write, Seek
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## Overview
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AmigaDOS file I/O is synchronous from the caller's perspective. All functions use `BPTR` file handles and communicate errors via `IoErr()`.
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---
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## Core Functions
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| LVO | Function | Registers | Returns |
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|---|---|---|---|
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| −30 | `Open(name, mode)` | D1=name, D2=mode | D0=BPTR handle (0=fail) |
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| −36 | `Close(fh)` | D1=handle | D0=BOOL |
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| −42 | `Read(fh, buf, len)` | D1=handle, D2=buf, D3=len | D0=actual bytes (−1=error) |
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| −48 | `Write(fh, buf, len)` | D1=handle, D2=buf, D3=len | D0=actual bytes (−1=error) |
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| −66 | `Seek(fh, pos, mode)` | D1=handle, D2=pos, D3=mode | D0=old position (−1=error) |
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---
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## Access Modes
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```c
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/* dos/dos.h — NDK39 */
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#define MODE_READWRITE 1004 /* open existing, read+write */
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#define MODE_OLDFILE 1005 /* open existing, read only */
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#define MODE_NEWFILE 1006 /* create new (truncate if exists) */
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```
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---
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## Seek Modes
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```c
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#define OFFSET_BEGINNING -1 /* from start of file */
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#define OFFSET_CURRENT 0 /* from current position */
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#define OFFSET_END 1 /* from end of file */
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```
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---
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## File Handle (BPTR)
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The returned handle is a BPTR to a `struct FileHandle`:
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```c
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struct FileHandle {
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struct Message *fh_Link;
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struct MsgPort *fh_Interactive; /* non-NULL if console */
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struct MsgPort *fh_Type; /* handler process port */
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BPTR fh_Buf; /* I/O buffer (BPTR) */
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LONG fh_Pos; /* current position in buffer */
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LONG fh_End; /* end of valid data in buffer */
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LONG fh_Funcs; /* unused */
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LONG fh_Func2; /* unused */
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LONG fh_Func3; /* unused */
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LONG fh_Args; /* packet args */
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BPTR fh_Arg2;
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};
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```
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To access as a C pointer: `struct FileHandle *fh = BADDR(handle);`
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---
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## Usage Example
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```c
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BPTR fh = Open("RAM:test.txt", MODE_NEWFILE);
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if (fh) {
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Write(fh, "Hello Amiga\n", 12);
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Close(fh);
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}
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fh = Open("RAM:test.txt", MODE_OLDFILE);
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if (fh) {
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UBYTE buf[64];
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LONG n = Read(fh, buf, sizeof(buf));
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if (n > 0) Write(Output(), buf, n); /* echo to stdout */
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Close(fh);
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} else {
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PrintFault(IoErr(), "Open failed");
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}
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```
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---
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## Error Checking
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```c
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LONG err = IoErr(); /* LVO −66 — returns last DOS error code */
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/* Common codes: */
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#define ERROR_OBJECT_NOT_FOUND 205
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#define ERROR_OBJECT_EXISTS 203
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#define ERROR_DISK_FULL 221
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#define ERROR_SEEK_ERROR 219
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```
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---
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## References
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- NDK39: `dos/dos.h`, `dos/dosextens.h`
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- `07_dos/error_handling.md` — full error code list
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- ADCD 2.1: `Open`, `Close`, `Read`, `Write`, `Seek`
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