amiga-bootcamp/07_dos/file_io.md

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File I/O — Open, Close, Read, Write, Seek

Overview

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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Core Functions

LVO	Function	Registers	Returns
−30	Open(name, mode)	D1=name, D2=mode	D0=BPTR handle (0=fail)
−36	Close(fh)	D1=handle	D0=BOOL
−42	Read(fh, buf, len)	D1=handle, D2=buf, D3=len	D0=actual bytes (−1=error)
−48	Write(fh, buf, len)	D1=handle, D2=buf, D3=len	D0=actual bytes (−1=error)
−66	Seek(fh, pos, mode)	D1=handle, D2=pos, D3=mode	D0=old position (−1=error)

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Access Modes

/* dos/dos.h — NDK39 */
#define MODE_READWRITE  1004   /* open existing, read+write */
#define MODE_OLDFILE    1005   /* open existing, read only */
#define MODE_NEWFILE    1006   /* create new (truncate if exists) */

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Seek Modes

#define OFFSET_BEGINNING  -1   /* from start of file */
#define OFFSET_CURRENT     0   /* from current position */
#define OFFSET_END         1   /* from end of file */

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File Handle (BPTR)

The returned handle is a BPTR to a struct FileHandle:

struct FileHandle {
    struct Message *fh_Link;
    struct MsgPort *fh_Interactive; /* non-NULL if console */
    struct MsgPort *fh_Type;       /* handler process port */
    BPTR   fh_Buf;                 /* I/O buffer (BPTR) */
    LONG   fh_Pos;                 /* current position in buffer */
    LONG   fh_End;                 /* end of valid data in buffer */
    LONG   fh_Funcs;               /* unused */
    LONG   fh_Func2;               /* unused */
    LONG   fh_Func3;               /* unused */
    LONG   fh_Args;                /* packet args */
    BPTR   fh_Arg2;
};

To access as a C pointer: struct FileHandle *fh = BADDR(handle);

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Usage Example

BPTR fh = Open("RAM:test.txt", MODE_NEWFILE);
if (fh) {
    Write(fh, "Hello Amiga\n", 12);
    Close(fh);
}

fh = Open("RAM:test.txt", MODE_OLDFILE);
if (fh) {
    UBYTE buf[64];
    LONG n = Read(fh, buf, sizeof(buf));
    if (n > 0) Write(Output(), buf, n);  /* echo to stdout */
    Close(fh);
} else {
    PrintFault(IoErr(), "Open failed");
}

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Error Checking

LONG err = IoErr();   /* LVO −66 — returns last DOS error code */
/* Common codes: */
#define ERROR_OBJECT_NOT_FOUND    205
#define ERROR_OBJECT_EXISTS       203
#define ERROR_DISK_FULL           221
#define ERROR_SEEK_ERROR          219

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References

• NDK39: dos/dos.h, dos/dosextens.h
• error_handling.md — full error code list
• ADCD 2.1: Open, Close, Read, Write, Seek