DevHlp_ReadFileAt is used by base device drivers to read a file previously opened using DevHlp_OpenFile from a specified file location.

Calling Sequence in Assembler

LES   DI, ReadFileAt
MOV   DL, DevHlp_ReadFileAt

CALL  [Device_Help]

ES DI points to a FILEIOINFO structure defined as follows

FILEIOINFO struc
length        dw   12    ; length of imbedded fle system operation structure
;
FREADAT struc
Buffer        dd   ?    ; 16 16 pointer to input buffer
Readsize      dd   ?    ; length of data to read
StartPosition dd   ?    ; starting position relative to the beginning of the file
FREADAT       ends
;
FILEIOINFO    ends

Results in Assembler

C Clear if file is closed. AX = zero.

C Set if error. AX = Error code. Possible errors

24 ERROR_BAD_LENGTH

The length in the FILEIOINFO structure is invalid.

Calling Sequence in C

#include  "dhcalls.h"

USHORT APIENTRY DevHelp_FileReadAt

(PFILEIOINFO pFileReadAt)
Pointer to the FILEIOINFO structure defined as follows

typedef struct FOPEN {
        PSZ    FileName;   /* (input) pointer to file name */
        ULONG  FileSize;   /* (output) size of file returned by FileOpen */
} FILEOPEN;

typedef struct FCLOSE {
        USHORT  reserved   /* reserved */
} FILECLOSE;

typedef struct FREAD {
        PBYTE Buffer;     /* (input) pointer to input buffer */
        ULONG ReadSize;   /* (input) number of bytes to read from file */
} FILEREAD;

typedef struct FREADAT {
        PBYTE Buffer;        /* (input) pointer to input buffer */
        ULONG ReadSize;      /* (input) number of bytes to read from file */
        ULONG StartPosition  /* (input) starting file position relative to
                                 the beginning of the file */
} FILEREADAT;

typedef union FILEIOOP {
        struct FOPEN FileOpen;
        struct FCLOSE FileClose;
        struct FREAD FileRead;
        struct FREADAT FileReadAt;
} FILEIOOP;

typedef struc _DDFileIo {
        USHORT Length; /* (input) length of imbedded structure */
        FILEIOOP Data; /* (input) imbedded file system operation structure */
} FILEIOINFO, FAR * PFILEIOINFO

Results in C

Success Indicator 0 if file was read.

24 ERROR_BAD_LENGTH

Length in the FILEIOINFO structure is invalid.

Remarks

DevHlp_ReadFileAt may be called at initialization time only. It provides a primitive interface to the mini-IFS or micro_IFS at initialization time.

The current file position is set according to the StartPosition. The file is read from that file position. After a successful read, the current file position is updated.

Using this interface, one file only may be opened at a time. No handle is assigend by open. The file read is assumed to be the most recent opened using DevHlp_OpenFile.

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