LOC / SEEK Functions

Syntax

Syntax: result = LOC(#n) | result = SEEK(#n)
Source: PowerBASIC built-in functions, implemented via SetFilePointer

Description

Both LOC and SEEK query the current file pointer position, but they return different values depending on the file mode:

LOC(#n) returns the logical position: current record number in RANDOM mode, or current byte position in BINARY mode.
SEEK(#n) returns the 1-based byte position regardless of mode, with BINARY files returning position and RANDOM files returning the position of the next record (1-based).

Internally, both functions call the Win32 SetFilePointer with a distance of 0 and FILE_CURRENT, which returns the current file pointer without moving it. The result is then adjusted based on the file mode (dividing by record length for LOC in RANDOM mode).

Parameters

Parameter	Description
`n`	The integer file number to query. The file must be open for BINARY or RANDOM access.

Return Values by File Mode

Function	BINARY Mode	RANDOM Mode	OUTPUT/APPEND
`LOC(#n)`	Current byte position (0-based, then +1?) Returns bytes read/written at current position	Current record number (1-based) `LOC = (byte_pos / LEN) + 1`	Number of bytes written since last OPEN
`SEEK(#n)`	1-based byte position of next GET/PUT `SEEK = byte_pos + 1`	1-based byte position `SEEK = byte_pos + 1`	Not applicable

Example

TYPE Employee
    id AS LONG
    nam AS STRING * 32
END TYPE

FUNCTION PBMAIN() AS LONG
    LOCAL emp AS Employee
    LOCAL pos AS LONG

    OPEN "employees.dat" FOR RANDOM AS #1 LEN = 36

    ' Read through all records, tracking position
    WHILE NOT EOF(1)
        pos = LOC(1)              ' current record number
        GET #1, , emp
        PRINT "Record "; pos; ": "; emp.id; " "; emp.nam
    WEND

    PRINT "Total records read: "; LOC(1) - 1
    CLOSE #1
    FUNCTION = 0
END FUNCTION

Binary Mode Position Tracking

OPEN "data.bin" FOR BINARY AS #2

GET #2, , val1
PRINT "LOC after read: "; LOC(2)     ' bytes read so far (e.g. 4)
PRINT "SEEK position: "; SEEK(2)     ' next byte position (e.g. 5)

GET #2, 10, val2                     ' seek to position 10
PRINT "LOC after seek: "; LOC(2)     ' 10
PRINT "SEEK position: "; SEEK(2)     ' 14 (after reading 4 bytes)

CLOSE #2

Generated Assembly

LOC(#n) - Query Current Position via SetFilePointer

; LOC(#n) implementation using SetFilePointer with FILE_CURRENT (no move)
mov    rcx, [rbp - handle_offset]       ; hFile = stored file handle
xor    rdx, rdx                         ; lDistanceToMove = 0 (no movement)
xor    r8, r8                           ; lpDistanceToMoveHigh = NULL
mov    r9, 1                            ; dwMoveMethod = FILE_CURRENT (1)
call   [__imp_SetFilePointer]           ; returns current position in EAX

; In RANDOM mode: divide by record length to get record number
;   record_num = (current_pos / record_len) + 1
; In BINARY mode: return the byte position directly
;   (some implementations add 1 for 1-based or keep 0-based)

; For RANDOM mode adjustment:
xor    rdx, rdx                         ; clear high for division
mov    rcx, <record_length>            ; divisor = LEN from OPEN
div    rcx                             ; RAX = position / record_length
inc    rax                             ; 0-based -> 1-based record number
mov    [rbp - loc_result], rax          ; store LOC() result

SEEK(#n) - Query as Function (vs Statement)

; SEEK(#n) function implementation
mov    rcx, [rbp - handle_offset]       ; hFile = stored file handle
xor    rdx, rdx                         ; lDistanceToMove = 0
xor    r8, r8                           ; lpDistanceToMoveHigh = NULL
mov    r9, 1                            ; dwMoveMethod = FILE_CURRENT
call   [__imp_SetFilePointer]           ; returns current byte position in EAX

; Convert 0-based byte position to 1-based
inc    rax                             ; SEEK = current_pos + 1
mov    [rbp - seek_result], rax         ; store SEEK() result

LOC vs SEEK Comparison

Aspect	LOC(#n)	SEEK Statement	SEEK(#n) Function
Type	Function (returns value)	Statement (no return)	Function (returns value)
Read/Write	Read current position	Set new position	Read current position
RANDOM mode	Record number (1-based)	Set record number (1-based)	Byte position (1-based)
BINARY mode	Byte position	Set byte position (1-based)	Byte position (1-based)
Win32 API	`SetFilePointer` (query)	`SetFilePointer` (set)	`SetFilePointer` (query)
MoveMethod	FILE_CURRENT, dist=0	FILE_BEGIN	FILE_CURRENT, dist=0

Imported WinAPI Functions

Function	DLL	Purpose
`SetFilePointer`	kernel32.dll	Query current file pointer position (when called with distance=0 and FILE_CURRENT), or set position (when called with FILE_BEGIN)

Notes

LOC(#n) and SEEK(#n) are read-only queries—they do not move the file pointer.
SEEK #n, pos (statement form) does move the file pointer. See the SEEK Statement documentation for details.
In RANDOM mode, LOC(#n) returns the record number that was last read or written, not the next record. After GET #1, 5, rec, LOC(1) returns 5.
In OUTPUT/APPEND mode, LOC(#n) returns the number of bytes written since the file was opened.

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