Loc Function

Returns a Long specifying the current read/write position within an open file.

Syntax

Loc(filenumber)

Parameters

filenumber (Required): Integer file number used in the Open statement
Must be a valid file number from a currently open file
File numbers typically obtained from FreeFile function

Return Value

Returns a Long: - For Random mode: Record number of last record read or written - For Sequential mode: Current byte position divided by 128 - For Binary mode: Position of last byte read or written - Returns 0 if no read/write operations have occurred yet - Returns value based on last I/O operation

Remarks

The Loc function returns the current position in an open file:

Behavior varies based on file access mode
For Random access: Returns record number (1-based)
For Sequential access: Returns byte position / 128 (approximation)
For Binary access: Returns byte position (0-based)
Does not move the file pointer
Read-only operation (non-destructive)
Useful for tracking progress in file operations
Returns position of last operation, not next operation
For Random files, increments after Get/Put
For Binary files, tracks exact byte position
For Sequential files, provides approximate position
Essential for file I/O progress tracking
Used with Seek to navigate files
Different from Seek function (which also sets position)
LOF function returns file length, Loc returns position
Error 52 if file number not open
Error 68 if device unavailable
Common in loops reading/writing files
Helps detect end-of-file conditions
Used for progress bars during file operations

Typical Uses

Track Random File Position

currentRecord = Loc(1)

Track Binary File Position

bytesProcessed = Loc(fileNum)

Progress Calculation

percentComplete = (Loc(1) / LOF(1)) * 100

Check if Data Written

If Loc(fileNum) > 0 Then
' File has been written to
End If

Loop Until End

Do While Loc(1) < LOF(1)
Get #1, , record
Loop

Record Number Display

lblRecordNum.Caption = "Record: " & Loc(1)

Byte Position Check

Debug.Print "Position: " & Loc(fileNum)

Progress Bar Update

ProgressBar1.Value = (Loc(1) / totalRecords) * 100

Basic Examples

Example 1: Random File Position

Type CustomerRecord
ID As Long
Name As String * 50
End Type

Dim customer As CustomerRecord
Dim fileNum As Integer

fileNum = FreeFile
Open "customers.dat" For Random As #fileNum Len = Len(customer)

' Read records
Do While Not EOF(fileNum)
Get #fileNum, , customer
Debug.Print "Record: " & Loc(fileNum)
Loop

Close #fileNum

Example 2: Binary File Progress

Dim fileNum As Integer
Dim data As Byte
Dim fileSize As Long

fileNum = FreeFile
Open "data.bin" For Binary As #fileNum
fileSize = LOF(fileNum)

Do While Loc(fileNum) < fileSize
Get #fileNum, , data

' Update progress
If Loc(fileNum) Mod 1024 = 0 Then
Debug.Print "Progress: " & (Loc(fileNum) / fileSize) * 100 & "%"
End If
Loop

Close #fileNum

Example 3: Sequential File Position

Dim fileNum As Integer
Dim line As String

fileNum = FreeFile
Open "log.txt" For Input As #fileNum

Do While Not EOF(fileNum)
Line Input #fileNum, line

' Approximate position (bytes / 128)
Debug.Print "Position: " & Loc(fileNum)
Loop

Close #fileNum

Example 4: Track Write Position

Dim fileNum As Integer
Dim i As Integer

fileNum = FreeFile
Open "output.bin" For Binary As #fileNum

For i = 1 To 100
Put #fileNum, , i
Debug.Print "Wrote to position: " & Loc(fileNum)
Next i

Close #fileNum

Common Patterns

Pattern 1: `CalculateProgress`

Function CalculateProgress(ByVal fileNum As Integer) As Single
Dim currentPos As Long
Dim totalSize As Long

currentPos = Loc(fileNum)
totalSize = LOF(fileNum)

If totalSize > 0 Then
CalculateProgress = (currentPos / totalSize) * 100
Else
CalculateProgress = 0
End If
End Function

Pattern 2: `IsFilePositionChanged`

Function IsFilePositionChanged(ByVal fileNum As Integer, _
ByVal lastPosition As Long) As Boolean
IsFilePositionChanged = (Loc(fileNum) <> lastPosition)
End Function

Pattern 3: `GetCurrentRecord`

Function GetCurrentRecord(ByVal fileNum As Integer) As Long
' For Random access files
GetCurrentRecord = Loc(fileNum)
End Function

Pattern 4: `GetBytesProcessed`

Function GetBytesProcessed(ByVal fileNum As Integer) As Long
' For Binary access files
GetBytesProcessed = Loc(fileNum)
End Function

Pattern 5: `UpdateProgressBar`

Sub UpdateProgressBar(ByVal fileNum As Integer, _
ByVal progressBar As ProgressBar)
Dim percent As Single
percent = (Loc(fileNum) / LOF(fileNum)) * 100

If percent <= 100 Then
progressBar.Value = percent
End If
DoEvents
End Sub

Pattern 6: `ReadFileWithProgress`

Sub ReadFileWithProgress(ByVal filename As String)
Dim fileNum As Integer
Dim data As Byte
Dim lastPercent As Integer
Dim currentPercent As Integer

fileNum = FreeFile
Open filename For Binary As #fileNum

Do While Loc(fileNum) < LOF(fileNum)
Get #fileNum, , data
ProcessByte data

currentPercent = Int((Loc(fileNum) / LOF(fileNum)) * 100)
If currentPercent <> lastPercent Then
Debug.Print "Progress: " & currentPercent & "%"
lastPercent = currentPercent
End If
Loop

Close #fileNum
End Sub

Pattern 7: `GetRecordPosition`

Function GetRecordPosition(ByVal fileNum As Integer) As String
Dim current As Long
Dim total As Long

current = Loc(fileNum)
total = LOF(fileNum) / Len(recordVariable)

GetRecordPosition = current & " of " & total
End Function

Pattern 8: `SafeLoc`

Function SafeLoc(ByVal fileNum As Integer) As Long
On Error Resume Next
SafeLoc = Loc(fileNum)
If Err.Number <> 0 Then
SafeLoc = -1
Err.Clear
End If
End Function

Pattern 9: `IsAtEndOfFile`

Function IsAtEndOfFile(ByVal fileNum As Integer) As Boolean
' For Binary mode
IsAtEndOfFile = (Loc(fileNum) >= LOF(fileNum))
End Function

Pattern 10: `LogFilePosition`

Sub LogFilePosition(ByVal fileNum As Integer, _
ByVal operation As String)
Debug.Print operation & " - Position: " & Loc(fileNum) & _
" of " & LOF(fileNum)
End Sub

Advanced Examples

Example 1: File Reader with Progress

' Class: BinaryFileReader
Private m_fileNum As Integer
Private m_filename As String
Private m_fileSize As Long

Public Sub OpenFile(ByVal filename As String)
m_filename = filename
m_fileNum = FreeFile
Open filename For Binary As #m_fileNum
m_fileSize = LOF(m_fileNum)
End Sub

Public Function ReadByte() As Byte
If Not IsEOF Then
Get #m_fileNum, , ReadByte
End If
End Function

Public Property Get Position() As Long
Position = Loc(m_fileNum)
End Property

Public Property Get Size() As Long
Size = m_fileSize
End Property

Public Property Get Progress() As Single
If m_fileSize > 0 Then
Progress = (Loc(m_fileNum) / m_fileSize) * 100
Else
Progress = 0
End If
End Property

Public Property Get IsEOF() As Boolean
IsEOF = (Loc(m_fileNum) >= m_fileSize)
End Property

Public Sub CloseFile()
If m_fileNum > 0 Then
Close #m_fileNum
m_fileNum = 0
End If
End Sub

Private Sub Class_Terminate()
CloseFile
End Sub

Example 2: Random File Navigator

' Class: RandomFileNavigator
Private m_fileNum As Integer
Private m_recordLength As Integer
Private m_totalRecords As Long

Public Sub OpenFile(ByVal filename As String, _
ByVal recordLength As Integer)
m_recordLength = recordLength
m_fileNum = FreeFile
Open filename For Random As #m_fileNum Len = recordLength
m_totalRecords = LOF(m_fileNum) / recordLength
End Sub

Public Property Get CurrentRecord() As Long
CurrentRecord = Loc(m_fileNum)
End Property

Public Property Get TotalRecords() As Long
TotalRecords = m_totalRecords
End Property

Public Property Get ProgressPercent() As Single
If m_totalRecords > 0 Then
ProgressPercent = (Loc(m_fileNum) / m_totalRecords) * 100
Else
ProgressPercent = 0
End If
End Property

Public Function IsFirstRecord() As Boolean
IsFirstRecord = (Loc(m_fileNum) = 1)
End Function

Public Function IsLastRecord() As Boolean
IsLastRecord = (Loc(m_fileNum) = m_totalRecords)
End Function

Public Sub CloseFile()
If m_fileNum > 0 Then
Close #m_fileNum
m_fileNum = 0
End If
End Sub

Private Sub Class_Terminate()
CloseFile
End Sub

Example 3: File Copy with Progress

Sub CopyFileWithProgress(ByVal sourceFile As String, _
ByVal destFile As String, _
Optional progressBar As ProgressBar = Nothing)
Dim sourceNum As Integer, destNum As Integer
Dim buffer As Byte
Dim totalSize As Long
Dim lastPercent As Integer
Dim currentPercent As Integer

sourceNum = FreeFile
Open sourceFile For Binary As #sourceNum
totalSize = LOF(sourceNum)

destNum = FreeFile
Open destFile For Binary As #destNum

Do While Loc(sourceNum) < totalSize
Get #sourceNum, , buffer
Put #destNum, , buffer

If Not progressBar Is Nothing Then
currentPercent = Int((Loc(sourceNum) / totalSize) * 100)
If currentPercent <> lastPercent Then
progressBar.Value = currentPercent
lastPercent = currentPercent
DoEvents
End If
End If
Loop

Close #sourceNum
Close #destNum
End Sub

Example 4: File Processing Monitor

' Form with lblStatus, lblProgress, ProgressBar1
Private m_fileNum As Integer
Private m_totalSize As Long

Private Sub ProcessLargeFile(ByVal filename As String)
Dim data As Byte
Dim startTime As Single

m_fileNum = FreeFile
Open filename For Binary As #m_fileNum
m_totalSize = LOF(m_fileNum)

startTime = Timer
Timer1.Enabled = True

Do While Loc(m_fileNum) < m_totalSize
Get #m_fileNum, , data
ProcessData data
Loop

Timer1.Enabled = False
Close #m_fileNum

lblStatus.Caption = "Complete!"
End Sub

Private Sub Timer1_Timer()
UpdateProgress
End Sub

Private Sub UpdateProgress()
Dim percent As Single
Dim bytesProcessed As Long

On Error Resume Next
bytesProcessed = Loc(m_fileNum)

If m_totalSize > 0 Then
percent = (bytesProcessed / m_totalSize) * 100
ProgressBar1.Value = percent
lblProgress.Caption = Format(percent, "0.0") & "% - " & _
FormatBytes(bytesProcessed) & " of " & _
FormatBytes(m_totalSize)
End If
End Sub

Private Function FormatBytes(ByVal bytes As Long) As String
If bytes < 1024 Then
FormatBytes = bytes & " bytes"
ElseIf bytes < 1048576 Then
FormatBytes = Format(bytes / 1024, "0.0") & " KB"
Else
FormatBytes = Format(bytes / 1048576, "0.0") & " MB"
End If
End Function

Error Handling

' Error 52: Bad file name or number
On Error Resume Next
pos = Loc(999)
If Err.Number = 52 Then
MsgBox "File not open!"
End If

' Error 68: Device unavailable
pos = Loc(fileNum)
If Err.Number = 68 Then
MsgBox "Device unavailable!"
End If

' Safe position retrieval
Function GetSafePosition(ByVal fileNum As Integer) As Long
On Error Resume Next
GetSafePosition = Loc(fileNum)
If Err.Number <> 0 Then
GetSafePosition = -1
Err.Clear
End If
End Function

Performance Considerations

Very Fast: Loc is a simple file pointer query
No I/O: Does not perform actual file operations
Frequent Calls: Safe to call in tight loops
Progress Updates: Use modulo to update UI less frequently
DoEvents: Call DoEvents when updating UI to maintain responsiveness

Best Practices

Use with LOF for calculating percentage complete
Check file is open before calling Loc
Update progress periodically not on every byte
Cache in variable if using multiple times
Use for Binary/Random files (Sequential returns approximation)
Combine with EOF for robust loop conditions
Handle errors for unopened files
Use DoEvents when updating UI in loops
Consider mode when interpreting return value
Document units (bytes, records, or approximation)

Function	Purpose	Read/Write	Return Value
Loc	Get position	Read-only	Position
Seek (function)	Get position	Read-only	Position + 1
Seek (statement)	Set position	Write	N/A
LOF	Get file length	Read-only	Total bytes
EOF	Check end	Read-only	Boolean

Loc vs Seek Function

' Loc - returns position of last operation
currentPos = Loc(fileNum)

' Seek - returns next read/write position (Loc + 1)
nextPos = Seek(fileNum)

' For Binary mode:
' After reading byte at position 100:
' Loc returns 100
' Seek returns 101

Mode-Specific Behavior

' Random mode - record number
Open "data.dat" For Random As #1 Len = 128
Get #1, 5, record
Debug.Print Loc(1)  ' Returns 5 (record number)

' Binary mode - byte position
Open "data.bin" For Binary As #1
Get #1, , buffer
Debug.Print Loc(1)  ' Returns bytes read

' Sequential mode - approximate (bytes / 128)
Open "text.txt" For Input As #1
Line Input #1, line
Debug.Print Loc(1)  ' Returns approximate position

Platform Notes

Available in all VB6 versions
Part of VBA core library
Returns Long (max 2GB file support)
For files > 2GB, result may overflow
Windows-specific file I/O
Behavior identical across Windows versions
Sequential mode approximation may vary
Random mode most reliable
Binary mode exact for files < 2GB

Limitations

2GB Limit: Long type limits file size to ~2GB
Sequential Approximation: Not exact for Input/Output/Append modes
Division by 128: Sequential mode uses this approximation
No String Files: Works with file numbers only
Requires Open File: Error if file not open
Mode Dependent: Return value meaning varies by mode
No Directory: Only for files, not directories
Last Operation: Returns position of last I/O, not current

Seek: Get/set file position (next position, not last)
LOF: Get length of file
EOF: Check if at end of file
Open: Open file for I/O
Close: Close file
Get: Read from file (updates Loc)
Put: Write to file (updates Loc)
FreeFile: Get available file number