FreeBasic Open Keyword

last modified June 16, 2025

The FreeBasic Open keyword is used for file operations. It allows reading from and writing to files on disk. File handling is essential for persistent data storage in applications.

Basic Definition

In FreeBasic, Open creates a connection between a program and a file. It specifies the file name, access mode, and file number. The file number is used to reference the opened file in subsequent operations.

The Open statement must be paired with a Close statement. Files can be opened for reading, writing, or both. Different modes control how the file is accessed and whether it's created if missing.

Opening a File for Reading

This example shows how to open a file for reading text data.

open_read.bas

Dim fileNum As Integer = FreeFile()
Open "data.txt" For Input As #fileNum

Dim line As String
While Not EOF(fileNum)
    Line Input #fileNum, line
    Print line
Wend

Close #fileNum

Here we open "data.txt" for input (reading). FreeFile() gets an available file number. We read lines until EOF (end-of-file) is reached. Each line is printed to the console. Finally, we close the file.

Opening a File for Writing

This example demonstrates opening a file for writing new content.

open_write.bas

Dim fileNum As Integer = FreeFile()
Open "output.txt" For Output As #fileNum

Print #fileNum, "First line of text"
Print #fileNum, "Second line of text"

Close #fileNum

We open "output.txt" for output (writing). If the file exists, it's truncated. If it doesn't exist, it's created. We write two lines using Print # and close the file. This is useful for creating logs.

Appending to a File

This example shows how to append data to an existing file.

open_append.bas

Dim fileNum As Integer = FreeFile()
Open "log.txt" For Append As #fileNum

Print #fileNum, "New log entry: "; Date()
Print #fileNum, "---------------------"

Close #fileNum

Using For Append mode preserves existing content. New data is added at the end. This is ideal for log files or accumulating data. The Date() function adds a timestamp to our log entry.

Binary File Access

Binary mode allows reading and writing raw bytes to a file.

open_binary.bas

Dim fileNum As Integer = FreeFile()
Open "data.bin" For Binary As #fileNum

Dim buffer As String * 10
Get #fileNum, , buffer
Put #fileNum, , "NewData1234"

Close #fileNum

Binary mode is used for non-text files or precise byte control. Get reads bytes into a buffer. Put writes bytes from a string. The position can be specified or left to advance sequentially.

Random Access Files

Random access allows reading/writing fixed-length records at any position.

open_random.bas

Type Person
    name As String * 20
    age As Integer
End Type

Dim fileNum As Integer = FreeFile()
Open "people.dat" For Random As #fileNum Len = Len(Person)

Dim p As Person
p.name = "John Doe"
p.age = 30
Put #fileNum, 1, p

Get #fileNum, 1, p
Print "Name: "; p.name; ", Age: "; p.age

Close #fileNum

Random access files use fixed-length records. We define a Type structure. Len = Len(Person) sets the record size. Records are accessed by number (1-based). This is efficient for database-like files.

Checking File Existence Before Opening

It's good practice to check if a file exists before opening it.

open_exists.bas

Dim filename As String = "config.ini"

If Dir(filename) = "" Then
    Print "File not found, creating new one"
    Dim fileNum As Integer = FreeFile()
    Open filename For Output As #fileNum
    Print #fileNum, "[settings]"
    Close #fileNum
Else
    Print "File exists, opening for reading"
    Dim fileNum As Integer = FreeFile()
    Open filename For Input As #fileNum
    ' Read file contents...
    Close #fileNum
End If

Dir() checks if a file exists. If it doesn't, we create it. If it exists, we open it for reading. This prevents errors when opening non-existent files. Always handle both cases in file operations.

File Locking for Multi-Process Access

File locking prevents conflicts when multiple processes access a file.

open_lock.bas

Dim fileNum As Integer = FreeFile()
Open "shared.txt" For Binary Access Read Write Lock Read Write As #fileNum

' Exclusive access granted
Print #fileNum, "Process 1 was here"
Sleep 2000  ' Simulate work

Close #fileNum

The Lock clause controls access. Lock Read Write requests exclusive access. Other processes will wait until the file is unlocked. This is crucial for multi-process applications sharing files.

Best Practices

Error Handling: Always check for file operation success.
Resource Management: Close files promptly when done.
File Numbers: Use FreeFile() to avoid conflicts.
Modes: Choose the appropriate access mode for your needs.
Paths: Use absolute paths or handle relative paths carefully.

This tutorial covered the FreeBasic Open keyword with practical examples showing file operations in different scenarios.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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