close

Close the file and release associated resources.

    
    
    
close()

Examples

Example of proper file operation workflow.

    javascript
    
    
function main() {
    let file = os.open("data.txt", "w")
    file.write("Hello FMZ!")
    file.close()  // 必须关闭文件
}

See Also

Remarks

This method must be called after using the file object to release system resources.

puts

Write one or more strings to a file.

    
    
    
puts(data1, data2, ...dataN)

Examples

Write multiple strings to a file.

    javascript
    
    
function main() {
    let file = os.open("output.txt", "w+")
    let bytes = file.puts("Hello", " ", "World", "!")
    Log("Bytes written:", bytes)  // Bytes written: 12

    file.seek(0, 0)
    let data = file.read()
    Log("data:", data)            // If using os.open("output.txt", "w") may result in data being undefined
                                  // data: Hello World!

    // file.puts()                // error: puts requires at least 1 argument at main
    file.puts(", Hello FMZ!")

    file.seek(0, 0)
    data = file.read()
    Log("data:", data)            // data: Hello World!, Hello FMZ!
    file.close()
}

Returns

Arguments

See Also

Remarks

Multiple string parameters can be written at once, they will be concatenated in order and then written.

printf

Write formatted data to file.

    
    
    
printf(format)

    
    
    
printf(format, arg1, arg2, ...argN)

Examples

Write formatted trading data.

    javascript
    
    
function main() {
    let file = os.open("trade_log.txt", "w+")
    let price = 100.25
    let volume = 1000
    let bytes = file.printf("Price: %.2f, Volume: %d\n", price, volume)
    Log("Formatted bytes written:", bytes)

    file.seek(0, 0)
    let data = file.read()
    Log("data:", data)                              // data: Price: 100.25, Volume: 1000

    // file.printf("| Price: %.2f, Volume: %d\n")   // "| Price: %!f(MISSING), Volume: %!d(MISSING)"
    // file.seek(0, 0)
    // data = file.read()
    // Log("data:", data)

    file.close()
}

Returns

Arguments

See Also

flush

Flush the file buffer to ensure data is written to disk.

    
    
    
flush()

Examples

Write important log data in real-time.

    javascript
    
    
function main() {
    let logFile = os.open("critical.log", "a")
    logFile.printf("[%s] Critical event occurred\n", new Date().toISOString())   // [2025-09-09T03:15:43.895Z] Critical event occurred
    logFile.flush()  // Immediately write data to disk
    // Continue with other operations...
    logFile.close()
}

See Also

tell

Get the current file pointer position.

    
    
    
tell()

Examples

Track position during file operations.

    javascript
    
    
function main() {
    let file = os.open("data.txt", "r+")
    Log("Initial position:", file.tell())      // Initial position: 0

    file.write("Hello")
    Log("After write position:", file.tell())  // After write position: 5
    file.close()
}

Returns

See Also

Remarks

Returns the byte offset of the current file pointer relative to the beginning of the file.

seek

Move the file pointer to a specified position.

    
    
    
seek(offset, whence)

Examples

Read characters in reverse order.

    javascript
    
    
function main() {
    let str = "Hello FMZ!"
    let file = os.open("data.txt", "w+")
    file.write(str)

    // If i > str.length: will throw InternalError: seek .../xxx/data.txt: invalid argument at main
    for (let i = 1; i <= str.length; i++) {
        file.seek(-i, 2)
        let data = file.read(1)
        Log("i:", i, ", data:", data)
    }

    file.close()
}

Returns

Arguments

See Also

Remarks

Used to position the file pointer, the offset parameter can be negative (indicating backward movement).

eof

Check if the file pointer has reached the end of file.

    
    
    
eof()

Examples

Read file line by line until end of file.

    javascript
    
    
function main() {
    let file = os.open("data.txt", "r")
    let lineCount = 0

    while (!file.eof()) {
        let line = file.getline()
        if (line) {
            lineCount++
            Log("Line", lineCount + ":", line)
        }
    }

    Log("Total lines:", lineCount)
    file.close()
}

Returns

See Also

Remarks

Used to determine whether all content has been read when reading a file.

read

Read data from a file.

    
    
    
read()

    
    
    
read(size)

Examples

Read file content in chunks.

    javascript
    
    
function main() {
    let file = os.open("data.txt", "r")

    // data.txt
    // This is a test line: Line 1.
    // This is a test line: Line 2.
    // ...

    let chunkSize = 29
    let totalBytes = 0

    while (!file.eof()) {
        let chunk = file.read(chunkSize)
        if (chunk) {
            totalBytes += chunk.length || chunk.byteLength
            Log("Read chunk, total bytes so far:", totalBytes, ", chunk:", chunk)
        }
    }

    file.close()
}

Returns

Arguments

See Also

Remarks

Can read a specified number of bytes or all remaining content in the file. The return type may be string or ArrayBuffer.

write

Write string data to a file.

    
    
    
write(data)

Examples

Writing trade records

    javascript
    
    
function main() {
    let file = os.open("trades.log", "a")
    let timestamp = new Date().toISOString()
    let tradeInfo = `${timestamp},BUY,50000,1\n`
    let bytes = file.write(tradeInfo)
    Log("Trade record written, bytes:", bytes)
    file.close()
}

Returns

Arguments

See Also

Remarks

Writes string data to the current position in the file.

getline

Read the next line from the file.

    
    
    
getline()

Examples

Read file line by line until the end.

    javascript
    
    
function main() {
    let file = os.open("data.txt", "r")
    let lineCount = 0

    while (!file.eof()) {
        let line = file.getline()
        if (line) {
            lineCount++
            Log("Line", lineCount + ":", line)
        }
    }

    Log("Total lines:", lineCount)
    file.close()
}

Returns

See Also

Remarks

Read file content line by line in sequential order.

toString

Get the string representation of the file object.

    
    
    
toString()

Examples

Get the description information of the file object.

    javascript
    
    
function main() {
    let file = os.open("data/data.txt", "r")
    Log("File info:", file.toString())           // File info: File(data/data.txt)
    file.close()
}

Returns

See Also

Remarks

Returns the description information of the file object.