QIODevice Class

Member Function Documentation

Reads a line from this device into buffer, and returns the subset of buffer that contains the data read.

If buffer's size is smaller than the length of the line, only the characters that fit within buffer are read and returned. In this case, calling readLineInto() again will retrieve the remainder of the line. To determine whether the entire line was read, first check if the device is atEnd(), in case the last line didn't end with a newline. If not atEnd(), verify whether the returned view ends with '\n'. Otherwise, another call to readLineInto() is required.

The resulting line can have trailing end-of-line characters ("\n" or "\r\n"), so calling QByteArrayView::trimmed() may be necessary.

In case an error occurred, this function returns a null QByteArrayView. Otherwise it is a sub-span of buffer. If no data was currently available for reading or the device is atEnd(), this function returns an empty QByteArrayView.

Note that the return value is not null terminated. If you want null-termination, you can pass buffer.chopped(1) and then insert '\0' at buffer[result.size()].

This function was introduced in Qt 6.9.

See also readLine().

QIODevice::QIODevice()

Constructs a QIODevice object.

[explicit] QIODevice::QIODevice(QObject *parent)

Constructs a QIODevice object with the given parent.

[virtual noexcept] QIODevice::~QIODevice()

The destructor is virtual, and QIODevice is an abstract base class. This destructor does not call close(), but the subclass destructor might. If you are in doubt, call close() before destroying the QIODevice.

[signal] void QIODevice::aboutToClose()

This signal is emitted when the device is about to close. Connect this signal if you have operations that need to be performed before the device closes (e.g., if you have data in a separate buffer that needs to be written to the device).

[virtual] bool QIODevice::atEnd() const

Returns true if the current read and write position is at the end of the device (i.e. there is no more data available for reading on the device); otherwise returns false.

For some devices, atEnd() can return true even though there is more data to read. This special case only applies to devices that generate data in direct response to you calling read() (e.g., /dev or /proc files on Unix and macOS, or console input / stdin on all platforms).

See also bytesAvailable(), read(), and isSequential().

[virtual] qint64 QIODevice::bytesAvailable() const

Returns the number of bytes that are available for reading. This function is commonly used with sequential devices to determine the number of bytes to allocate in a buffer before reading.

Subclasses that reimplement this function must call the base implementation in order to include the size of the buffer of QIODevice. Example:

qint64 CustomDevice::bytesAvailable() const
{
    return buffer.size() + QIODevice::bytesAvailable();
}

See also bytesToWrite(), readyRead(), and isSequential().

[virtual] qint64 QIODevice::bytesToWrite() const

For buffered devices, this function returns the number of bytes waiting to be written. For devices with no buffer, this function returns 0.

Subclasses that reimplement this function must call the base implementation in order to include the size of the buffer of QIODevice.

See also bytesAvailable(), bytesWritten(), and isSequential().

[signal] void QIODevice::bytesWritten(qint64 bytes)

This signal is emitted every time a payload of data has been written to the device's current write channel. The bytes argument is set to the number of bytes that were written in this payload.

bytesWritten() is not emitted recursively; if you reenter the event loop or call waitForBytesWritten() inside a slot connected to the bytesWritten() signal, the signal will not be reemitted (although waitForBytesWritten() may still return true).

See also readyRead().

[virtual] bool QIODevice::canReadLine() const

Returns true if a complete line of data can be read from the device; otherwise returns false.

Note that unbuffered devices, which have no way of determining what can be read, always return false.

This function is often called in conjunction with the readyRead() signal.

Subclasses that reimplement this function must call the base implementation in order to include the contents of the QIODevice's buffer. Example:

bool CustomDevice::canReadLine() const
{
    return buffer.contains('\n') || QIODevice::canReadLine();
}

See also readyRead() and readLine().

[signal] void QIODevice::channelBytesWritten(int channel, qint64 bytes)

This signal is emitted every time a payload of data has been written to the device. The bytes argument is set to the number of bytes that were written in this payload, while channel is the channel they were written to. Unlike bytesWritten(), it is emitted regardless of the current write channel.

channelBytesWritten() can be emitted recursively - even for the same channel.

See also bytesWritten() and channelReadyRead().

[signal] void QIODevice::channelReadyRead(int channel)

This signal is emitted when new data is available for reading from the device. The channel argument is set to the index of the read channel on which the data has arrived. Unlike readyRead(), it is emitted regardless of the current read channel.

channelReadyRead() can be emitted recursively - even for the same channel.

See also readyRead() and channelBytesWritten().

[virtual] void QIODevice::close()

First emits aboutToClose(), then closes the device and sets its OpenMode to NotOpen. The error string is also reset.

See also setOpenMode() and QIODeviceBase::OpenMode.

void QIODevice::commitTransaction()

Completes a read transaction.

For sequential devices, all data recorded in the internal buffer during the transaction will be discarded.

See also startTransaction() and rollbackTransaction().

int QIODevice::currentReadChannel() const

Returns the index of the current read channel.

See also setCurrentReadChannel(), readChannelCount(), and QProcess.

int QIODevice::currentWriteChannel() const

Returns the index of the current write channel.

See also setCurrentWriteChannel() and writeChannelCount().

QString QIODevice::errorString() const

Returns a human-readable description of the last device error that occurred.

See also setErrorString().

bool QIODevice::getChar(char *c)

Reads one character from the device and stores it in c. If c is nullptr, the character is discarded. Returns true on success; otherwise returns false.

See also read(), putChar(), and ungetChar().

bool QIODevice::isOpen() const

Returns true if the device is open; otherwise returns false. A device is open if it can be read from and/or written to. By default, this function returns false if openMode() returns NotOpen.

See also openMode() and QIODeviceBase::OpenMode.

bool QIODevice::isReadable() const

Returns true if data can be read from the device; otherwise returns false. Use bytesAvailable() to determine how many bytes can be read.

This is a convenience function which checks if the OpenMode of the device contains the ReadOnly flag.

See also openMode() and OpenMode.

[virtual] bool QIODevice::isSequential() const

Returns true if this device is sequential; otherwise returns false.

Sequential devices, as opposed to a random-access devices, have no concept of a start, an end, a size, or a current position, and they do not support seeking. You can only read from the device when it reports that data is available. The most common example of a sequential device is a network socket. On Unix, special files such as /dev/zero and fifo pipes are sequential.

Regular files, on the other hand, do support random access. They have both a size and a current position, and they also support seeking backwards and forwards in the data stream. Regular files are non-sequential.

See also bytesAvailable().

bool QIODevice::isTextModeEnabled() const

Returns true if the Text flag is enabled; otherwise returns false.

See also setTextModeEnabled().

bool QIODevice::isTransactionStarted() const

Returns true if a transaction is in progress on the device, otherwise false.

See also startTransaction().

bool QIODevice::isWritable() const

Returns true if data can be written to the device; otherwise returns false.

This is a convenience function which checks if the OpenMode of the device contains the WriteOnly flag.

See also openMode() and OpenMode.

[virtual] bool QIODevice::open(QIODeviceBase::OpenMode mode)

Opens the device and sets its OpenMode to mode. Returns true if successful; otherwise returns false. This function should be called from any reimplementations of open() or other functions that open the device.

See also openMode() and QIODeviceBase::OpenMode.

QIODeviceBase::OpenMode QIODevice::openMode() const

Returns the mode in which the device has been opened; i.e. ReadOnly or WriteOnly.

See also setOpenMode() and OpenMode.

qint64 QIODevice::peek(char *data, qint64 maxSize)

Reads at most maxSize bytes from the device into data, without side effects (i.e., if you call read() after peek(), you will get the same data). Returns the number of bytes read. If an error occurs, such as when attempting to peek a device opened in WriteOnly mode, this function returns -1.

0 is returned when no more data is available for reading.

Example:

bool isExeFile(QFile *file)
{
    char buf[2];
    if (file->peek(buf, sizeof(buf)) == sizeof(buf))
        return (buf[0] == 'M' && buf[1] == 'Z');
    return false;
}

See also read().

QByteArray QIODevice::peek(qint64 maxSize)

This is an overloaded function.

Peeks at most maxSize bytes from the device, returning the data peeked as a QByteArray.

Example:

bool isExeFile(QFile *file)
{
    return file->peek(2) == "MZ";
}

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for peeking, or that an error occurred.

See also read().

[virtual] qint64 QIODevice::pos() const

For random-access devices, this function returns the position that data is written to or read from. For sequential devices or closed devices, where there is no concept of a "current position", 0 is returned.

The current read/write position of the device is maintained internally by QIODevice, so reimplementing this function is not necessary. When subclassing QIODevice, use QIODevice::seek() to notify QIODevice about changes in the device position.

See also isSequential() and seek().

bool QIODevice::putChar(char c)

Writes the character c to the device. Returns true on success; otherwise returns false.

See also write(), getChar(), and ungetChar().

qint64 QIODevice::read(char *data, qint64 maxSize)

Reads at most maxSize bytes from the device into data, and returns the number of bytes read. If an error occurs, such as when attempting to read from a device opened in WriteOnly mode, this function returns -1.

0 is returned when no more data is available for reading. However, reading past the end of the stream is considered an error, so this function returns -1 in those cases (that is, reading on a closed socket or after a process has died).

See also readData(), readLine(), and write().

QByteArray QIODevice::read(qint64 maxSize)

This is an overloaded function.

Reads at most maxSize bytes from the device, and returns the data read as a QByteArray.

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for reading, or that an error occurred.

QByteArray QIODevice::readAll()

Reads all remaining data from the device, and returns it as a byte array.

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for reading, or that an error occurred. This function also has no way of indicating that more data may have been available and couldn't be read.

int QIODevice::readChannelCount() const

Returns the number of available read channels if the device is open; otherwise returns 0.

See also writeChannelCount() and QProcess.

[signal] void QIODevice::readChannelFinished()

This signal is emitted when the input (reading) stream is closed in this device. It is emitted as soon as the closing is detected, which means that there might still be data available for reading with read().

See also atEnd() and read().

[pure virtual protected] qint64 QIODevice::readData(char *data, qint64 maxSize)

Reads up to maxSize bytes from the device into data, and returns the number of bytes read or -1 if an error occurred.

If there are no bytes to be read and there can never be more bytes available (examples include socket closed, pipe closed, sub-process finished), this function returns -1.

This function is called by QIODevice. Reimplement this function when creating a subclass of QIODevice.

When reimplementing this function it is important that this function reads all the required data before returning. This is required in order for QDataStream to be able to operate on the class. QDataStream assumes all the requested information was read and therefore does not retry reading if there was a problem.

This function might be called with a maxSize of 0, which can be used to perform post-reading operations.

See also read(), readLine(), and writeData().

qint64 QIODevice::readLine(char *data, qint64 maxSize)

This function reads a line of ASCII characters from the device, up to a maximum of maxSize - 1 bytes, stores the characters in data, and returns the number of bytes read. If a line could not be read but no error occurred, this function returns 0. If an error occurs, this function returns the length of what could be read, or -1 if nothing was read.

A terminating '\0' byte is always appended to data, so maxSize must be larger than 1.

Data is read until either of the following conditions are met:

• The first '\n' character is read.
• maxSize - 1 bytes are read.
• The end of the device data is detected.

For example, the following code reads a line of characters from a file:

QFile file("box.txt");
if (file.open(QFile::ReadOnly)) {
    char buf[1024];
    qint64 lineLength = file.readLine(buf, sizeof(buf));
    if (lineLength != -1) {
        // the line is available in buf
    }
}

The newline character ('\n') is included in the buffer. If a newline is not encountered before maxSize - 1 bytes are read, a newline will not be inserted into the buffer. On windows newline characters are replaced with '\n'.

Note that on sequential devices, data may not be immediately available, which may result in a partial line being returned. By calling the canReadLine() function before reading, you can check whether a complete line (including the newline character) can be read.

This function calls readLineData(), which is implemented using repeated calls to getChar(). You can provide a more efficient implementation by reimplementing readLineData() in your own subclass.

See also getChar(), read(), canReadLine(), and write().

QByteArray QIODevice::readLine(qint64 maxSize = 0)

This is an overloaded function.

Reads a line from the device, but no more than maxSize characters, and returns the result as a byte array.

If maxSize is 0 or not specified, the line can be of any length, thereby enabling unlimited reading.

The resulting line can have trailing end-of-line characters ("\n" or "\r\n"), so calling QByteArray::trimmed() may be necessary.

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for reading, or that an error occurred.

[virtual protected] qint64 QIODevice::readLineData(char *data, qint64 maxSize)

Reads up to maxSize characters into data and returns the number of characters read.

This function is called by readLine(), and provides its base implementation, using getChar(). Buffered devices can improve the performance of readLine() by reimplementing this function.

readLine() appends a '\0' byte to data; readLineData() does not need to do this.

If you reimplement this function, be careful to return the correct value: it should return the number of bytes read in this line, including the terminating newline, or 0 if there is no line to be read at this point. If an error occurs, it should return -1 if and only if no bytes were read. Reading past EOF is considered an error.

[since 6.9] bool QIODevice::readLineInto(QByteArray *line, qint64 maxSize = 0)

Reads a line from the device, but no more than maxSize characters. and stores it as a byte array in line.

If maxSize is 0 or not specified, the line can be of any length, thereby enabling unlimited reading.

The resulting line can have trailing end-of-line characters ("\n" or "\r\n"), so calling QByteArray::trimmed() may be necessary.

If no data was currently available for reading, or in case an error occurred, this function returns false and sets line to empty. Otherwise it returns true.

Note that the contents of line before the call are discarded in any case but its capacity() is never reduced.

This function was introduced in Qt 6.9.

See also readAll(), readLine(), and QTextStream::readLineInto().

[signal] void QIODevice::readyRead()

This signal is emitted once every time new data is available for reading from the device's current read channel. It will only be emitted again once new data is available, such as when a new payload of network data has arrived on your network socket, or when a new block of data has been appended to your device.

readyRead() is not emitted recursively; if you reenter the event loop or call waitForReadyRead() inside a slot connected to the readyRead() signal, the signal will not be reemitted (although waitForReadyRead() may still return true).

Note for developers implementing classes derived from QIODevice: you should always emit readyRead() when new data has arrived (do not emit it only because there's data still to be read in your buffers). Do not emit readyRead() in other conditions.

See also bytesWritten().

[virtual] bool QIODevice::reset()

Seeks to the start of input for random-access devices. Returns true on success; otherwise returns false (for example, if the device is not open).

Note that when using a QTextStream on a QFile, calling reset() on the QFile will not have the expected result because QTextStream buffers the file. Use the QTextStream::seek() function instead.

See also seek().

void QIODevice::rollbackTransaction()

Rolls back a read transaction.

Restores the input stream to the point of the startTransaction() call. This function is commonly used to rollback the transaction when an incomplete read was detected prior to committing the transaction.

See also startTransaction() and commitTransaction().

[virtual] bool QIODevice::seek(qint64 pos)

For random-access devices, this function sets the current position to pos, returning true on success, or false if an error occurred. For sequential devices, the default behavior is to produce a warning and return false.

When subclassing QIODevice, you must call QIODevice::seek() at the start of your function to ensure integrity with QIODevice's built-in buffer.

See also pos() and isSequential().

void QIODevice::setCurrentReadChannel(int channel)

Sets the current read channel of the QIODevice to the given channel. The current input channel is used by the functions read(), readAll(), readLine(), and getChar(). It also determines which channel triggers QIODevice to emit readyRead().

See also currentReadChannel(), readChannelCount(), and QProcess.

void QIODevice::setCurrentWriteChannel(int channel)

Sets the current write channel of the QIODevice to the given channel. The current output channel is used by the functions write(), putChar(). It also determines which channel triggers QIODevice to emit bytesWritten().

See also currentWriteChannel() and writeChannelCount().

[protected] void QIODevice::setErrorString(const QString &str)

Sets the human readable description of the last device error that occurred to str.

See also errorString().

[protected] void QIODevice::setOpenMode(QIODeviceBase::OpenMode openMode)

Sets the OpenMode of the device to openMode. Call this function to set the open mode if the flags change after the device has been opened.

See also openMode() and OpenMode.

void QIODevice::setTextModeEnabled(bool enabled)

If enabled is true, this function sets the Text flag on the device; otherwise the Text flag is removed. This feature is useful for classes that provide custom end-of-line handling on a QIODevice.

The IO device should be opened before calling this function.

See also isTextModeEnabled(), open(), and setOpenMode().

[virtual] qint64 QIODevice::size() const

For open random-access devices, this function returns the size of the device. For open sequential devices, bytesAvailable() is returned.

If the device is closed, the size returned will not reflect the actual size of the device.

See also isSequential() and pos().

qint64 QIODevice::skip(qint64 maxSize)

Skips up to maxSize bytes from the device. Returns the number of bytes actually skipped, or -1 on error.

This function does not wait and only discards the data that is already available for reading.

If the device is opened in text mode, end-of-line terminators are translated to '\n' symbols and count as a single byte identically to the read() and peek() behavior.

This function works for all devices, including sequential ones that cannot seek(). It is optimized to skip unwanted data after a peek() call.

For random-access devices, skip() can be used to seek forward from the current position. Negative maxSize values are not allowed.

See also skipData(), peek(), seek(), and read().

[virtual protected, since 6.0] qint64 QIODevice::skipData(qint64 maxSize)

Skips up to maxSize bytes from the device. Returns the number of bytes actually skipped, or -1 on error.

This function is called by QIODevice. Consider reimplementing it when creating a subclass of QIODevice.

The base implementation discards the data by reading into a dummy buffer. This is slow, but works for all types of devices. Subclasses can reimplement this function to improve on that.

This function was introduced in Qt 6.0.

See also skip(), peek(), seek(), and read().

void QIODevice::startTransaction()

Starts a new read transaction on the device.

Defines a restorable point within the sequence of read operations. For sequential devices, read data will be duplicated internally to allow recovery in case of incomplete reads. For random-access devices, this function saves the current position. Call commitTransaction() or rollbackTransaction() to finish the transaction.

See also commitTransaction() and rollbackTransaction().

void QIODevice::ungetChar(char c)

Puts the character c back into the device, and decrements the current position unless the position is 0. This function is usually called to "undo" a getChar() operation, such as when writing a backtracking parser.

If c was not previously read from the device, the behavior is undefined.

[virtual] bool QIODevice::waitForBytesWritten(int msecs)

For buffered devices, this function waits until a payload of buffered written data has been written to the device and the bytesWritten() signal has been emitted, or until msecs milliseconds have passed. If msecs is -1, this function will not time out. For unbuffered devices, it returns immediately.

Returns true if a payload of data was written to the device; otherwise returns false (i.e. if the operation timed out, or if an error occurred).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

If called from within a slot connected to the bytesWritten() signal, bytesWritten() will not be reemitted.

Reimplement this function to provide a blocking API for a custom device. The default implementation does nothing, and returns false.

See also waitForReadyRead().

[virtual] bool QIODevice::waitForReadyRead(int msecs)

Blocks until new data is available for reading and the readyRead() signal has been emitted, or until msecs milliseconds have passed. If msecs is -1, this function will not time out.

Returns true if new data is available for reading; otherwise returns false (if the operation timed out or if an error occurred).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

If called from within a slot connected to the readyRead() signal, readyRead() will not be reemitted.

Reimplement this function to provide a blocking API for a custom device. The default implementation does nothing, and returns false.

See also waitForBytesWritten().

qint64 QIODevice::write(const char *data, qint64 maxSize)

Writes at most maxSize bytes of data from data to the device. Returns the number of bytes that were actually written, or -1 if an error occurred.

See also read() and writeData().

qint64 QIODevice::write(const QByteArray &data)

This is an overloaded function.

Writes the content of data to the device. Returns the number of bytes that were actually written, or -1 if an error occurred.

See also read() and writeData().

qint64 QIODevice::write(const char *data)

This is an overloaded function.

Writes data from a zero-terminated string of 8-bit characters to the device. Returns the number of bytes that were actually written, or -1 if an error occurred. This is equivalent to

...
QIODevice::write(data, qstrlen(data));
...

See also read() and writeData().

int QIODevice::writeChannelCount() const

Returns the number of available write channels if the device is open; otherwise returns 0.

See also readChannelCount().

[pure virtual protected] qint64 QIODevice::writeData(const char *data, qint64 maxSize)

Writes up to maxSize bytes from data to the device. Returns the number of bytes written, or -1 if an error occurred.

This function is called by QIODevice. Reimplement this function when creating a subclass of QIODevice.

When reimplementing this function it is important that this function writes all the data available before returning. This is required in order for QDataStream to be able to operate on the class. QDataStream assumes all the information was written and therefore does not retry writing if there was a problem.

See also read() and write().