Standard I/O and Console Output

ESP-IDF provides C standard I/O facilities, such as stdin, stdout, and stderr streams, as well as C standard library functions such as printf() which operate on these streams.

As common in POSIX systems, these streams are buffering wrappers around file descriptors:

  • stdin is a buffered stream for reading input from the user, wrapping file descriptor STDIN_FILENO (0).

  • stdout is a buffered stream for writing output to the user, wrapping STDOUT_FILENO (1).

  • stderr is a buffered stream for writing error messages to the user, wrapping STDERR_FILENO (2).

In ESP-IDF, there is no practical distinction between stdout and stderr, as both streams are sent to the same physical interface. Most applications will use only stdout. For example, ESP-IDF logging functions always write to stdout regardless of the log level.

The underlying stdin, stdout, and stderr file descriptors are implemented based on VFS drivers.

On ESP32-S2, ESP-IDF provides implementations of VFS drivers for I/O over:

  • UART

  • USB CDC (using USB_OTG peripheral)

  • "Null" (no output)

Standard I/O is not limited to these options, though. See below on enabling custom destinations for standard I/O.

Configuration

Built-in implementations of standard I/O can be selected using several Kconfig options:

Enabling one of these option will cause the corresponding VFS driver to be built into the application and used to open stdin, stdout, and stderr streams. Data written to stdout and stderr will be sent over the selected interface, and input from the selected interface will be available on stdin.

Standard Streams and FreeRTOS Tasks

In ESP-IDF, to save RAM, FILE objects for stdin, stdout, and stderr are shared between all FreeRTOS tasks, but the pointers to these objects are unique for every task. This means that:

  • It is possible to change stdin, stdout, and stderr for any given task without affecting other tasks, e.g., by doing stdin = fopen("/dev/uart/1", "r").

  • To change the default stdin, stdout, stderr streams for new tasks, modify _GLOBAL_REENT->_stdin (_stdout, _stderr) before creating the task.

  • Closing default stdin, stdout, or stderr using fclose closes the FILE stream object, which will affect all other tasks.

Each stream (stdin, stdout, stderr) has a mutex associated with it. This mutex is used to protect the stream from concurrent access by multiple tasks. For example, if two tasks are writing to stdout at the same time, the mutex will ensure that the outputs from each task are not mixed together.

Blocking and non-blocking I/O

UART

By default, UART VFS uses simplified functions for reading from and writing to UART. Writes busy-wait until all data is put into UART FIFO, and reads are non-blocking, returning only the data present in the FIFO. Due to this non-blocking read behavior, higher level C library calls, such as fscanf("%d\n", &var);, might not have desired results.

Applications which use the UART driver can instruct VFS to use the driver's interrupt driven, blocking read and write functions instead. This can be done using a call to the uart_vfs_dev_use_driver() function. It is also possible to revert to the basic non-blocking functions using a call to uart_vfs_dev_use_nonblocking().

When the interrupt-driven driver is installed, it is also possible to enable/disable non-blocking behavior using fcntl function with O_NONBLOCK flag.

USB CDC (using USB_OTG peripheral)

USB CDC VFS driver provides blocking I/O behavior by default. It is possible to enable non-blocking behavior using fcntl function with O_NONBLOCK flag.

Newline conversion

VFS drivers provide an optional newline conversion feature for input and output. Internally, most applications send and receive lines terminated by the LF (\n) character. Different terminal programs may require different line termination, such as CR or CRLF.

Applications can configure this behavior globally using the following Kconfig options:

It is also possible to configure line ending conversion for the specific VFS driver:

Buffering

By default, standard I/O streams are line buffered. This means that data written to the stream is not sent to the underlying device until a newline character is written, or the buffer is full. This means, for example, that if you call printf("Hello"), the text will not be sent to the UART until you call printf("\n") or the stream buffer fills up due to other prints.

This behavior can be changed using the setvbuf() function. For example, to disable buffering for stdout:

setvbuf(stdout, NULL, _IONBF, 0);

You can also use setvbuf() to increase the buffer size, or switch to fully buffered mode.

Custom channels for standard I/O

To send application output to a custom channel (for example, a WebSocket connection), it is possible to create a custom VFS driver. See the VFS documentation for details. The VFS driver has to implement at least the following functions:

  • open() and close()

  • write()

  • read() — only if the custom channel is also used for input

  • fstat() — recommended, to provide correct buffering behavior for the I/O streams

  • fcntl() — only if non-blocking I/O has to be supported

Once you have created a custom VFS driver, use esp_vfs_register_fs() to register it with VFS. Then, use fopen() to redirect stdout and stderr to the custom channel. For example:

FILE *f = fopen("/dev/mychannel", "w");
if (f == NULL) {
    // handle the error here
}
stdout = f;
stderr = f;

Note that logging functions (ESP_LOGE(), etc.) write their output to stdout. Keep this in mind when using logging within the implementation of your custom VFS (or any components which it calls). For example, if the custom VFS driver's write() operation fails and uses ESP_LOGE() to log the error, this will cause the output to be sent to stdout, which would again call the custom VFS driver's write() operation. This would result in an infinite loop. It is recommended to keep track of this re-entry condition in the VFS driver's write() implementation, and return immediately if the write operation is still in progress.

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