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Virtual filesystem component

:link_to_translation:zh_CN:[中文]

Overview

Virtual filesystem (VFS) component provides a unified interface for drivers which can perform operations on file-like objects. These can be real filesystems (FAT, SPIFFS, etc.) or device drivers which provide a file-like interface.

This component allows C library functions, such as fopen and fprintf, to work with FS drivers. At a high level, each FS driver is associated with some path prefix. When one of C library functions needs to open a file, the VFS component searches for the FS driver associated with the file path and forwards the call to that driver. VFS also forwards read, write, and other calls for the given file to the same FS driver.

For example, one can register a FAT filesystem driver with the /fat prefix and call fopen("/fat/file.txt", "w"). The VFS component will then call the function open of the FAT driver and pass the argument /file.txt to it together with appropriate mode flags. All subsequent calls to C library functions for the returned FILE* stream will also be forwarded to the FAT driver.

FS registration

To register an FS driver, an application needs to define an instance of the :cppesp_vfs_t structure and populate it with function pointers to FS APIs:

c

esp_vfs_t myfs = {
    .flags = ESP_VFS_FLAG_DEFAULT,
    .write = &myfs_write,
    .open = &myfs_open,
    .fstat = &myfs_fstat,
    .close = &myfs_close,
    .read = &myfs_read,
};

ESP_ERROR_CHECK(esp_vfs_register("/data", &myfs, NULL));

Depending on the way how the FS driver declares its API functions, either read, write, etc., or read_p, write_p, etc., should be used.

Case 1: API functions are declared without an extra context pointer (the FS driver is a singleton):

ssize_t myfs_write(int fd, const void * data, size_t size);

// In definition of esp_vfs_t:
    .flags = ESP_VFS_FLAG_DEFAULT,
    .write = &myfs_write,
// ... other members initialized

// When registering FS, context pointer (third argument) is NULL:
ESP_ERROR_CHECK(esp_vfs_register("/data", &myfs, NULL));

Case 2: API functions are declared with an extra context pointer (the FS driver supports multiple instances):

ssize_t myfs_write(myfs_t* fs, int fd, const void * data, size_t size);

// In definition of esp_vfs_t:
    .flags = ESP_VFS_FLAG_CONTEXT_PTR,
    .write_p = &myfs_write,
// ... other members initialized

// When registering FS, pass the FS context pointer into the third argument
// (hypothetical myfs_mount function is used for illustrative purposes)
myfs_t* myfs_inst1 = myfs_mount(partition1->offset, partition1->size);
ESP_ERROR_CHECK(esp_vfs_register("/data1", &myfs, myfs_inst1));

// Can register another instance:
myfs_t* myfs_inst2 = myfs_mount(partition2->offset, partition2->size);
ESP_ERROR_CHECK(esp_vfs_register("/data2", &myfs, myfs_inst2));

Synchronous input/output multiplexing

Synchronous input/output multiplexing by :cppselect is supported in the VFS component. The implementation works in the following way.

  1. :cppselect is called with file descriptors which could belong to various VFS drivers.
  2. The file descriptors are divided into groups each belonging to one VFS driver.
  3. The file descriptors belonging to non-socket VFS drivers are handed over to the given VFS drivers by :cppstart_select, described later on this page. This function represents the driver-specific implementation of :cppselect for the given driver. This should be a non-blocking call which means the function should immediately return after setting up the environment for checking events related to the given file descriptors.
  4. The file descriptors belonging to the socket VFS driver are handed over to the socket driver by :cppsocket_select described later on this page. This is a blocking call which means that it will return only if there is an event related to socket file descriptors or a non-socket driver signals :cppsocket_select to exit.
  5. Results are collected from each VFS driver and all drivers are stopped by de-initialization of the environment for checking events.
  6. The :cppselect call ends and returns the appropriate results.

Non-socket VFS drivers

If you want to use :cppselect with a file descriptor belonging to a non-socket VFS driver, then you need to register the driver with functions :cppstart_select and :cppend_select similarly to the following example:

c

// In definition of esp_vfs_t:
    .start_select = &uart_start_select,
    .end_select = &uart_end_select,
// ... other members initialized

:cppstart_select is called for setting up the environment for detection of read/write/error conditions on file descriptors belonging to the given VFS driver.

:cppend_select is called to stop/deinitialize/free the environment which was setup by :cppstart_select.

Note

:cppend_select might be called without a previous :cppstart_select call in some rare circumstances. :cppend_select should fail gracefully if this is the case (i.e., should not crash but return an error instead).

Please refer to the reference implementation for the UART peripheral in :component_file:`vfs/vfs_uart.c and most particularly to the functions :cpp:func:`esp_vfs_dev_uart_register, :cppuart_start_select, and :cppuart_end_select for more information.

Please check the following examples that demonstrate the use of :cppselect with VFS file descriptors:

  • peripherals/uart/uart_select
  • system/select

Socket VFS drivers

A socket VFS driver is using its own internal implementation of :cppselect and non-socket VFS drivers notify it upon read/write/error conditions.

A socket VFS driver needs to be registered with the following functions defined:

c

// In definition of esp_vfs_t:
    .socket_select = &lwip_select,
    .get_socket_select_semaphore = &lwip_get_socket_select_semaphore,
    .stop_socket_select = &lwip_stop_socket_select,
    .stop_socket_select_isr = &lwip_stop_socket_select_isr,
// ... other members initialized

:cppsocket_select is the internal implementation of :cppselect for the socket driver. It works only with file descriptors belonging to the socket VFS.

:cppget_socket_select_semaphore returns the signalization object (semaphore) which will be used in non-socket drivers to stop the waiting in :cppsocket_select.

:cppstop_socket_select call is used to stop the waiting in :cppsocket_select by passing the object returned by :cppget_socket_select_semaphore.

:cppstop_socket_select_isr has the same functionality as :cppstop_socket_select but it can be used from ISR.

Please see :component_file:`lwip/port/esp32xx/vfs_lwip.c` for a reference socket driver implementation using LWIP.

Note

If you use :cppselect for socket file descriptors only then you can disable the CONFIG_VFS_SUPPORT_SELECT option to reduce the code size and improve performance. You should not change the socket driver during an active :cppselect call or you might experience some undefined behavior.

Paths

Each registered FS has a path prefix associated with it. This prefix can be considered as a "mount point" of this partition.

In case when mount points are nested, the mount point with the longest matching path prefix is used when opening the file. For instance, suppose that the following filesystems are registered in VFS:

  • FS 1 on /data
  • FS 2 on /data/static

Then:

  • FS 1 will be used when opening a file called /data/log.txt
  • FS 2 will be used when opening a file called /data/static/index.html
  • Even if /index.html" does not exist in FS 2, FS 1 will not be searched for /static/index.html.

As a general rule, mount point names must start with the path separator (/) and must contain at least one character after path separator. However, an empty mount point name is also supported and might be used in cases when an application needs to provide a "fallback" filesystem or to override VFS functionality altogether. Such filesystem will be used if no prefix matches the path given.

VFS does not handle dots (.) in path names in any special way. VFS does not treat .. as a reference to the parent directory. In the above example, using a path /data/static/../log.txt will not result in a call to FS 1 to open /log.txt. Specific FS drivers (such as FATFS) might handle dots in file names differently.

When opening files, the FS driver receives only relative paths to files. For example:

  1. The myfs driver is registered with /data as a path prefix.
  2. The application calls fopen("/data/config.json", ...).
  3. The VFS component calls myfs_open("/config.json", ...).
  4. The myfs driver opens the /config.json file.

VFS does not impose any limit on total file path length, but it does limit the FS path prefix to ESP_VFS_PATH_MAX characters. Individual FS drivers may have their own filename length limitations.

File descriptors

File descriptors are small positive integers from 0 to FD_SETSIZE - 1, where FD_SETSIZE is defined in newlib's sys/types.h. The largest file descriptors (configured by CONFIG_LWIP_MAX_SOCKETS) are reserved for sockets. The VFS component contains a lookup-table called s_fd_table for mapping global file descriptors to VFS driver indexes registered in the s_vfs array.

Standard IO streams (stdin, stdout, stderr)

If the menuconfig option UART for console output is not set to None, then stdin, stdout, and stderr are configured to read from, and write to, a UART. It is possible to use UART0 or UART1 for standard IO. By default, UART0 is used with 115200 baud rate; TX pin is GPIO1; RX pin is GPIO3. These parameters can be changed in menuconfig.

Writing to stdout or stderr will send characters to the UART transmit FIFO. Reading from stdin will retrieve characters from the UART receive FIFO.

By default, VFS uses simple 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 esp_vfs_dev_uart_use_driver function. It is also possible to revert to the basic non-blocking functions using a call to esp_vfs_dev_uart_use_nonblocking.

VFS also provides 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 separately for input and output either via menuconfig, or by calls to the functions esp_vfs_dev_uart_port_set_rx_line_endings and esp_vfs_dev_uart_port_set_tx_line_endings.

Standard streams and FreeRTOS tasks

FILE objects for stdin, stdout, and stderr are shared between all FreeRTOS tasks, but the pointers to these objects are stored in per-task struct _reent.

The following code is transferred to fprintf(__getreent()->_stderr, "42\n"); by the preprocessor:

c

fprintf(stderr, "42\n");

The __getreent() function returns a per-task pointer to struct _reent in newlib libc. This structure is allocated on the TCB of each task. When a task is initialized, _stdin, _stdout, and _stderr members of struct _reent are set to the values of _stdin, _stdout, and _stderr of _GLOBAL_REENT (i.e., the structure which is used before FreeRTOS is started).

Such a design has the following consequences:

  • It is possible to set stdin, stdout, and stderr for any given task without affecting other tasks, e.g., by doing stdin = fopen("/dev/uart/1", "r").
  • Closing default stdin, stdout, or stderr using fclose will close the FILE stream object, which will affect all other tasks.
  • To change the default stdin, stdout, stderr streams for new tasks, modify _GLOBAL_REENT->_stdin (_stdout, _stderr) before creating the task.

Event fds

eventfd() call is a powerful tool to notify a select() based loop of custom events. The eventfd() implementation in ESP-IDF is generally the same as described in man(2) eventfd except for:

  • esp_vfs_eventfd_register() has to be called before calling eventfd()
  • Options EFD_CLOEXEC, EFD_NONBLOCK and EFD_SEMAPHORE are not supported in flags.
  • Option EFD_SUPPORT_ISR has been added in flags. This flag is required to read and write the eventfd in an interrupt handler.

Note that creating an eventfd with EFD_SUPPORT_ISR will cause interrupts to be temporarily disabled when reading, writing the file and during the beginning and the ending of the select() when this file is set.

API Reference

inc/esp_vfs.inc

inc/esp_vfs_dev.inc

inc/esp_vfs_eventfd.inc