Virtual filesystem component¶

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 esp_vfs_t structure and populate it with function pointers to FS APIs:

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 select() is supported in the VFS component. The implementation works in the following way.

select() is called with file descriptors which could belong to various VFS drivers.
The file descriptors are divided into groups each belonging to one VFS driver.
The file descriptors belonging to non-socket VFS drivers are handed over to the given VFS drivers by start_select() described later on this page. This function represents the driver-specific implementation of select() 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.
The file descriptors belonging to the socket VFS driver are handed over to the socket driver by socket_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 socket_select() to exit.
Results are collected from each VFS driver and all drivers are stopped by deinitiazation of the environment for checking events.
The select() call ends and returns the appropriate results.

Non-socket VFS drivers¶

If you want to use select() with a file descriptor belonging to a non-socket VFS driver then you need to register the driver with functions start_select() and end_select() similarly to the following example:

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

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

end_select() is called to stop/deinitialize/free the environment which was setup by start_select().

注解

end_select() might be called without a previous start_select() call in some rare circumstances. end_select() should fail gracefully if this is the case.

Please refer to the reference implementation for the UART peripheral in vfs/vfs_uart.c and most particularly to the functions esp_vfs_dev_uart_register(), uart_start_select(), and uart_end_select() for more information.

Please check the following examples that demonstrate the use of select() with VFS file descriptors:

Socket VFS drivers¶

A socket VFS driver is using its own internal implementation of select() 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:

// 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

socket_select() is the internal implementation of select() for the socket driver. It works only with file descriptors belonging to the socket VFS.

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

stop_socket_select() call is used to stop the waiting in socket_select() by passing the object returned by get_socket_select_semaphore().

stop_socket_select_isr() has the same functionality as stop_socket_select() but it can be used from ISR.

Please see lwip/port/esp32/vfs_lwip.c for a reference socket driver implementation using LWIP.

注解

If you use select() for socket file descriptors only then you can enable the CONFIG_LWIP_USE_ONLY_LWIP_SELECT option to reduce the code size and improve performance.

注解

Don’t change the socket driver during an active select() 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:

The myfs driver is registered with /data as a path prefix.
The application calls fopen("/data/config.json", ...).
The VFS component calls myfs_open("/config.json", ...).
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:

fprintf(stderr, "42\n");

The __getreent() function returns a per-task pointer to struct _reent (newlib/include/sys/reent.h#L370-L417). 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.

Application Example¶

Instructions

API Reference¶

Header File¶

vfs/include/esp_vfs.h

Functions¶

ssize_t esp_vfs_write(struct _reent *r, int fd, const void *data, size_t size)¶: These functions are to be used in newlib syscall table. They will be called by newlib when it needs to use any of the syscalls.

off_t esp_vfs_lseek(struct _reent *r, int fd, off_t size, int mode)¶

ssize_t esp_vfs_read(struct _reent *r, int fd, void *dst, size_t size)¶

int esp_vfs_open(struct _reent *r, const char *path, int flags, int mode)¶

int esp_vfs_close(struct _reent *r, int fd)¶

int esp_vfs_fstat(struct _reent *r, int fd, struct stat *st)¶

int esp_vfs_stat(struct _reent *r, const char *path, struct stat *st)¶

int esp_vfs_link(struct _reent *r, const char *n1, const char *n2)¶

int esp_vfs_unlink(struct _reent *r, const char *path)¶

int esp_vfs_rename(struct _reent *r, const char *src, const char *dst)¶

int esp_vfs_utime(const char *path, const struct utimbuf *times)¶

esp_err_t esp_vfs_register(const char *base_path, const esp_vfs_t *vfs, void *ctx)¶

Register a virtual filesystem for given path prefix.

Return

ESP_OK if successful, ESP_ERR_NO_MEM if too many VFSes are registered.

Parameters

base_path: file path prefix associated with the filesystem. Must be a zero-terminated C string, up to ESP_VFS_PATH_MAX characters long, and at least 2 characters long. Name must start with a “/” and must not end with “/”. For example, “/data” or “/dev/spi” are valid. These VFSes would then be called to handle file paths such as “/data/myfile.txt” or “/dev/spi/0”.
vfs: Pointer to esp_vfs_t, a structure which maps syscalls to the filesystem driver functions. VFS component doesn’t assume ownership of this pointer.
ctx: If vfs->flags has ESP_VFS_FLAG_CONTEXT_PTR set, a pointer which should be passed to VFS functions. Otherwise, NULL.

esp_err_t esp_vfs_register_fd_range(const esp_vfs_t *vfs, void *ctx, int min_fd, int max_fd)¶

Special case function for registering a VFS that uses a method other than open() to open new file descriptors from the interval <min_fd; max_fd).

This is a special-purpose function intended for registering LWIP sockets to VFS.

Return

ESP_OK if successful, ESP_ERR_NO_MEM if too many VFSes are registered, ESP_ERR_INVALID_ARG if the file descriptor boundaries are incorrect.

Parameters

vfs: Pointer to esp_vfs_t. Meaning is the same as for esp_vfs_register().
ctx: Pointer to context structure. Meaning is the same as for esp_vfs_register().
min_fd: The smallest file descriptor this VFS will use.
max_fd: Upper boundary for file descriptors this VFS will use (the biggest file descriptor plus one).

esp_err_t esp_vfs_register_with_id(const esp_vfs_t *vfs, void *ctx, esp_vfs_id_t *vfs_id)¶

Special case function for registering a VFS that uses a method other than open() to open new file descriptors. In comparison with esp_vfs_register_fd_range, this function doesn’t pre-registers an interval of file descriptors. File descriptors can be registered later, by using esp_vfs_register_fd.

Return

ESP_OK if successful, ESP_ERR_NO_MEM if too many VFSes are registered, ESP_ERR_INVALID_ARG if the file descriptor boundaries are incorrect.

Parameters

vfs: Pointer to esp_vfs_t. Meaning is the same as for esp_vfs_register().
ctx: Pointer to context structure. Meaning is the same as for esp_vfs_register().
vfs_id: Here will be written the VFS ID which can be passed to esp_vfs_register_fd for registering file descriptors.

esp_err_t esp_vfs_unregister(const char *base_path)¶

Unregister a virtual filesystem for given path prefix

Return

ESP_OK if successful, ESP_ERR_INVALID_STATE if VFS for given prefix hasn’t been registered

Parameters

base_path: file prefix previously used in esp_vfs_register call

esp_err_t esp_vfs_register_fd(esp_vfs_id_t vfs_id, int *fd)¶

Special function for registering another file descriptor for a VFS registered by esp_vfs_register_with_id.

Return

ESP_OK if the registration is successful, ESP_ERR_NO_MEM if too many file descriptors are registered, ESP_ERR_INVALID_ARG if the arguments are incorrect.

Parameters

vfs_id: VFS identificator returned by esp_vfs_register_with_id.
fd: The registered file descriptor will be written to this address.

esp_err_t esp_vfs_unregister_fd(esp_vfs_id_t vfs_id, int fd)¶

Special function for unregistering a file descriptor belonging to a VFS registered by esp_vfs_register_with_id.

Return

ESP_OK if the registration is successful, ESP_ERR_INVALID_ARG if the arguments are incorrect.

Parameters

vfs_id: VFS identificator returned by esp_vfs_register_with_id.
fd: File descriptor which should be unregistered.

int esp_vfs_select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *timeout)¶

Synchronous I/O multiplexing which implements the functionality of POSIX select() for VFS.

Return

The number of descriptors set in the descriptor sets, or -1 when an error (specified by errno) have occurred.

Parameters

nfds: Specifies the range of descriptors which should be checked. The first nfds descriptors will be checked in each set.
readfds: If not NULL, then points to a descriptor set that on input specifies which descriptors should be checked for being ready to read, and on output indicates which descriptors are ready to read.
writefds: If not NULL, then points to a descriptor set that on input specifies which descriptors should be checked for being ready to write, and on output indicates which descriptors are ready to write.
errorfds: If not NULL, then points to a descriptor set that on input specifies which descriptors should be checked for error conditions, and on output indicates which descriptors have error conditions.
timeout: If not NULL, then points to timeval structure which specifies the time period after which the functions should time-out and return. If it is NULL, then the function will not time-out.

void esp_vfs_select_triggered(esp_vfs_select_sem_t sem)¶

Notification from a VFS driver about a read/write/error condition.

This function is called when the VFS driver detects a read/write/error condition as it was requested by the previous call to start_select.

Parameters

sem: semaphore structure which was passed to the driver by the start_select call

void esp_vfs_select_triggered_isr(esp_vfs_select_sem_t sem, BaseType_t *woken)¶

Notification from a VFS driver about a read/write/error condition (ISR version)

This function is called when the VFS driver detects a read/write/error condition as it was requested by the previous call to start_select.

Parameters

sem: semaphore structure which was passed to the driver by the start_select call
woken: is set to pdTRUE if the function wakes up a task with higher priority

int esp_vfs_poll(struct pollfd *fds, nfds_t nfds, int timeout)¶

Implements the VFS layer for synchronous I/O multiplexing by poll()

The implementation is based on esp_vfs_select. The parameters and return values are compatible with POSIX poll().

Return

A positive return value indicates the number of file descriptors that have been selected. The 0 return value indicates a timed-out poll. -1 is return on failure and errno is set accordingly.

Parameters

fds: Pointer to the array containing file descriptors and events poll() should consider.
nfds: Number of items in the array fds.
timeout: Poll() should wait at least timeout milliseconds. If the value is 0 then it should return immediately. If the value is -1 then it should wait (block) until the event occurs.

ssize_t esp_vfs_pread(int fd, void *dst, size_t size, off_t offset)¶

Implements the VFS layer of POSIX pread()

Return

A positive return value indicates the number of bytes read. -1 is return on failure and errno is set accordingly.

Parameters

fd: File descriptor used for read
dst: Pointer to the buffer where the output will be written
size: Number of bytes to be read
offset: Starting offset of the read

ssize_t esp_vfs_pwrite(int fd, const void *src, size_t size, off_t offset)¶

Implements the VFS layer of POSIX pwrite()

Return

A positive return value indicates the number of bytes written. -1 is return on failure and errno is set accordingly.

Parameters

fd: File descriptor used for write
src: Pointer to the buffer from where the output will be read
size: Number of bytes to write
offset: Starting offset of the write

Structures¶

struct esp_vfs_select_sem_t¶

VFS semaphore type for select()

Public Members

bool is_sem_local¶: type of “sem” is SemaphoreHandle_t when true, defined by socket driver otherwise

void *sem¶: semaphore instance

struct esp_vfs_t¶

VFS definition structure.

This structure should be filled with pointers to corresponding FS driver functions.

VFS component will translate all FDs so that the filesystem implementation sees them starting at zero. The caller sees a global FD which is prefixed with an pre-filesystem-implementation.

Some FS implementations expect some state (e.g. pointer to some structure) to be passed in as a first argument. For these implementations, populate the members of this structure which have _p suffix, set flags member to ESP_VFS_FLAG_CONTEXT_PTR and provide the context pointer to esp_vfs_register function. If the implementation doesn’t use this extra argument, populate the members without _p suffix and set flags member to ESP_VFS_FLAG_DEFAULT.

If the FS driver doesn’t provide some of the functions, set corresponding members to NULL.

Public Members

int flags¶: ESP_VFS_FLAG_CONTEXT_PTR or ESP_VFS_FLAG_DEFAULT

esp_err_t (*start_select)(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, esp_vfs_select_sem_t sem, void **end_select_args)¶: start_select is called for setting up synchronous I/O multiplexing of the desired file descriptors in the given VFS

int (*socket_select)(int nfds, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *timeout)¶: socket select function for socket FDs with the functionality of POSIX select(); this should be set only for the socket VFS

void (*stop_socket_select)(void *sem)¶: called by VFS to interrupt the socket_select call when select is activated from a non-socket VFS driver; set only for the socket driver

void (*stop_socket_select_isr)(void *sem, BaseType_t *woken)¶: stop_socket_select which can be called from ISR; set only for the socket driver

void *(*get_socket_select_semaphore)()¶: end_select is called to stop the I/O multiplexing and deinitialize the environment created by start_select for the given VFS

esp_err_t (*end_select)(void *end_select_args)¶: get_socket_select_semaphore returns semaphore allocated in the socket driver; set only for the socket driver

Macros¶

MAX_FDS¶: Maximum number of (global) file descriptors.

ESP_VFS_PATH_MAX¶: Maximum length of path prefix (not including zero terminator)

ESP_VFS_FLAG_DEFAULT¶: Default value of flags member in esp_vfs_t structure.

ESP_VFS_FLAG_CONTEXT_PTR¶: Flag which indicates that FS needs extra context pointer in syscalls.

Type Definitions¶

typedef int esp_vfs_id_t¶

Header File¶

vfs/include/esp_vfs_dev.h

Functions¶

void esp_vfs_dev_uart_register()¶

add /dev/uart virtual filesystem driver

This function is called from startup code to enable serial output

void esp_vfs_dev_uart_set_rx_line_endings(esp_line_endings_t mode)¶

Set the line endings expected to be received on UART.

This specifies the conversion between line endings received on UART and newlines (‘

’, LF) passed into stdin:

ESP_LINE_ENDINGS_CRLF: convert CRLF to LF
ESP_LINE_ENDINGS_CR: convert CR to LF
ESP_LINE_ENDINGS_LF: no modification

Note

this function is not thread safe w.r.t. reading from UART

Parameters

mode: line endings expected on UART

void esp_vfs_dev_uart_set_tx_line_endings(esp_line_endings_t mode)¶

Set the line endings to sent to UART.

This specifies the conversion between newlines (‘

’, LF) on stdout and line endings sent over UART:

ESP_LINE_ENDINGS_CRLF: convert LF to CRLF
ESP_LINE_ENDINGS_CR: convert LF to CR
ESP_LINE_ENDINGS_LF: no modification

Note

this function is not thread safe w.r.t. writing to UART

Parameters

mode: line endings to send to UART

int esp_vfs_dev_uart_port_set_rx_line_endings(int uart_num, esp_line_endings_t mode)¶

Set the line endings expected to be received on specified UART.

This specifies the conversion between line endings received on UART and newlines (‘

’, LF) passed into stdin:

ESP_LINE_ENDINGS_CRLF: convert CRLF to LF
ESP_LINE_ENDINGS_CR: convert CR to LF
ESP_LINE_ENDINGS_LF: no modification

Note

this function is not thread safe w.r.t. reading from UART

Return

0 if successed, or -1 when an error (specified by errno) have occurred.

Parameters

uart_num: the UART number
mode: line endings to send to UART

int esp_vfs_dev_uart_port_set_tx_line_endings(int uart_num, esp_line_endings_t mode)¶

Set the line endings to sent to specified UART.

This specifies the conversion between newlines (‘

’, LF) on stdout and line endings sent over UART:

ESP_LINE_ENDINGS_CRLF: convert LF to CRLF
ESP_LINE_ENDINGS_CR: convert LF to CR
ESP_LINE_ENDINGS_LF: no modification

Note

this function is not thread safe w.r.t. writing to UART

Return

0 if successed, or -1 when an error (specified by errno) have occurred.

Parameters

uart_num: the UART number
mode: line endings to send to UART

void esp_vfs_dev_uart_use_nonblocking(int uart_num)¶

set VFS to use simple functions for reading and writing UART Read is non-blocking, write is busy waiting until TX FIFO has enough space. These functions are used by default.

Parameters

uart_num: UART peripheral number

void esp_vfs_dev_uart_use_driver(int uart_num)¶

set VFS to use UART driver for reading and writing

Note

application must configure UART driver before calling these functions With these functions, read and write are blocking and interrupt-driven.

Parameters

uart_num: UART peripheral number

Enumerations¶

enum esp_line_endings_t¶

Line ending settings.

Values:

ESP_LINE_ENDINGS_CRLF¶: CR + LF.

ESP_LINE_ENDINGS_CR¶: CR.

ESP_LINE_ENDINGS_LF¶: LF.

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