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FileSystem

FileSystem class hierarchy

The file system API provides a common interface for implementing a file system on a block-based storage device. The file system API is a class-based interface, but implementing the file system API provides the standard POSIX file API familiar to C users.

The main purpose of a FileSystem is to be instantiated. The FileSystem's constructor can take in a BlockDevice and mount point specified as a string, as well as other implementation-specific configuration options. The mount point can then act as the first directory in any paths in Mbed OS when used with the POSIX API. This gives you access to files inside the file system. The mount point can be NULL if you only need to use the FileSystem as a C++ object.

The FileSystem's file and dir functions are protected because you should not use the FileSystem file and dir functions directly. They are only a convenience for implementors. Instead, the File and Dir classes provide access to file and directory operations in a C++ class that respects RAII and other C++ conventions.

File system get default instance

The Mbed OS configuration allows you to add block devices as components using the targets.json file or target overrides in the application configuration file.

For details regarding how to configure the default file system or override its implemetation, please refer to the storage configuration guide.

File system class reference

Public Member Functions
 FileSystem (const char *name=NULL)
 FileSystem lifetime. More...
virtual int mount (BlockDevice *bd)=0
 Mounts a filesystem to a block device. More...
virtual int unmount ()=0
 Unmounts a filesystem from the underlying block device. More...
virtual int reformat (BlockDevice *bd=NULL)
 Reformats a filesystem, results in an empty and mounted filesystem. More...
virtual int remove (const char *path)
 Remove a file from the filesystem. More...
virtual int rename (const char *path, const char *newpath)
 Rename a file in the filesystem. More...
virtual int stat (const char *path, struct stat *st)
 Store information about the file in a stat structure. More...
virtual int mkdir (const char *path, mode_t mode)
 Create a directory in the filesystem. More...
virtual int statvfs (const char *path, struct statvfs *buf)
 Store information about the mounted filesystem in a statvfs structure. More...
FileHandleopen (const char *path, int flags)
 Open a file on the filesystem. More...
DirHandleopendir (const char *path)
 Open a directory on the filesystem. More...
Static Public Member Functions
static FileSystemget_default_instance ()
 Return the default filesystem. More...
Protected Member Functions
virtual int file_open (fs_file_t *file, const char *path, int flags)=0
 Open a file on the filesystem. More...
virtual int file_close (fs_file_t file)=0
 Close a file. More...
virtual ssize_t file_read (fs_file_t file, void *buffer, size_t size)=0
 Read the contents of a file into a buffer. More...
virtual ssize_t file_write (fs_file_t file, const void *buffer, size_t size)=0
 Write the contents of a buffer to a file. More...
virtual int file_sync (fs_file_t file)
 Flush any buffers associated with the file. More...
virtual int file_isatty (fs_file_t file)
 Check if the file in an interactive terminal device If so, line buffered behaviour is used by default. More...
virtual off_t file_seek (fs_file_t file, off_t offset, int whence)=0
 Move the file position to a given offset from from a given location. More...
virtual off_t file_tell (fs_file_t file)
 Get the file position of the file. More...
virtual void file_rewind (fs_file_t file)
 Rewind the file position to the beginning of the file. More...
virtual off_t file_size (fs_file_t file)
 Get the size of the file. More...
virtual int dir_open (fs_dir_t *dir, const char *path)
 Open a directory on the filesystem. More...
virtual int dir_close (fs_dir_t dir)
 Close a directory. More...
virtual ssize_t dir_read (fs_dir_t dir, struct dirent *ent)
 Read the next directory entry. More...
virtual void dir_seek (fs_dir_t dir, off_t offset)
 Set the current position of the directory. More...
virtual off_t dir_tell (fs_dir_t dir)
 Get the current position of the directory. More...
virtual void dir_rewind (fs_dir_t dir)
 Rewind the current position to the beginning of the directory. More...
virtual size_t dir_size (fs_dir_t dir)
 Get the sizeof the directory. More...
virtual int open (FileHandle **file, const char *path, int flags)
 Open a file on the filesystem. More...
virtual int open (DirHandle **dir, const char *path)
 Open a directory on the filesystem. More...

File system example

/* mbed Microcontroller Library
 * Copyright (c) 2006-2013 ARM Limited
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#include "mbed.h"
#include <stdio.h>
#include <errno.h>

#include "BlockDevice.h"

// This will take the system's default block device
BlockDevice *bd = BlockDevice::get_default_instance();

// Instead of the default block device, you can define your own block device.
// For example: HeapBlockDevice with size of 2048 bytes, read size 1, write size 1 and erase size 512.
// #include "HeapBlockDevice.h"
// BlockDevice *bd = new HeapBlockDevice(2048, 1, 1, 512);


// This example uses LittleFileSystem as the default file system
#include "LittleFileSystem.h"
LittleFileSystem fs("fs");

// Uncomment the following two lines and comment the previous two to use FAT file system.
// #include "FATFileSystem.h"
// FATFileSystem fs("fs");


// Set up the button to trigger an erase
InterruptIn irq(BUTTON1);
void erase() {
    printf("Initializing the block device... ");
    fflush(stdout);
    int err = bd->init();
    printf("%s\n", (err ? "Fail :(" : "OK"));
    if (err) {
        error("error: %s (%d)\n", strerror(-err), err);
    }

    printf("Erasing the block device... ");
    fflush(stdout);
    err = bd->erase(0, bd->size());
    printf("%s\n", (err ? "Fail :(" : "OK"));
    if (err) {
        error("error: %s (%d)\n", strerror(-err), err);
    }

    printf("Deinitializing the block device... ");
    fflush(stdout);
    err = bd->deinit();
    printf("%s\n", (err ? "Fail :(" : "OK"));
    if (err) {
        error("error: %s (%d)\n", strerror(-err), err);
    }
}


// Entry point for the example
int main() {
    printf("--- Mbed OS filesystem example ---\n");

    // Setup the erase event on button press, use the event queue
    // to avoid running in interrupt context
    irq.fall(mbed_event_queue()->event(erase));

    // Try to mount the filesystem
    printf("Mounting the filesystem... ");
    fflush(stdout);
    int err = fs.mount(bd);
    printf("%s\n", (err ? "Fail :(" : "OK"));
    if (err) {
        // Reformat if we can't mount the filesystem
        // this should only happen on the first boot
        printf("No filesystem found, formatting... ");
        fflush(stdout);
        err = fs.reformat(bd);
        printf("%s\n", (err ? "Fail :(" : "OK"));
        if (err) {
            error("error: %s (%d)\n", strerror(-err), err);
        }
    }

    // Open the numbers file
    printf("Opening \"/fs/numbers.txt\"... ");
    fflush(stdout);
    FILE *f = fopen("/fs/numbers.txt", "r+");
    printf("%s\n", (!f ? "Fail :(" : "OK"));
    if (!f) {
        // Create the numbers file if it doesn't exist
        printf("No file found, creating a new file... ");
        fflush(stdout);
        f = fopen("/fs/numbers.txt", "w+");
        printf("%s\n", (!f ? "Fail :(" : "OK"));
        if (!f) {
            error("error: %s (%d)\n", strerror(errno), -errno);
        }

        for (int i = 0; i < 10; i++) {
            printf("\rWriting numbers (%d/%d)... ", i, 10);
            fflush(stdout);
            err = fprintf(f, "    %d\n", i);
            if (err < 0) {
                printf("Fail :(\n");
                error("error: %s (%d)\n", strerror(errno), -errno);
            }
        }
        printf("\rWriting numbers (%d/%d)... OK\n", 10, 10);

        printf("Seeking file... ");
        fflush(stdout);
        err = fseek(f, 0, SEEK_SET);
        printf("%s\n", (err < 0 ? "Fail :(" : "OK"));
        if (err < 0) {
            error("error: %s (%d)\n", strerror(errno), -errno);
        }
    }

    // Go through and increment the numbers
    for (int i = 0; i < 10; i++) {
        printf("\rIncrementing numbers (%d/%d)... ", i, 10);
        fflush(stdout);

        // Get current stream position
        long pos = ftell(f);

        // Parse out the number and increment
        int32_t number;
        fscanf(f, "%d", &number);
        number += 1;

        // Seek to beginning of number
        fseek(f, pos, SEEK_SET);
    
        // Store number
        fprintf(f, "    %d\n", number);

        // Flush between write and read on same file
        fflush(f);
    }
    printf("\rIncrementing numbers (%d/%d)... OK\n", 10, 10);

    // Close the file which also flushes any cached writes
    printf("Closing \"/fs/numbers.txt\"... ");
    fflush(stdout);
    err = fclose(f);
    printf("%s\n", (err < 0 ? "Fail :(" : "OK"));
    if (err < 0) {
        error("error: %s (%d)\n", strerror(errno), -errno);
    }
    
    // Display the root directory
    printf("Opening the root directory... ");
    fflush(stdout);
    DIR *d = opendir("/fs/");
    printf("%s\n", (!d ? "Fail :(" : "OK"));
    if (!d) {
        error("error: %s (%d)\n", strerror(errno), -errno);
    }

    printf("root directory:\n");
    while (true) {
        struct dirent *e = readdir(d);
        if (!e) {
            break;
        }

        printf("    %s\n", e->d_name);
    }

    printf("Closing the root directory... ");
    fflush(stdout);
    err = closedir(d);
    printf("%s\n", (err < 0 ? "Fail :(" : "OK"));
    if (err < 0) {
        error("error: %s (%d)\n", strerror(errno), -errno);
    }

    // Display the numbers file
    printf("Opening \"/fs/numbers.txt\"... ");
    fflush(stdout);
    f = fopen("/fs/numbers.txt", "r");
    printf("%s\n", (!f ? "Fail :(" : "OK"));
    if (!f) {
        error("error: %s (%d)\n", strerror(errno), -errno);
    }

    printf("numbers:\n");
    while (!feof(f)) {
        int c = fgetc(f);
        printf("%c", c);
    }

    printf("\rClosing \"/fs/numbers.txt\"... ");
    fflush(stdout);
    err = fclose(f);
    printf("%s\n", (err < 0 ? "Fail :(" : "OK"));
    if (err < 0) {
        error("error: %s (%d)\n", strerror(errno), -errno);
    }

    // Tidy up
    printf("Unmounting... ");
    fflush(stdout);
    err = fs.unmount();
    printf("%s\n", (err < 0 ? "Fail :(" : "OK"));
    if (err < 0) {
        error("error: %s (%d)\n", strerror(-err), err);
    }
        
    printf("Mbed OS filesystem example done!\n");
}


Sector requirements

LittleFS requirements:

  • 4 blocks root dir.
  • 2 blocks per dir.
  • 1 block per file.

FATFS requirements:

  • Hard minimum: 256 blocks.
  • 128 blocks for FAT table.
  • 1 block per dir.
  • 1 block per file.
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