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File

File class hierarchy

The File class provides access to the underlying storage of a file system in a generic manner. This acts as a buffer between the file system and any technical limitations of the physical storage medium. On the storage, each file is a blob of data referenced by a file name represented as a string. By providing a file name to the open function in the File class, you can access the blob of data stored on disk.

The File API allows operations on stored data without ever needing to buffer the data in RAM. Because flash is several orders of magnitude cheaper than RAM, this can significantly increase the amount of data a device can store without significantly increasing the device's cost.

Note: To get and set a buffer to a file, jump to the read and write functions. In the File class, the read and write functions always read or write the entire file at a time.

A File operates like an infinitely long reel of tape. Each File object keeps track of its current position in the data, and this position updates as you read from or write to the file. As data is written to the File, the File grows unless the underlying storage runs out of memory. If supported by the underlying file system, you can explicitly move the File position with the seek and rewind functions.

Combining read, write, and seek on a File allows both streaming and random access to data stored on external memory without necessarily buffering the file contents on the device's RAM.

File class reference

Public Member Functions
 File ()
 Create an uninitialized file. More...
 File (FileSystem *fs, const char *path, int flags=O_RDONLY)
 Create a file on a filesystem. More...
virtual ~File ()
 Destroy a file. More...
virtual int open (FileSystem *fs, const char *path, int flags=O_RDONLY)
 Open a file on the filesystem. More...
virtual int close ()
 Close a file. More...
virtual ssize_t read (void *buffer, size_t size)
 Read the contents of a file into a buffer. More...
virtual ssize_t write (const void *buffer, size_t size)
 Write the contents of a buffer to a file. More...
virtual int sync ()
 Flush any buffers associated with the file. More...
virtual int isatty ()
 Check if the file in an interactive terminal device. More...
virtual off_t seek (off_t offset, int whence=SEEK_SET)
 Move the file position to a given offset from from a given location. More...
virtual off_t tell ()
 Get the file position of the file. More...
virtual void rewind ()
 Rewind the file position to the beginning of the file. More...
virtual off_t size ()
 Get the size of the file. More...
virtual off_t lseek (off_t offset, int whence)
 Move the file position to a given offset from a given location. More...
virtual int fsync ()
 Flush any buffers associated with the FileHandle, ensuring it is up to date on disk. More...
virtual off_t flen ()
 Find the length of the file. More...
virtual int set_blocking (bool blocking)
 Set blocking or nonblocking mode of the file operation like read/write. More...
virtual bool is_blocking () const
 Check current blocking or nonblocking mode for file operations. More...
virtual short poll (short events) const
 Check for poll event flags You can use or ignore the input parameter. More...
bool writable () const
 Definition depends on the subclass implementing FileHandle. More...
bool readable () const
 Definition depends on the subclass implementing FileHandle. More...
virtual void sigio (Callback< void()> func)
 Register a callback on state change of the file. 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>

// Block devices
#include "SPIFBlockDevice.h"
#include "DataFlashBlockDevice.h"
#include "SDBlockDevice.h"
#include "HeapBlockDevice.h"

// File systems
#include "LittleFileSystem.h"
#include "FATFileSystem.h"


// Physical block device, can be any device that supports the BlockDevice API
SPIFBlockDevice bd(
        MBED_CONF_SPIF_DRIVER_SPI_MOSI,
        MBED_CONF_SPIF_DRIVER_SPI_MISO,
        MBED_CONF_SPIF_DRIVER_SPI_CLK,
        MBED_CONF_SPIF_DRIVER_SPI_CS);

// File system declaration
LittleFileSystem 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");
}


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