LocalFileSystem

A filesystem for accessing the local mbed Microcontroller USB disk drive.

This allows programs to read and write files on the same disk drive that is used to program the mbed Microcontroller. Once created, the standard C file access functions are used to open, read and write files.

Hello World!

Code

#include "mbed.h"

LocalFileSystem local("local");               // Create the local filesystem under the name "local"

int main() {
    FILE *fp = fopen("/local/out.txt", "w");  // Open "out.txt" on the local file system for writing
    fprintf(fp, "Hello World!");
    fclose(fp);
}

Notes

If the microcontroller program opens a file on the local drive, it will be marked as “removed” on the Host computer. This means the PC will often display a message such as "insert a disk into drive" if you try to access it at this time; this is normal, and stops both the mbed and the PC trying to access the disk at the same time.

Warning

The USB drive will only re-appear when all file handles are closed in your program, or the microcontroller program exits.

If a running program on the mbed does not exit or does not release it's open file handles, you will no longer be able to see the USB drive when you plug the mbed into your PC. To allow you to see the drive again (and load a new program), use the following procedure:

Unplug the microcontroller
Hold the reset button down
While still holding the button, plug in the microcontroller. The mbed USB drive should appear.
Keep holding the button until the new program is saved onto the USB drive.

Here is an example program that shows the behaviour clearly:

Code

#include "mbed.h"

LocalFileSystem local("local");

int main() {
	printf("Hello World!\n");
	
	wait(5.0);
	
	printf("Opening File...\n"); // Drive should be marked as removed
	FILE *fp = fopen("/local/test.txt", "w");
	if(!fp) {
		fprintf(stderr, "File /local/test.txt could not be opened!\n");
		exit(1);
	}
	
	wait(5.0);
	
	printf("Writing Data...\n");	
	fprintf(fp, "Hello World!");
	
	wait(5.0);

	printf("Closing File...\n");
	fclose(fp);

        // Drive should be restored. this is the same as just returning from main

        wait(5);
}

Implementation Details

The LocalFileSystem actually accesses the mbed USB disk by making "semihosting" calls to the mbed Interface, which does the actual accesses on behalf of your program. This means the "filesystem" in this case is running on the interface. The underlying mechanism is the interface acting like a debugger, and the "semihost" calls are effectively breakpoints that the interface spots and does the requested operation.

Warning

The LocalFilesystem has a few restrictions:

Only 8.3 filenames are supported
Sub-directories are not supported
fseek is not supported for files opened for writing ("w")
File access calls (fread, fwrite) will block, including interrupts, as semihosting is effectively a debug breakpoint

Other filesystems (such as ones to talk to an SD card, or a USB FLASH drive) run on the target itself, so don't talk to the mbed Interface at all and these restrictions don't necessarily apply.

LocalFileSystem

Hello World!

Code

Notes

Warning

Code

Implementation Details

Warning

12 comments on LocalFileSystem:

Patrick Grady wrote: