What Is Needed To Use fopen() On An Embedded System?

Question

I am quite new to the FILE family of functions that the standard C library provides.

I recently stumbled across fopen() and the similar functions after researching how stdout, stdin and stderr work alongside functions like printf().

I was wondering, what is needed to use fopen() on an embedded system (which doesn't necessarily have operating system support). After reading more about it, is seems like a cool thing to do on more powerful embedded systems to hook into say, a UART/SPI interface, so that calling printf() would print data out of the UART. Simarly, you could read data from a UART buffer by calling scanf().

This would also increase portability! (code written for say, Linux, would be easier to port if printf() was supported). You could also print debug data to a file if it was running in a production environment, and read from it later.

Can you just use fopen() on a bare-bones embedded system? If so who/where/when is the "FILE" then created (as far as I now, fopen() does not malloc() space for the file, nor do you specify how much)? Or do you need a operating system with FAT file support. If so, would something like http://ultra-embedded.com/?fat_filelib work? Would using FreeRTOS help at all?

Answer 1

Check the documentation for your toolchain's C library - it should have something to say about re-targeting the library.

For example if you are using Newlib you must re-implement some or all of the [syscalls stubs][3] to suit your target. The low level open() syscall in this case will allow fopen() to work as necessary. At its simplest, you might implement open() to support higher-level stdio access to serial ports, but if you are expecting standard file-system access, then you will still need an underlying file-system to map it too.

Another example of re-targeting the Keil/ARM standard library can be found here.

Answer 2

Yes, it's often possible to use fopen() and similar routines in code for embedded systems. The way it often works is that the vendor supplies a C compiler and associated libraries targeted for their system, which implement some supported subset of the language in a way that's appropriate for that system (e.g. an implementation of printf() that outputs via a UART, or fopen() that uses RAM to simulate some sort of filesystem).