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You can interact with files on the microSD card (you can not interact with files on the V5 brain's flash) through standard C/C++ file I/O methods. For the most part, you can follow along with any standard C tutorial for file I/O and it will work with PROS. Here are a couple of recommended tutorials:

• https://www.cprogramming.com/tutorial/cfileio.html
• https://www.tutorialspoint.com/cprogramming/c_file_io.htm

The only additional detail needed for interacting with the filesystem in PROS is that any files on the microSD card must be prefaced with /usd/. A file on the microSD card can be written to in the following manner:

FILE* usd_file_write = fopen("/usd/example.txt", "w");
fputs("Example text", usd_file_write);
fclose(usd_file_write);

FILE* usd_file_read = fopen("/usd/example.txt", "r");
char buf[50]; // This just needs to be larger than the contents of the file
fread(buf, 1, 50, usd_file_read); // passing 1 because a `char` is 1 byte, and 50 b/c it's the length of buf
printf("%s\n", buf); // print the string read from the file
// Should print "Example text" to the terminal
fclose(usd_file_read); // always close files when you're done with them

FILE* stderr = fopen("serr", "w");
fputs("Example text", stderr);
fclose(usd_file_write);