(maybe solved) Mysterious mem corruption from some standard C code???

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>

long FileLength(char* path) {
long size;

FILE* stream = fopen(path ,"rb");
if( stream== NULL) {
size = -1;
} else {
fseek(stream, 0, SEEK_END);
size = ftell(stream);
fclose(stream);
}
return size;
}

int main() {
//char* path = "T:usb.log";
char* path = "RAM:does_not_exist";
printf("Size of '%s' is %ld bytes.\n", path, FileLength(path));
return 0;
}

it was being called on two paths

Directories do not exist in the C99 standard (or the C2011 one). So by definition, fopen-ing a directory is either implementation specific or undefined behavior.

One possibility might be that the code you think is at fault (FileLength) isn't.

Given that the code you pasted only contain a few calls, and neither of them are strange and rather basic, it's unlikely they're at fault to be honest.

So you had this problem for a long time while never rebooting the machine?

That seems to be a peculiar way to find out the size of a file.

Besides, you are assuming that the result will fit into 32 bits, whereas the result of (current) DOS calls is 64-bit.

I would suggest something like this (sorry, it's in C)