fopen and tests

tests/c/fopen.c

@@ -0,0 +1,16 @@
+#include <stdio.h>
+#include <errno.h>
+#include <assert.h>
+
+int main(int argc, char *argv[]) {
+    FILE *file1 = fopen("./c/fopen.c", "r");
+    assert(file1 && "failed to open file");
+    
+    FILE *file2 = fopen("./c/fopen.c", "meow");
+    assert(!file2 && "succeeded opening file despite invalid argument");
+    assert((errno == EINVAL) && "wrong errno");
+
+    FILE *file3 = fopen("/this-does-absolutely-not-exist-at-all-edhjkefhew98", "r");
+    assert(!file3 && "succeeded despite file not existing");
+    assert((errno == ENOENT) && "wrong errno");
+}