How use fwrite in Wasm

In main.cpp :
`#include
#include

int main() {
FILE *fh;

fh = fopen("a.txt", "wb");
if (!fh) exit(1);
fclose(fh);

fh = fopen("a.txt", "rb");
if (!fh) exit(1);

char data[] = "foobar";
size_t written = fwrite(data, 1, sizeof(data), fh);

printf("written=%zu\n", written);
}`

And compile it as:
emcc main.c -o main.html --preload-file a.txt

finally i run the html , why the a.txt is empty, how to use the fwrite, write something to the file?