FFI

# The door to the OS (design/ffi.md). The oracle cannot link C; it emulates
# write, exit, getenv, strlen, malloc, free so this runs.
extern.C fn write ['fd': int32, 'buf': ptr[uint8], 'len': uint64] -> int64;
extern.C fn strlen ['s': ptr[uint8]] -> uint64;
extern.C fn getenv ['name': ptr[uint8]] -> ptr[uint8];
extern.C fn malloc ['bytes': uint64] -> ptr.mut[uint8];
extern.C fn free ['p': ptr.mut[uint8]];

layout.C struct Timeval {
    sec: int64,
    usec: int64
}

fn say [lent 'text': str.utf8] -> fail[io] {
    no_check let 'n' := write[1, lend 'text', strlen[lend 'text']];   # a lend is the pointer; the call is a no_check site
    if 'n' < 0 { fail io; }
}

MAIN {
    say["hello from write" ++ newln] otherwise std::exit[1];
    say["hello again" ++ newln] otherwise std::exit[1];

    let 'key' := "HOME";                            # the lent text ends where C expects a 0
    no_check let 'home' := getenv[lend 'key'];
    if ptr.is_null['home'] { std::print.stored["no HOME", newln]; }
    else { no_check if strlen['home'] > 0 { std::print.stored["HOME is set", newln]; } }

    no_check let 'buf' := malloc[4];                 # bytes nobody owns
    no_check ptr.write['buf', value 72];
    no_check ptr.write[ptr.at['buf', by 1], value 105];
    no_check std::print.useful[ptr.read['buf'], " ", ptr.read[ptr.at['buf', by 1]], newln];
    no_check free['buf'];

    std::print.useful[size.of[int32], " ", size.of[Timeval], " ", align.of[int64], " ", size.of[ptr[uint8]], newln];
}