Implement getchar() syscall, remove read_i64()

In response to bug pointed out in #26

api/syscalls.json

@@ -169,15 +169,15 @@
         "description": "Print a newline to standard output."
       },
       {
-        "name": "read_i64",
+        "name": "getchar",
         "args": [],
         "returns": [
-          "i64",
+          "i32",
           "val"
         ],
         "permission": "default_allowed",
         "const_idx": 8,
-        "description": "Read an i64 value from standard input."
+        "description": "Read one byte from standard input. This is a blocking function. The value -1 is returned on end of file or error."
       }
     ],
     "constants": []

doc/syscalls.md

@@ -94,15 +94,15 @@ void print_endl()
 
 Print a newline to standard output.
 
-## read_i64
+## getchar
 
 ```
-i64 read_i64()
+i32 getchar()
 ```
 
-**Returns:** `i64 val`
+**Returns:** `i32 val`
 
-Read an i64 value from standard input.
+Read one byte from standard input. This is a blocking function. The value -1 is returned on end of file or error.
 
 # time
 

ncc/include/uvm/syscalls.h

@@ -41,9 +41,9 @@
 // Print a newline to standard output.
 #define print_endl() asm () -> void { syscall print_endl; }
 
-// i64 read_i64()
-// Read an i64 value from standard input.
-#define read_i64() asm () -> i64 { syscall read_i64; }
+// i32 getchar()
+// Read one byte from standard input. This is a blocking function. The value -1 is returned on end of file or error.
+#define getchar() asm () -> i32 { syscall getchar; }
 
 // u64 time_current_ms()
 // Get the UNIX time stamp in milliseconds.

vm/examples/factorial.asm

@@ -12,7 +12,7 @@ push PROMPT_STR;
 syscall print_str;
 
 # Read input number
-syscall read_i64;
+call READ_INT, 0;
 
 # Fall FACT with 1 argument
 call FACT, 1;
@@ -48,3 +48,46 @@ get_arg 0;
 mul_u64;
 
 ret;
+
+#
+# Read a positive integer from stdlin
+#
+READ_INT:
+
+push 0; # Current integer value
+
+LOOP:
+    # Read one character
+    syscall getchar;
+
+    # If < 0 done
+    dup;
+    push 48;
+    lt_i64;
+    jnz DONE;
+
+    # If > 9 done
+    dup;
+    push 57;
+    gt_i64;
+    jnz DONE;
+
+    # Convert to integer digit
+    push 48;
+    sub_u64;
+
+    # int_val * 10;
+    get_local 0;
+    push 10;
+    mul_u64;
+
+    # int_val + 10;
+    add_u64;
+    set_local 0;
+
+    jmp LOOP;
+
+DONE:
+
+get_local 0;
+ret;

vm/examples/fib.asm

@@ -12,7 +12,7 @@ push PROMPT_STR;
 syscall print_str;
 
 # Read input number
-syscall read_i64;
+call READ_INT, 0;
 
 # Call FIB with 1 argument
 call FIB, 1;
@@ -45,3 +45,46 @@ sub_u64;
 call FIB, 1;
 add_u64;
 ret;
+
+#
+# Read a positive integer from stdlin
+#
+READ_INT:
+
+push 0; # Current integer value
+
+LOOP:
+    # Read one character
+    syscall getchar;
+
+    # If < 0 done
+    dup;
+    push 48;
+    lt_i64;
+    jnz DONE;
+
+    # If > 9 done
+    dup;
+    push 57;
+    gt_i64;
+    jnz DONE;
+
+    # Convert to integer digit
+    push 48;
+    sub_u64;
+
+    # int_val * 10;
+    get_local 0;
+    push 10;
+    mul_u64;
+
+    # int_val + 10;
+    add_u64;
+    set_local 0;
+
+    jmp LOOP;
+
+DONE:
+
+get_local 0;
+ret;

vm/examples/guess.asm

@@ -40,7 +40,7 @@ set_local 0;
 # Ask user for their guess.
 push PROMPT_STR;
 syscall print_str;
-syscall read_i64;
+call READ_INT, 0;
 
 # if (guess == target) goto WIN;
 dup;
@@ -85,3 +85,46 @@ syscall print_endl;
 
 push 0;
 exit;
+
+#
+# Read a positive integer from stdlin
+#
+READ_INT:
+
+push 0; # Current integer value
+
+LOOP:
+    # Read one character
+    syscall getchar;
+
+    # If < 0 done
+    dup;
+    push 48;
+    lt_i64;
+    jnz DONE;
+
+    # If > 9 done
+    dup;
+    push 57;
+    gt_i64;
+    jnz DONE;
+
+    # Convert to integer digit
+    push 48;
+    sub_u64;
+
+    # int_val * 10;
+    get_local 0;
+    push 10;
+    mul_u64;
+
+    # int_val + 10;
+    add_u64;
+    set_local 0;
+
+    jmp LOOP;
+
+DONE:
+
+get_local 0;
+ret;

vm/src/sys/constants.rs

@@ -14,7 +14,7 @@ pub const MEMSET: u16 = 4;
 pub const PRINT_I64: u16 = 5;
 pub const PRINT_STR: u16 = 6;
 pub const PRINT_ENDL: u16 = 7;
-pub const READ_I64: u16 = 8;
+pub const GETCHAR: u16 = 8;
 pub const WINDOW_ON_KEYDOWN: u16 = 9;
 pub const WINDOW_DRAW_FRAME: u16 = 10;
 pub const WINDOW_ON_MOUSEMOVE: u16 = 11;
@@ -45,7 +45,7 @@ pub const SYSCALL_DESCS: [Option<SysCallDesc>; SYSCALL_TBL_LEN] = [
     Some(SysCallDesc { name: "print_i64", const_idx: 5, argc: 1, has_ret: false }),
     Some(SysCallDesc { name: "print_str", const_idx: 6, argc: 1, has_ret: false }),
     Some(SysCallDesc { name: "print_endl", const_idx: 7, argc: 0, has_ret: false }),
-    Some(SysCallDesc { name: "read_i64", const_idx: 8, argc: 0, has_ret: true }),
+    Some(SysCallDesc { name: "getchar", const_idx: 8, argc: 0, has_ret: true }),
     Some(SysCallDesc { name: "window_on_keydown", const_idx: 9, argc: 2, has_ret: false }),
     Some(SysCallDesc { name: "window_draw_frame", const_idx: 10, argc: 2, has_ret: false }),
     Some(SysCallDesc { name: "window_on_mousemove", const_idx: 11, argc: 2, has_ret: false }),

vm/src/sys/mod.rs

@@ -6,6 +6,7 @@ pub mod constants;
 extern crate sdl2;
 use std::collections::HashMap;
 use std::io::Write;
+use std::io::Read;
 use std::io::{stdout, stdin};
 use std::sync::{Arc, Weak, Mutex};
 use crate::vm::{Value, VM};
@@ -160,7 +161,7 @@ impl SysState
         self.reg_syscall(PRINT_F32, SysCallFn::Fn1_0(print_f32));
         self.reg_syscall(PRINT_STR, SysCallFn::Fn1_0(print_str));
         self.reg_syscall(PRINT_ENDL, SysCallFn::Fn0_0(print_endl));
-        self.reg_syscall(READ_I64, SysCallFn::Fn0_1(read_i64));
+        self.reg_syscall(GETCHAR, SysCallFn::Fn0_1(getchar));
 
         self.reg_syscall(TIME_CURRENT_MS, SysCallFn::Fn0_1(time_current_ms));
         self.reg_syscall(TIME_DELAY_CB, SysCallFn::Fn2_0(time_delay_cb));
@@ -232,38 +233,35 @@ fn print_i64(vm: &mut VM, v: Value)
 {
     let v = v.as_i64();
     print!("{}", v);
-    stdout().flush().unwrap();
 }
 
 fn print_f32(vm: &mut VM, v: Value)
 {
     let v = v.as_f32();
     print!("{}", v);
-    stdout().flush().unwrap();
 }
 
 /// Print a null-terminated UTF-8 string to stdout
 fn print_str(vm: &mut VM, str_ptr: Value)
 {
     let rust_str = vm.get_heap_str(str_ptr.as_usize());
     print!("{}", rust_str);
-    stdout().flush().unwrap();
 }
 
 /// Print a newline characted to stdout
 fn print_endl(vm: &mut VM)
 {
     println!();
-    stdout().flush().unwrap();
 }
 
-fn read_i64(vm: &mut VM) -> Value
+/// Read one byte of input from stdin.
+/// Analogous to C's getchar
+fn getchar(vm: &mut VM) -> Value
 {
-    let mut line_buf = String::new();
-    stdin()
-        .read_line(&mut line_buf)
-        .expect("failed to read input line");
-    let val: i64 = line_buf.trim().parse().expect("expected i64 input");
+    let ch = stdin().bytes().next();
 
-    return Value::from(val);
+    match ch {
+        Some(Ok(ch)) => Value::from(ch as i64),
+        None | Some(Err(_)) => Value::from(-1 as i64),
+    }
 }