snprintf

Makefile

@@ -1,5 +1,6 @@
 .PHONY: run
 run:
+	-rm build/boot.img.lock
 	-bochs -f bochs.conf
 
 build/boot.vdi: build/boot.img

include/kernel/stdio.h

@@ -1 +1,16 @@
 #pragma once
+
+#include <types/stdint.h>
+
+size_t
+snprint_decimal(
+    char* buf,
+    size_t buf_size,
+    int32_t num);
+
+size_t
+snprintf(
+    char* buf,
+    size_t buf_size,
+    const char* fmt,
+    ...);

src/kernel/stdio.c

@@ -0,0 +1,132 @@
+#include <kernel/stdio.h>
+
+#include <types/stdint.h>
+
+// where n is in the range of [0, 9]
+static inline char d_to_c(int32_t n)
+{
+    return '0' + n;
+}
+
+// this will check if there is still free space
+// in the buffer. if so, push the char into it,
+// change the value of buf_size and move pointer
+// forward
+//
+// x: char* buf
+// y: size_t buf_size
+// z: char c
+#define do_write_if_free(x, y, z) \
+    if ((y) > 1) {                \
+        *((x)++) = (z);           \
+        --(y);                    \
+    }
+
+// TODO: print negative numbers
+size_t
+snprint_decimal(
+    char* buf,
+    size_t buf_size,
+    int32_t num)
+{
+    size_t n_write = 0;
+    char* orig_buf = buf;
+
+    do {
+        do_write_if_free(buf, buf_size, d_to_c(num % 10));
+        num /= 10;
+        ++n_write;
+    } while (num != 0);
+
+    // prepend trailing '\0'
+    if (buf_size > 0)
+        *buf = 0x00;
+
+    // move buf pointer to the last digit of number
+    --buf;
+
+    // reverse output
+    while (orig_buf < buf) {
+        char c = *buf;
+        *buf = *orig_buf;
+        *orig_buf = c;
+        --buf;
+        ++orig_buf;
+    }
+
+    return n_write;
+}
+
+static inline size_t
+snprint_char(
+    char* buf,
+    size_t buf_size,
+    char c)
+{
+    if (buf_size > 1)
+        *buf = c;
+    return sizeof(c);
+}
+
+size_t
+snprintf(
+    char* buf,
+    size_t buf_size,
+    const char* fmt,
+    ...)
+{
+    size_t n_write = 0;
+    void* arg_ptr = ((void*)&buf) + sizeof(char*) + sizeof(size_t) + sizeof(const char*);
+
+    for (char c; (c = *fmt) != 0x00; ++fmt) {
+        if (c == '%') {
+            size_t n_tmp_write = 0;
+
+            switch (*(++fmt)) {
+
+            // int32 decimal
+            case 'd':
+                n_tmp_write = snprint_decimal(buf, buf_size, *(int32_t*)arg_ptr);
+                arg_ptr += sizeof(int32_t);
+                break;
+
+            // c string
+            case 's':
+                n_tmp_write = snprintf(buf, buf_size, *(const char**)arg_ptr);
+                arg_ptr += sizeof(const char*);
+                break;
+
+            // int8 char
+            case 'c':
+                n_tmp_write = snprint_char(buf, buf_size, *(char*)arg_ptr);
+                arg_ptr += sizeof(char);
+                break;
+            default:
+                n_tmp_write = snprint_char(buf, buf_size, *(fmt - 1));
+                break;
+            }
+
+            n_write += n_tmp_write;
+            if (buf_size > 1) {
+                if (buf_size > n_tmp_write) {
+                    buf += n_tmp_write;
+                    buf_size -= n_tmp_write;
+                } else {
+                    // no enough space
+                    // shrink buf_size to one
+                    buf += (buf_size - 1);
+                    buf_size = 1;
+                }
+            }
+
+        } else {
+            ++n_write;
+            do_write_if_free(buf, buf_size, c);
+        }
+    }
+
+    if (buf_size > 0)
+        *buf = 0x00;
+
+    return n_write;
+}

src/kernel_main.c

@@ -1,6 +1,7 @@
 #include <kernel_main.h>
 
 #include <asm/boot.h>
+#include <kernel/stdio.h>
 #include <kernel/vga.h>
 
 void kernel_main(void)