diff --git a/cia.c b/cia.c
index 4f99121..adbd990 100644
--- a/cia.c
+++ b/cia.c
@@ -31,6 +31,7 @@
 
 // Module stdlib_file
 #include "src/stdlib-file/file.c"
+#include "src/stdlib-file/fmt.c"
 
 // Module stdlib_string
 #include "src/stdlib-string/mem.c"
diff --git a/include/stdio.h b/include/stdio.h
index 3c795fd..f3e9b8d 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -4,6 +4,7 @@
 #include <cia/def.h>
 #include <cia/sync.h>
 #include <tinyrt.h>
+#include <stdarg.h>
 
 typedef struct FILE FILE;
 struct FILE {
@@ -23,3 +24,5 @@ int fputc(int c, FILE *file);
 size_t fread(void *restrict buf, size_t size, size_t count, FILE *restrict file);
 size_t fwrite(void const *restrict buf, size_t size, size_t count, FILE *restrict file);
 int fclose(FILE *file);
+
+int printf(char const *restrict fmt, ...);
\ No newline at end of file
diff --git a/src/stdlib-file/fmt.c b/src/stdlib-file/fmt.c
new file mode 100644
index 0000000..28cb017
--- /dev/null
+++ b/src/stdlib-file/fmt.c
@@ -0,0 +1,144 @@
+
+static int print_string(char *str) {
+    int str_len = 0;
+    while(str[str_len] != 0) {
+        str_len += 1;
+    }
+    u64 unused;
+    _RT_Status status = _rt_file_write(&stdout->rt_file, str_len, str, &unused);
+    if(status != _RT_STATUS_OK) {
+        return -1;
+    };
+    return str_len;
+}
+static int print_int(i64 number) {
+    bool negative = (number < 0);
+    if(negative) {
+        number = -number;
+    }
+    char buf[20];
+    buf[sizeof buf - 1] = 0;
+    char *p = buf + sizeof buf - 1;
+    do {
+        *--p = (number%10) + '0';
+        number /= 10;
+    } while(number > 0);
+    if(negative) {
+        *--p = '-';
+    }
+    u64 unused;
+    int buf_len = (int)(&buf[0] + sizeof buf - p);
+    _RT_Status status = _rt_file_write(&stdout->rt_file, buf_len, p, &unused);
+    if(status != _RT_STATUS_OK) {
+        return -1;
+    }
+    return buf_len;
+}
+
+static int print_uint(u64 number) {
+    char buf[20];
+    buf[sizeof buf - 1] = 0;
+    char *p = buf + sizeof buf - 1;
+    do {
+        *--p = (number%10) + '0';
+        number /= 10;
+    } while(number > 0);
+    u64 unused;
+    int buf_len = (int)(&buf[0] + sizeof buf - p);
+    _RT_Status status = _rt_file_write(&stdout->rt_file, buf_len, p, &unused);
+    if(status != _RT_STATUS_OK) {
+        return -1;
+    }
+    return buf_len;
+}
+static int print_hex(u64 number) {
+    char digits[] = "0123456789abcdef";
+    char buf[20];
+    buf[sizeof buf - 1] = 0;
+    char *p = buf + sizeof buf - 1;
+    for(int i = 0; i < 64; i += 4) {
+        // if(i != 0 && i % 16 == 0) {
+        //     *--p = '_';
+        // }
+        u8 bits = (number >> i) & 0x0f;
+        char digit = digits[bits];
+        *--p = digit;
+    }
+    *--p = 'x';
+    *--p = '0';
+    u64 unused;
+    int buf_len = (int)(&buf[0] + sizeof buf - p);
+    _RT_Status status = _rt_file_write(&stdout->rt_file, buf_len, p, &unused);
+    if(status != _RT_STATUS_OK) {
+        return -1;
+    }
+    return buf_len;
+}
+
+int printf(const char *restrict fmt, ...) {
+    i64 ret_printed = 0;
+    va_list args;
+    va_start(args, fmt);
+    i64 buf_start_idx = 0;
+    i64 buf_end_idx = 0;
+    u64 unused;
+    for(i64 i = 0; fmt[i] != 0;) {
+        buf_start_idx = i;
+        buf_end_idx = i;
+        while(fmt[i] != '%' && fmt[i] != 0) {
+            buf_end_idx += 1;
+            i += 1;
+        }
+        i64 buf_size = buf_end_idx - buf_start_idx;
+        if(buf_size != 0) {
+            _RT_Status status = _rt_file_write(
+                &stdout->rt_file
+                , buf_size
+                , (void *)&fmt[buf_start_idx]
+                , &unused
+            );
+            if(status != _RT_STATUS_OK) {
+                return -1;
+            }
+            ret_printed += buf_size;
+        }
+        if(fmt[i] == 0) {
+            break;
+        }
+        // fmt[i] == '%'
+        i += 1;
+        int arg_chars;
+        if(fmt[i] == 'd') {
+            int arg = va_arg(args, int);
+            arg_chars = print_int((i64)arg);
+            
+        }
+        else if(fmt[i] == 'u') {
+            unsigned int arg = va_arg(args, unsigned int);
+            arg_chars = print_uint((u64)arg);
+        }
+        else if(fmt[i] == 'p') {
+            void *arg = va_arg(args, void *);
+            arg_chars = print_hex((u64)arg);
+        }
+        else if(fmt[i] == 's') {
+            char *str = va_arg(args, char *);
+            arg_chars = print_string(str);
+        }
+        else if(fmt[i] == 'c') {
+            int ch = va_arg(args, int);
+            _RT_Status status = _rt_file_write(&stdout->rt_file, 1, &ch, &unused);
+            if(status != _RT_STATUS_OK) {
+                return -1;
+            }
+            arg_chars = 1;
+        }
+        i += 1;
+        if(arg_chars < 0) {
+            return -1;
+        }
+        ret_printed += arg_chars;
+    }
+    va_end(args);
+    return ret_printed;
+}
diff --git a/tests/hello.c b/tests/hello.c
index 063856f..8b90939 100644
--- a/tests/hello.c
+++ b/tests/hello.c
@@ -3,7 +3,8 @@
 #include <stdio.h>
 
 int main(int argc, char **argv, char **envp) {
-    char string[] = "Hello, world!\n";
-    fwrite(string, 1, sizeof string-1, stdout);
+    // char string[] = "Hello, world!\n";
+    // fwrite(string, 1, sizeof string-1, stdout);
+    printf("Hello, world! %d\n", 4);
     return 0;
 }