Add somewhat functioning printf

libc/stdio/printf.c

@@ -1,9 +1,130 @@
+#include <limits.h>
+#include <stdarg.h>
+#include <stdbool.h>
 #include <stdio.h>
+#include <string.h>
 
-int printf(const char* restrict fmt, ...)
+static bool print(const char* data, size_t len)
 {
+	const unsigned char* bytes = (const unsigned char*)data;
+	for(size_t i = 0; i < len; i++)
+		if (putchar(bytes[i]) == EOF)
+			return false;
+	return true;
+}
+
+static bool print_int(int value, int* out_len)
+{
+	if (value == -2147483648)
+	{
+		if (!print("-2147483648", 11))
+			return false;
+		*out_len = 11;
+		return true;
+	}
+
+	// Enough for any (32-bit) integer value
+	char buffer[11];
+	char* ptr = buffer + sizeof(buffer);
 	int len = 0;
-	while (fmt[len])
-		putchar(fmt[len++]);
-	return len;
+	bool sign = false;
+
+	if (value < 0)
+	{
+		sign = true;
+		value = -value;
+	}
+
+	while (value)
+	{
+		*(--ptr) = (value % 10) + '0';
+		value /= 10;
+		len++;
+	}
+
+	if (sign)
+	{
+		*(--ptr) = '-';
+		len++;
+	}
+
+	if (!print(ptr, len))
+		return false;
+
+	*out_len = len;
+	return true;
+}
+
+int printf(const char* restrict format, ...)
+{
+	va_list args;
+	va_start(args, format);
+
+	int written = 0;
+
+	while (*format)
+	{
+		size_t max_rem = INT_MAX - written;
+
+		if (format[0] != '%' || format[1] == '%')
+		{
+			if (format[0] == '%')
+				format++;
+			size_t len = 1;
+			while (format[len] && format[len] != '%')
+				len++;
+			if (!print(format, len))
+				return -1;
+			format += len;
+			written += len;
+			continue;
+		}
+
+		const char* format_start = format++;
+
+		if (*format == 'c')
+		{
+			format++;
+			char c = (char)va_arg(args, int);
+			if (!max_rem) // FIXME: EOVERFLOW
+				return -1;
+			if (!print(&c, sizeof(c)))
+				return -1;
+			written++;
+		}
+		else if (*format == 's')
+		{
+			format++;
+			const char* str = va_arg(args, const char*);
+			size_t len = strlen(str);
+			if (max_rem < len) // FIXME: EOVERFLOW
+				return -1;
+			if (!print(str, len))
+				return -1;
+			written += len;
+		}
+		else if (*format == 'd' || *format == 'i')
+		{
+			format++;
+			int value = va_arg(args, int);
+			int len;
+			if (!print_int(value, &len))
+				return -1;
+			written += len;
+		}
+		else
+		{
+			format = format_start;
+			size_t len = strlen(format);
+			if (max_rem < len) // FIXME: EOVERFLOW
+				return -1;
+			if (!print(format, len))
+				return -1;
+			written += len;
+			format += len;
+		}
+	}
+
+	va_end(args);
+	return written;
 }