Kernel+LibC: Add some errno codes

Kernel now returns ENOMEM and other errnos, so we dont have to write
error messages

libc/Makefile

@@ -41,9 +41,10 @@ string/memcpy.o \
 string/memmove.o \
 string/memset.o \
 string/strcmp.o \
-string/strncmp.o \
 string/strcpy.o \
+string/strerror.o \
 string/strlen.o \
+string/strncmp.o \
 string/strncpy.o \
 string/strstr.o \
  

libc/include/errno.h

@@ -0,0 +1,16 @@
+#pragma once
+
+#include <sys/cdefs.h>
+
+#define ENOMEM 1
+#define EINVAL 2
+#define ENOTDIR 3
+#define EISDIR 4
+#define ENOENT 5
+#define EIO 6
+
+__BEGIN_DECLS
+
+extern int errno;
+
+__END_DECLS

libc/include/string.h

@@ -19,4 +19,6 @@ char* strncpy(char* __restrict, const char* __restrict, size_t);
 
 char* strstr(const char*, const char*);
 
+char* strerror(int);
+
 __END_DECLS

libc/string/strerror.cpp

@@ -0,0 +1,32 @@
+#include <errno.h>
+#include <string.h>
+
+int errno = 0;
+
+char* strerror(int error)
+{
+	switch (error)
+	{
+	case ENOMEM:
+		return "Cannot allocate memory";
+	case EINVAL:
+		return "Invalid argument";
+	case EISDIR:
+		return "Is a directory";
+	case ENOTDIR:
+		return "Not a directory";
+	case ENOENT:
+		return "No such file or directory";
+	case EIO:
+		return "Input/output error";
+	default:
+		break;
+	}
+
+	// FIXME: sprintf
+	//static char buffer[26];
+	//sprintf(buffer, "Unknown error %d", error);
+	//return buffer;
+	errno = EINVAL;
+	return "Unknown error";
+}