app: Use C FILE instead of POSIX fd, and switch back to /etc/motd

apps/app.c

@@ -15,23 +15,20 @@ int main()
     atexit(bye);
     printf("Welcome to %s from userspace (pid %d)!\n", "Luna", getpid());
 
-    mkdir("/home", 0755);
-    mkdir("/home/user", 0755);
-
-    int fd = open("/home/user/notes.txt", O_RDWR | O_CREAT, 0644);
-    if (fd < 0)
+    FILE* f = fopen("/etc/motd", "r");
+    if (!f)
     {
-        perror("open");
+        perror("fopen");
         return 1;
     }
 
     char buffer[512];
-    ssize_t nread = read(fd, buffer, sizeof(buffer));
+    size_t nread = fread(buffer, 1, sizeof(buffer), f);
     buffer[nread] = 0;
 
-    printf("/home/user/notes.txt says: %s", buffer);
+    printf("/etc/motd says: %s", buffer);
 
-    close(fd);
+    fclose(f);
 
     time_t now = time(NULL);
     printf("date: %s", ctime(&now));

libc/src/stdio.cpp

@@ -10,6 +10,23 @@
 
 FILE* stderr = nullptr;
 
+static int fopen_parse_mode(const char* mode)
+{
+    int result = 0;
+    switch (*mode)
+    {
+    case 'r': result |= O_RDONLY; break;
+    case 'w': result |= (O_WRONLY | O_CREAT | O_TRUNC); break;
+    case 'a': result |= (O_WRONLY | O_CREAT | O_APPEND); break;
+
+    default: errno = EINVAL; return -1;
+    }
+
+    if (strchr(mode, '+')) result |= O_RDWR;
+
+    return result;
+}
+
 extern "C"
 {
     int console_write(const char* str, size_t size)
@@ -18,10 +35,13 @@ extern "C"
         __errno_return(rc, int);
     }
 
-    FILE* fopen(const char* path, const char*)
+    FILE* fopen(const char* path, const char* mode)
     {
-        // FIXME: Parse the mode string.
-        int fd = open(path, O_RDWR);
+        int flags;
+
+        if ((flags = fopen_parse_mode(mode)) < 0) return nullptr;
+
+        int fd = open(path, flags, 0666);
         if (fd < 0) return nullptr;
 
         FILE* f = (FILE*)malloc(sizeof(FILE));