Add source code files in their uncompleted state

The current state of the library contains support for the following
functions:
 - int luxinit(struct luxdisp *)     | Init a luxdisp struct
 - int luxget(struct luxdisp *)      | Get the current brightness
 - int luxset(struct luxdisp *, int) | Set the current brightness
 - int luxmax(struct luxdisp *)      | Get the maximum brightness
 - void luxfree(struct luxdisp *)    | Free a luxdisp struct

1 files changed, 168 insertions, 0 deletions

diff --git a/liblux.c b/liblux.c
new file mode 100644
index 0000000..3195342
--- /dev/null
+++ b/liblux.c
@@ -0,0 +1,168 @@
+/*
+ * BSD Zero Clause License
+ *
+ * Copyright (c) 2022 Thomas Voss
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+ * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+ * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+ * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+ * OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+ * PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#define _POSIX_C_SOURCE 200809L
+
+#include <sys/types.h>
+
+#include <dirent.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#include "lux.h"
+
+#ifdef O_PATH
+	#define GETDIR_FLAGS O_PATH
+#else
+	#define GETDIR_FLAGS (O_RDONLY | O_DIRECTORY)
+#endif
+
+static int intlen(int);
+static int getdir(void);
+static FILE *getbstream(int);
+
+int
+luxinit(struct luxdisp *disp)
+{
+	/* Set the default values for the luxdisp struct.  We want to be as lazy
+	 * as possible, so we don't bother with __bstream and __max yet.  We do
+	 * need __dirfd though.
+	 */
+	*disp = (struct luxdisp) {
+		.__bstream = NULL,
+		.__max     = -1,
+		.__dirfd   = getdir()
+	};
+	return disp->__dirfd;
+}
+
+int
+luxget(struct luxdisp *disp)
+{
+	/* If we don't have an open stream with the brightness file yet then
+	 * open one.  If we do, we instead rewind to the start of the file so
+	 * that we don't just read an EOF.
+	 */
+	if (disp->__bstream == NULL) {
+		if ((disp->__bstream = getbstream(disp->__dirfd)) == NULL)
+			return -1;
+	} else
+		rewind(disp->__bstream);
+
+	int n;
+	fscanf(disp->__bstream, "%d", &n);
+	return n;
+}
+
+int
+luxset(struct luxdisp *disp, int n)
+{
+	/* The same deal as in luxget() */
+	if (disp->__bstream == NULL) {
+		if ((disp->__bstream = getbstream(disp->__dirfd)) == NULL)
+			return -1;
+	} else
+		rewind(disp->__bstream);
+
+	/* After writing the new brightness value to the brightness file, we
+	 * need to truncate it so that luxset(disp, 1234); luxset(disp, 9);
+	 * gives us a final brightness of 2 and not 9234.
+	 */
+	fprintf(disp->__bstream, "%d", n);
+	ftruncate(fileno(disp->__bstream), intlen(n));
+	return n;
+}
+
+int
+luxmax(struct luxdisp *disp)
+{
+	/* The maximum brightness shouldn't change, so we can cache it in the
+	 * struct.
+	 */
+	if (disp->__max != -1)
+		return disp->__max;
+
+	int fd = openat(disp->__dirfd, "max_brightness", O_RDONLY);
+	if (fd == -1)
+		return -1;
+	FILE *stream = fdopen(fd, "r");
+	if (stream == NULL) {
+		close(fd);
+		return -1;
+	}
+
+	fscanf(stream, "%d", &disp->__max);
+	fclose(stream);
+	return disp->__max;
+}
+
+void
+luxfree(struct luxdisp *disp)
+{
+	if (disp->__dirfd != -1)
+		close(disp->__dirfd);
+	if (disp->__bstream != NULL)
+		fclose(disp->__bstream);
+}
+
+int
+intlen(int n)
+{
+	return    n < 10         ? 1
+		: n < 100        ? 2
+		: n < 1000       ? 3
+		: n < 10000      ? 4
+		: n < 100000     ? 5
+		: n < 1000000    ? 6
+		: n < 10000000   ? 7
+		: n < 100000000  ? 8
+		: n < 1000000000 ? 9
+		: 10;
+}
+
+int
+getdir(void)
+{
+	int fd, dfd;
+	struct dirent *dp;
+	DIR *dir;
+
+	/* To get the file descriptor for the directory where all the brightness
+	 * files are located, we first need to open BACKLIGHT_DIR which is the
+	 * directory where the backlight directories are located.  Then we need
+	 * to get a DIR* of that directory and get the 3rd entry.  We need the
+	 * third because the fist and second are the . and .. folders.  Finally
+	 * we open the folder so that we have a file descriptor we can return.
+	 */
+	if ((dfd = open(BACKLIGHT_DIR, O_RDONLY)) == -1
+			|| (dir = fdopendir(dfd)) == NULL
+			|| (dp  = readdir(dir))   == NULL
+			|| (dp  = readdir(dir))   == NULL
+			|| (dp  = readdir(dir))   == NULL
+			|| (fd  = openat(dfd, dp->d_name, GETDIR_FLAGS)) == -1)
+		return -1;
+
+	closedir(dir);
+	return fd;
+}
+
+FILE *
+getbstream(int dirfd)
+{
+	return fdopen(openat(dirfd, "brightness", O_RDWR), "r+");
+}