Use opendir/readdir instead of scandir

scandir is a relatively recent addition to POSIX and is not available on
older versions of Mac OS X.  opendir/readdir should be more portable.

util-unix.cpp

@@ -43,6 +43,8 @@
 #include <vector>
 #include <string>
 #include <cstring>
+#include <cstddef>
+#include <algorithm>
 
 std::string System_error::message () const
 {
@@ -310,25 +312,47 @@ int util_rename (const char* from, const char* to)
 	return rename(from, to);
 }
 
-static int dirfilter (const struct dirent* ent)
+static size_t sizeof_dirent_for (DIR* p)
 {
-	// filter out . and ..
-	return std::strcmp(ent->d_name, ".") != 0 && std::strcmp(ent->d_name, "..") != 0;
+	long name_max = fpathconf(dirfd(p), _PC_NAME_MAX);
+	if (name_max == -1) {
+		#ifdef NAME_MAX
+		name_max = NAME_MAX;
+		#else
+		name_max = 255;
+		#endif
+	}
+	return offsetof(struct dirent, d_name) + name_max + 1; // final +1 is for d_name's null terminator
 }
 
 std::vector<std::string> get_directory_contents (const char* path)
 {
-	struct dirent**		namelist;
-	int			n = scandir(path, &namelist, dirfilter, alphasort);
-	if (n == -1) {
-		throw System_error("scandir", path, errno);
-	}
-	std::vector<std::string>	contents(n);
-	for (int i = 0; i < n; ++i) {
-		contents[i] = namelist[i]->d_name;
-		free(namelist[i]);
-	}
-	free(namelist);
+	std::vector<std::string>		contents;
 
+	DIR*					dir = opendir(path);
+	if (!dir) {
+		throw System_error("opendir", path, errno);
+	}
+	try {
+		std::vector<unsigned char>	buffer(sizeof_dirent_for(dir));
+		struct dirent*			dirent_buffer = reinterpret_cast<struct dirent*>(&buffer[0]);
+		struct dirent*			ent = NULL;
+		int				err = 0;
+		while ((err = readdir_r(dir, dirent_buffer, &ent)) == 0 && ent != NULL) {
+			if (std::strcmp(ent->d_name, ".") == 0 || std::strcmp(ent->d_name, "..") == 0) {
+				continue;
+			}
+			contents.push_back(ent->d_name);
+		}
+		if (err != 0) {
+			throw System_error("readdir_r", path, errno);
+		}
+	} catch (...) {
+		closedir(dir);
+		throw;
+	}
+	closedir(dir);
+
+	std::sort(contents.begin(), contents.end());
 	return contents;
 }