Add fopenat utility function

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>

lib/Makemodule.am

@@ -8,7 +8,8 @@ libinit_a_CFLAGS = $(AM_CFLAGS)
 
 libutil_a_SOURCES = lib/util/argv_exec.c lib/util/enum_by_name.c
 libutil_a_SOURCES += lib/util/enum_to_name.c lib/util/mksock.c
-libutil_a_SOURCES += lib/util/print_version.c $(HEADRS)
+libutil_a_SOURCES += lib/util/print_version.c lib/util/fopenat.c
+libutil_a_SOURCES += lib/include/util.h
 libutil_a_CPPFLAGS = $(AM_CPPFLAGS)
 libutil_a_CFLAGS = $(AM_CFLAGS)
 

lib/include/util.h

@@ -21,6 +21,7 @@
 #include <sys/types.h>
 #include <stdbool.h>
 #include <stddef.h>
+#include <stdio.h>
 
 #include "config.h"
 
@@ -83,5 +84,11 @@ int setup_tty(const char *tty, bool truncate);
 
 NORETURN void argv_exec(exec_t *e);
 
+/*
+	Similar to openat: opens a file relative to a dirfd, but returns
+	a FILE pointer instead of an fd.
+ */
+FILE *fopenat(int fd, const char *filename, const char *mode);
+
 #endif /* UTIL_H */
 

lib/util/fopenat.c

@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+/*
+ * Copyright (C) 2018 - David Oberhollenzer
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <errno.h>
+
+#include "util.h"
+
+FILE *fopenat(int dirfd, const char *filename, const char *mode)
+{
+	const char *ptr = mode;
+	int fd, flags = 0;
+	FILE *fp;
+
+	switch (*(ptr++)) {
+	case 'r':
+		flags = O_RDONLY;
+		break;
+	case 'w':
+		flags = O_WRONLY | O_CREAT | O_TRUNC;
+		break;
+	case 'a':
+		flags = O_WRONLY | O_CREAT | O_APPEND;
+		break;
+	default:
+		errno = EINVAL;
+		return NULL;
+	}
+
+	if (*ptr == '+') {
+		flags = (flags & ~(O_RDONLY | O_WRONLY)) | O_RDWR;
+		++ptr;
+	}
+
+	if (*ptr == 'b')
+		++ptr;
+
+	if (*ptr != '\0') {
+		errno = EINVAL;
+		return NULL;
+	}
+
+	fd = openat(dirfd, filename, flags, 0644);
+	if (fd == -1)
+		return NULL;
+
+	fp = fdopen(fd, mode);
+	if (fp == NULL)
+		close(fd);
+
+	return fp;
+}