add util.c

Signed-off-by: Mattias Andrée <maandree@operamail.com>

1 files changed, 82 insertions, 0 deletions

diff --git a/src/libmdsserver/util.c b/src/libmdsserver/util.c
new file mode 100644
index 0000000..c4606e8
--- /dev/null
+++ b/src/libmdsserver/util.c
@@ -0,0 +1,82 @@
+/**
+ * mds — A micro-display server
+ * Copyright © 2014  Mattias Andrée (maandree@member.fsf.org)
+ * 
+ * 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 <http://www.gnu.org/licenses/>.
+ */
+#include "util.h"
+#include "config.h"
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <limits.h>
+#include <string.h>
+
+
+/**
+ * Read an environment variable, but handle it as undefined if empty
+ * 
+ * @param   var  The environment variable's name
+ * @return       The environment variable's value, `NULL` if empty or not defined
+ */
+char* getenv_nonempty(const char* var)
+{
+  char* rc = getenv(var);
+  if ((rc == NULL) || (*rc == '\0'))
+    return NULL;
+  return rc;
+}
+
+
+/**
+ * Re-exec the server.
+ * This function only returns on failure.
+ * 
+ * @param  argc      The number of elements in `argv`
+ * @param  argv      The command line arguments
+ * @param  reexeced  Whether the server has previously been re-exec:ed
+ */
+void reexec_server(int argc, char** argv, int reexeced)
+{
+  char readlink_buf[PATH_MAX];
+  ssize_t readlink_ptr;
+  char** reexec_args;
+  char** reexec_args_;
+  int i;
+  
+  /* Re-exec the server. */
+  readlink_ptr = readlink(SELF_EXE, readlink_buf, (sizeof(readlink_buf) / sizeof(char)) - 1);
+  if (readlink_ptr < 0)
+    return;
+  /* ‘readlink() does not append a null byte to buf.’ */
+  readlink_buf[readlink_ptr] = '\0';
+  reexec_args = alloca(((size_t)argc + 2) * sizeof(char*));
+  reexec_args_ = reexec_args;
+  if (reexeced == 0)
+    {
+      *reexec_args_++ = *argv;
+      *reexec_args_ = strdup("--re-exec");
+      if (*reexec_args_)
+	return;
+      for (i = 1; i < argc; i++)
+	reexec_args_[i] = argv[i];
+    }
+  else /* Don't let the --re-exec:s accumulate. */
+    *reexec_args_ = *argv;
+  for (i = 1; i < argc; i++)
+    reexec_args_[i] = argv[i];
+  reexec_args_[argc] = NULL;
+  execv(readlink_buf, reexec_args);
+}
+