configure: check whether gettid() is declared

configure.ac

@@ -620,7 +620,7 @@ AC_CHECK_DECLS([PR_CAPBSET_DROP], [], [], [#include <sys/prctl.h>])
 AC_CHECK_HEADERS([sys/signalfd.h pty.h ifaddrs.h sys/capability.h sys/personality.h utmpx.h sys/timerfd.h])
 
 # Check for some syscalls functions
-AC_CHECK_FUNCS([setns pivot_root sethostname unshare rand_r confstr faccessat])
+AC_CHECK_FUNCS([setns pivot_root sethostname unshare rand_r confstr faccessat gettid])
 
 # Check for some functions
 AC_CHECK_LIB(pthread, main)

src/lxc/log.c

@@ -42,6 +42,20 @@
 
 #define LXC_LOG_DATEFOMAT_SIZE  15
 
+#ifndef HAVE_GETTID
+static inline pid_t gettid(void)
+{
+#ifdef __NR_gettid
+	return (pid_t)syscall(SYS_gettid);
+#else
+	errno = ENOSYS;
+	return -1;
+#endif
+}
+#else
+extern pid_t gettid(void);
+#endif
+
 int lxc_log_fd = -1;
 int lxc_quiet_specified;
 int lxc_log_use_global_fd;