perf record: auto detect when stdout is a pipe

This patch gives the ability to 'perf record' to detect when its stdout
has been redirected to a pipe. There's now no more need to add '-o -'
switch in this case.

However '-o <path>' option has always precedence, that is if specified
and stdout has been connected via a pipe then the output will go into
the specified output.

LKML-Reference: <m3ipxo966i.fsf@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-record.c

@@ -48,7 +48,7 @@ static unsigned int		user_freq 			= UINT_MAX;
 static int			freq				=   1000;
 static int			output;
 static int			pipe_output			=      0;
-static const char		*output_name			= "perf.data";
+static const char		*output_name			= NULL;
 static int			group				=      0;
 static int			realtime_prio			=      0;
 static bool			nodelay				=  false;
@@ -497,6 +497,13 @@ static int __cmd_record(int argc, const char **argv)
 		exit(-1);
 	}
 
+	if (!output_name) {
+		if (!fstat(STDOUT_FILENO, &st) && S_ISFIFO(st.st_mode))
+			pipe_output = 1;
+		else
+			output_name = "perf.data";
+	}
+	if (output_name) {
 		if (!strcmp(output_name, "-"))
 			pipe_output = 1;
 		else if (!stat(output_name, &st) && st.st_size) {
@@ -510,6 +517,7 @@ static int __cmd_record(int argc, const char **argv)
 		} else if (write_mode == WRITE_APPEND) {
 			write_mode = WRITE_FORCE;
 		}
+	}
 
 	flags = O_CREAT|O_RDWR;
 	if (write_mode == WRITE_APPEND)