main: refuse to dump the result into tty

Closes: https://codeberg.org/vyivel/dulcepan/issues/17

src/main.c

@@ -1,6 +1,7 @@
 #include <getopt.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 #include <wayland-client-core.h>
 #include <wayland-client-protocol.h>
 #include <xkbcommon/xkbcommon.h>
@@ -167,8 +168,9 @@ static void help(const char *prog) {
 			"  -o <path>    Specify the output file path.\n"
 			"\n"
 			"If the output file path is not specified, the resuling image will be printed to\n"
-			"the standard output. If the output file format is not specified, it is guessed\n"
-			"from the output file path if it's specified, and assumed to be PNG otherwise.\n"
+			"the standard output, which is expected to not be a terminal. If the output file\n"
+			"format is not specified, it is guessed from the output file path if it's\n"
+			"specified, and assumed to be PNG otherwise.\n"
 			"\n"
 			"Supported formats: png, ppm.\n"
 			"\n"
@@ -210,9 +212,13 @@ int main(int argc, char **argv) {
 		}
 	}
 
-	state.basedir_ctx = sfdo_basedir_ctx_create();
-
-	dp_config_load(&state, config_path);
+	if (state.output_path == NULL && isatty(STDOUT_FILENO)) {
+		fprintf(stderr,
+				"Refusing to run as the standard output is a terminal and there's no output file\n"
+				"path specified.\n\n");
+		help(argv[0]);
+		exit(1);
+	}
 
 	if (state.output_format == DP_FILE_UNKNOWN) {
 		if (state.output_path != NULL) {
@@ -227,6 +233,10 @@ int main(int argc, char **argv) {
 		}
 	}
 
+	state.basedir_ctx = sfdo_basedir_ctx_create();
+
+	dp_config_load(&state, config_path);
+
 	wl_list_init(&state.outputs);
 	wl_list_init(&state.seats);