libsigrok/cli: Implement loglevel support.

libsigrok can now be told at which loglevel to work, i.e., how many
debug/error/warning output to generate. You can also query the current
loglevel.

In sigrok-cli it is now possible to set the loglevel via -l. For example:

 - Disable all output: sigrok-cli -l 0
 - Only show errors: sigrok-cli -l 1
 - Show errors, warnings, info, and debug messages: sigrok-cli -l 4

log.c

@@ -23,12 +23,49 @@
 #include <sigrok.h>
 #include <sigrok-internal.h>
 
+static int sr_loglevel = SR_LOG_WARN; /* Show errors+warnings per default. */
+
+/**
+ * Set the libsigrok loglevel.
+ *
+ * This influences the amount of log messages (debug messages, error messages,
+ * and so on) libsigrok will output. Using SR_LOG_NONE disables all messages.
+ *
+ * @param loglevel The loglevel to set (SR_LOG_NONE, SR_LOG_ERR, SR_LOG_WARN,
+ *                 SR_LOG_INFO, or SR_LOG_DBG).
+ * @return SR_OK upon success, SR_ERR_ARG upon invalid loglevel.
+ */
+int sr_set_loglevel(int loglevel)
+{
+	if (loglevel < SR_LOG_NONE || loglevel > SR_LOG_DBG) {
+		sr_err("log: %s: invalid loglevel %d", __func__, loglevel);
+		return SR_ERR_ARG;
+	}
+
+	sr_loglevel = loglevel;
+
+	sr_dbg("log: %s: libsigrok loglevel set to %d", __func__, loglevel);
+
+	return SR_OK;
+}
+
+/**
+ * Get the libsigrok loglevel.
+ *
+ * @return The currently configured libsigrok loglevel.
+ */
+int sr_get_loglevel(void)
+{
+	return sr_loglevel;
+}
+
 static int sr_logv(int loglevel, const char *format, va_list args)
 {
 	int ret;
 
-	/* Avoid compiler warnings. */
-	loglevel = loglevel;
+	/* Only output messages of at least the selected loglevel(s). */
+	if (loglevel > sr_loglevel)
+		return SR_OK; /* TODO? */
 
 	ret = vfprintf(stderr, format, args);
 	fprintf(stderr, "\n");

sigrok-internal.h

@@ -41,13 +41,6 @@ int load_hwplugins(void);
 
 /*--- log.c -----------------------------------------------------------------*/
 
-/* Log levels for sr_log() and friends. */
-#define SR_LOG_NONE	0
-#define SR_LOG_DBG	1
-#define SR_LOG_INFO	2
-#define SR_LOG_WARN	3
-#define SR_LOG_ERR	4
-
 int sr_log(int loglevel, const char *format, ...);
 int sr_dbg(const char *format, ...);
 int sr_info(const char *format, ...);

sigrok-proto.h

@@ -25,6 +25,11 @@
 int sr_init(void);
 int sr_exit(void);
 
+/*--- log.c -----------------------------------------------------------------*/
+
+int sr_set_loglevel(int loglevel);
+int sr_get_loglevel(void);
+
 /*--- datastore.c -----------------------------------------------------------*/
 
 int sr_datastore_new(int unitsize, struct sr_datastore **ds);

sigrok.h

@@ -67,6 +67,13 @@ extern "C" {
 
 #define SR_HZ_TO_NS(n) (1000000000 / (n))
 
+/* libsigrok loglevels. */
+#define SR_LOG_NONE	0
+#define SR_LOG_ERR	1
+#define SR_LOG_WARN	2
+#define SR_LOG_INFO	3
+#define SR_LOG_DBG	4
+
 typedef int (*sr_receive_data_callback) (int fd, int revents, void *user_data);
 
 /* Data types used by hardware plugins for set_configuration() */