fix all GCC -Wall, -Wextra, -Wpedantic and -D_FORTIFY_SOURCE=2 warnings

Approach used to fix each error type is as follows:

warning: unused parameter ‘x’ [-Wunused-parameter]
- add `__attribute__((unused))` to parameter signature in .c file (but not on its .h counterpart)

warning: ignoring return value of ‘f’, declared with attribute warn_unused_result [-Wunused-result]
- wrap function call in an `if (f(...) ...)` with appopriate error test condition, followed by an
  `fprintf(stderr, "Warning: ...\n")` with some informative error message

warning: ‘x’ may be used uninitialized in this function [-Wmaybe-uninitialized]
- initialize the variable in its declaration with a dummy value such as 0

warning: pointer targets in passing argument 1 of ‘read_d64_le’ differ in signedness [-Wpointer-sign]
- change passed argument type to match signedness. Luckly the argument exists solely for that function call

audio.c

@@ -509,7 +509,8 @@ int aiff_id(unsigned char *buf, int len)
 	return 1;
 }
 
-int aiff_open(FILE *in, wavegain_opt *opt, unsigned char *buf, int buflen)
+int aiff_open(FILE *in, wavegain_opt *opt, unsigned char *buf,
+              int buflen __attribute__((unused)))
 {
 	int aifc; /* AIFC or AIFF? */
 	unsigned int len;
@@ -636,7 +637,9 @@ int wav_id(unsigned char *buf, int len)
 	return 1;
 }
 
-int wav_open(FILE *in, wavegain_opt *opt, unsigned char *oldbuf, int buflen)
+int wav_open(FILE *in, wavegain_opt *opt,
+             unsigned char *oldbuf __attribute__((unused)),
+             int buflen __attribute__((unused)))
 {
 	unsigned char buf[81];
 	Int64_t len;
@@ -692,9 +695,10 @@ int wav_open(FILE *in, wavegain_opt *opt, unsigned char *oldbuf, int buflen)
 		if (!find_gain_chunk(in, &len))
 			FSEEK64(in, current_pos, SEEK_SET);
 		else {
-			char buf_double[8];
+			unsigned char buf_double[8];
 			opt->gain_chunk = 1;
-			fread(buf_double, 1, 8, in);
+			if (fread(buf_double, 1, 8, in) < 8)
+				fprintf(stderr, "Warning: Failed to read WAV gain chunk\n");
 			opt->gain_scale = READ_D64(buf_double);
 		}
 	}
@@ -712,7 +716,8 @@ int wav_open(FILE *in, wavegain_opt *opt, unsigned char *oldbuf, int buflen)
 			fprintf(stderr, "Error: unable to allocate memory for header\n");
 		else {
 			opt->header_size = current_pos;
-			fread(opt->header, 1, opt->header_size, in);
+			if (fread(opt->header, 1, opt->header_size, in) < (size_t)opt->header_size)
+				fprintf(stderr, "Warning: Failed to read WAV header when applying gain\n");
 		}
 		FSEEK64(in, current_pos, SEEK_SET);
 	}
@@ -1065,7 +1070,8 @@ void close_audio_file( FILE *in, audio_file *aufile, wavegain_opt *opt)
 					FSEEK64 (in, current_pos_t, SEEK_SET);
 					ch = malloc (sizeof(char) * (pos - current_pos_t));
 
-					fread (ch, 1, pos - current_pos_t, in);
+					if (fread (ch, 1, pos - current_pos_t, in) < (pos - current_pos_t))
+						fprintf(stderr, "Warning: Failed to read input audio file when closing output file\n");
 					fwrite (ch, pos - current_pos_t, 1, aufile->sndfile);
 
 					if (ch)

main.c

@@ -158,7 +158,7 @@ const char* ftos(double f, const char* format)
  * \return  0 if successful and -1 if an error occured (in which case a
  *          message has been printed).
  */
-int process_files(FILE_LIST* file_list, SETTINGS* settings, const char* dir)
+int process_files(FILE_LIST* file_list, SETTINGS* settings, const char* dir __attribute__((unused)))
 {
 	FILE_LIST* file;
 	double     factor_clip,

recurse.c

@@ -96,7 +96,7 @@ static int contains_pattern(const char* string)
 			case '\\':
 				/* Accept a terminating \ as a literal \ */
 				if (string[1])
-					string++;
+					string++; // @suppress("No break at end of case")
 				/* Fall through */
 
 			default:
@@ -169,7 +169,7 @@ static int match(const char* pattern, const char* text)
 			case '\\':
 				/* Accept a terminating \ as a literal \ */
 				if (pattern[1])
-					++pattern;
+					++pattern; // @suppress("No break at end of case")
 				/* Fall through */
 
 			default:

wavegain.c

@@ -381,7 +381,8 @@ int get_gain(const char *filename, double *track_peak, double *track_gain,
  * If audiophile_gain is selected, that value is used, otherwise the
  * radio_gain value is used.
  */
-int write_gains(const char *filename, double radio_gain, double audiophile_gain, double TitlePeak, 
+int write_gains(const char *filename, double radio_gain, double audiophile_gain,
+                double TitlePeak __attribute__((unused)),
                 double *dc_offset, double *album_dc_offset, SETTINGS *settings)
 {
 	wavegain_opt *wg_opts = malloc(sizeof(wavegain_opt));
@@ -393,8 +394,8 @@ int write_gains(const char *filename, double radio_gain, double audiophile_gain,
 	double       Gain;
 	double       scale;
 	double       total_read = 0.;
-	double       wrap_prev_pos;
-	double       wrap_prev_neg;
+	double       wrap_prev_pos = 0;
+	double       wrap_prev_neg = 0;
 	void         *sample_buffer;
 	input_format *format;
 
@@ -674,9 +675,16 @@ int write_gains(const char *filename, double radio_gain, double audiophile_gain,
 #endif
 #ifndef _WIN32
 			/* copy owner, group, permissions from original to output */
+			/* TODO: add -p|--preserve=[=ATTR_LIST] (see `man cp`) and only copy
+			 * attributes if enabled. Silently ignore errors on EPERM
+			 */
 			stat(filename, &fst);
-			chown(tempName, fst.st_uid, -1);
-			chown(tempName, -1 ,fst.st_gid);
+			if (
+				/* FIXME: Could/Should both calls be perfomed in a single step? */
+				chown(tempName, fst.st_uid, -1) != 0 ||
+				chown(tempName, -1 ,fst.st_gid) != 0
+			)
+				fprintf(stderr, "Warning: could not change file owner: %s (%s)\n", tempName, filename);
 			chmod(tempName, fst.st_mode);
 #endif
 			if (rename(tempName, filename) != 0) {