improve error message

jpeg2png.c

@@ -20,10 +20,12 @@ int main(int argc, char *argv[]) {
                 die("usage: jpeg2png in.jpg out.png");
         }
 
-        FILE *in = fopen(argv[1], "rb");
-        if(!in) { die(NULL); }
-        FILE *out = fopen(argv[2], "wb");
-        if(!out) { die(NULL); }
+        const char *infilename = argv[1];
+        const char *outfilename = argv[2];
+        FILE *in = fopen(infilename, "rb");
+        if(!in) { die_perror("could not open input file `%s`", infilename); }
+        FILE *out = fopen(outfilename, "wb");
+        if(!out) { die_perror("could not open output file `%s`", outfilename); }
 
         struct jpeg jpeg;
         read_jpeg(in, &jpeg);

utils.c

@@ -5,16 +5,23 @@
 #include "utils.h"
 
 noreturn void die(const char *msg, ...)  {
-        if(msg) {
-                fprintf(stderr, "jpeg2png: ");
-                va_list l;
-                va_start(l, msg);
-                vfprintf(stderr, msg, l);
-                va_end(l);
-                fprintf(stderr, "\n");
-        } else {
-                perror("jpeg2png");
-        }
+        fprintf(stderr, "jpeg2png: ");
+        va_list l;
+        va_start(l, msg);
+        vfprintf(stderr, msg, l);
+        va_end(l);
+        fprintf(stderr, "\n");
+        exit(EXIT_FAILURE);
+}
+
+noreturn void die_perror(const char *msg, ...)  {
+        fprintf(stderr, "jpeg2png: ");
+        va_list l;
+        va_start(l, msg);
+        vfprintf(stderr, msg, l);
+        va_end(l);
+        fprintf(stderr, ": ");
+        perror(NULL);
         exit(EXIT_FAILURE);
 }
 

utils.h

@@ -7,6 +7,7 @@
 #include <math.h>
 
 noreturn void die(const char *msg, ...);
+noreturn void die_perror(const char *msg, ...);
 clock_t start_timer(const char *name);
 void stop_timer(clock_t t, const char *n);