Parameterize the number of random bytes

Allow to specify the number of random bytes to obtain from a single
parameter on the command line.

src/scdrand.c

@@ -45,7 +45,15 @@
 static void
 usage(int status)
 {
-    puts("Usage: scdrand\n");
+    printf("Usage: scdrand [options] [BYTES]\n\
+Read the specified number of random bytes from a smartcard\n\
+and feed them to the kernel entropy pool. If BYTES is not\n\
+specified, the default is %d bytes.\n\n", DEFAULT_RANDOM_BYTES);
+
+    puts("Options:\n\
+  -h, --help            Display this help message.\n\
+  -v, --version         Display the version message.\n\
+");
 
     printf("Report bugs to <%s>.\n", PACKAGE_BUGREPORT);
 
@@ -278,10 +286,25 @@ start_privileged_process(void)
     return 0;   /* Not reached. */
 }
 
+static int
+get_uinteger_or_die(const char *arg)
+{
+    unsigned val;
+    char *endptr;
+
+    errno = 0;
+    val = strtoul(arg, &endptr, 10);
+    if ( errno != 0 || endptr == arg )
+        errx(EXIT_FAILURE, "Invalid argument, unsigned integer expected: %s", arg);
+
+    return val;
+}
+
 int
 main(int argc, char **argv)
 {
     int c, fd;
+    unsigned nbytes;
     char *scd_socket_name;
     unsigned char random_buffer[MAX_RANDOM_BYTES];
 
@@ -292,6 +315,7 @@ main(int argc, char **argv)
     };
 
     setprogname(argv[0]);
+    nbytes = DEFAULT_RANDOM_BYTES;
 
     while ( (c = getopt_long(argc, argv, "hv", options, NULL)) != -1 ) {
         switch ( c ) {
@@ -311,6 +335,16 @@ main(int argc, char **argv)
 
     fd = start_privileged_process();
 
+    if ( optind < argc ) {
+        nbytes = get_uinteger_or_die(argv[optind++]);
+
+        if ( nbytes == 0 )  /* Funny guy? */
+            exit(EXIT_FAILURE);
+
+        if ( nbytes > MAX_RANDOM_BYTES )
+            nbytes = MAX_RANDOM_BYTES;
+    }
+
 #ifndef GPG_ERR_INITIALIZED
     gpg_err_init();
 #endif
@@ -320,7 +354,7 @@ main(int argc, char **argv)
     if ( scd_socket_name ) {
         int n, i;
 
-        n = get_challenge(scd_socket_name, random_buffer, DEFAULT_RANDOM_BYTES);
+        n = get_challenge(scd_socket_name, random_buffer, nbytes);
         write(fd, random_buffer, n);
 
         free(scd_socket_name);