Trying this simple change...

Author: EricEngle-NOAA

src/g2cio.c

@@ -41,7 +41,7 @@ int
 g2c_file_io(FILE *f, int write, int g2ctype, void *var)
 {
     void *void_be;
-    char *bvar = NULL;
+    signed char *bvar = NULL;
     short *svar = NULL;
     int *ivar = NULL;
     long long int *i64var = NULL;
@@ -64,7 +64,15 @@ g2c_file_io(FILE *f, int write, int g2ctype, void *var)
         void_be = &byte_be;
         if (write)
         {
-            byte_tmp = *bvar;
+            /* Are we writing a negative number? */
+            if (g2ctype == G2C_BYTE && *bvar < 0)
+            {
+                byte_tmp = -1 * *bvar;         /* Store as positive. */
+                byte_tmp |= 1UL << BITSHIFT_7; /* Set sign bit. */
+            }
+            else
+                byte_tmp = *bvar;
+
             /* Convert result to big-endian. */
             byte_be = byte_tmp;
         }
@@ -150,6 +158,13 @@ g2c_file_io(FILE *f, int write, int g2ctype, void *var)
         case G2C_UBYTE:
             /* No conversion needed for one-byte values. */
             *bvar = byte_be;
+
+            /* Did we read a negative number? Check the sign bit... */
+            if (g2ctype == G2C_BYTE && *bvar & 1 << BITSHIFT_7)
+            {
+                *bvar &= ~(1UL << BITSHIFT_7); /* Clear sign bit. */
+                *bvar *= -1;                   /* Make it negative. */
+            }
             break;
         case G2C_SHORT:
         case G2C_USHORT: