read_instr.c: Report offset on invalid instruction

This should help with troubleshooting.

1 files changed, 10 insertions, 4 deletions

diff --git a/read_instr.c b/read_instr.c
index 7f7d2a6..907069d 100644
--- a/read_instr.c
+++ b/read_instr.c
@@ -642,10 +642,16 @@ int read_instr(FILE *const f, struct parse *const p)
 
     for (;;)
     {
-        const size_t n = fread(&op, sizeof op, 1, f);
+        size_t n;
         varuint32 relative_depth;
+        const long offset = ftell(f);
 
-        if (!n)
+        if (offset < 0)
+        {
+            fprintf(stderr, "%s: ftell(3): %s\n", __func__, strerror(errno));
+            goto failure;
+        }
+        else if (!(n = fread(&op, sizeof op, 1, f)))
         {
             fprintf(stderr, "%s: fread(3) failed, ferror(3)=%d, feof(3)=%d\n",
                 __func__, ferror(f), feof(f));
@@ -653,8 +659,8 @@ int read_instr(FILE *const f, struct parse *const p)
         }
         else if (op >= sizeof table / sizeof *table || !table[op])
         {
-            fprintf(stderr, "%s: invalid or unsupported opcode %#" PRIx8 "\n",
-                __func__, op);
+            fprintf(stderr, "%s: invalid or unsupported opcode %#" PRIx8 ", "
+                "offset=%#lx\n", __func__, op, offset);
             goto failure;
         }
         else if (table[op](f, &relative_depth))