What causes "Comment text may not exceed %u bytes\n" in DrKLO/Telegram?

Limit comment source to under 65000 bytes before feeding it in.

Comment text may not exceed %u bytes\n

Error message

Comment text may not exceed %u bytes\n

What it means

If no -comment and no -cfile was supplied, wrjpgcom reads the comment text from comment_file (or stdin) byte-by-byte into a MAX_COM_LENGTH (65000) buffer. If the loop reaches comment_length >= 65000 before EOF, it prints this message and exits EXIT_FAILURE. This bounds the JPEG COM marker payload to the JPEG spec ceiling.

Source

Thrown at TMessagesProj/jni/mozjpeg/wrjpgcom.c:556

  }
#else
  outfile = stdout;
#endif
#endif /* TWO_FILE_COMMANDLINE */

  /* Collect comment text from comment_file or stdin, if necessary */
  if (comment_arg == NULL) {
    FILE *src_file;
    int c;

    comment_arg = (char *)malloc((size_t)MAX_COM_LENGTH);
    if (comment_arg == NULL)
      ERREXIT("Insufficient memory");
    comment_length = 0;
    src_file = (comment_file != NULL ? comment_file : stdin);
    while ((c = getc(src_file)) != EOF) {
      if (comment_length >= (unsigned int)MAX_COM_LENGTH) {
        fprintf(stderr, "Comment text may not exceed %u bytes\n",
                (unsigned int)MAX_COM_LENGTH);
        exit(EXIT_FAILURE);
      }
      comment_arg[comment_length++] = (char)c;
    }
    if (comment_file != NULL)
      fclose(comment_file);
  }

  /* Copy JPEG headers until SOFn marker;
   * we will insert the new comment marker just before SOFn.
   * This (a) causes the new comment to appear after, rather than before,
   * existing comments; and (b) ensures that comments come after any JFIF
   * or JFXX markers, as required by the JFIF specification.
   */
  marker = scan_JPEG_header(keep_COM);
  /* Insert the new COM marker, but only if nonempty text has been supplied */
  if (comment_length > 0) {

View on GitHub (pinned to 45ab8f4308)

Solutions

Limit comment source to under 65000 bytes before feeding it in.
Pre-truncate the input: `head -c 64999 comment.txt | wrjpgcom ...`.
If a larger comment is truly needed, note JPEG COM markers cannot exceed ~65533 bytes; split across multiple COM markers via custom tooling instead.

Example fix

// before
wrjpgcom -cfile big.txt photo.jpg   # big.txt > 65000 bytes
// after
head -c 65000 big.txt > /tmp/c.txt && wrjpgcom -cfile /tmp/c.txt photo.jpg

Defensive patterns

Strategy: validation

Validate before calling

#define MAX_COM_LENGTH 65000L
/* bound the comment source fed via stdin/comment_file */
if (source_byte_count >= (size_t)MAX_COM_LENGTH) {
    /* truncate or reject before feeding wrjpgcom */
}

Prevention

Pre-truncate comment input to < 65000 bytes (e.g. head -c 64999).
Check the source file size before redirecting it as comment input.
Remember JPEG COM markers are hard-capped near 64KB.

When it happens

Trigger: Running `wrjpgcom [input]` (reading comment from stdin/comment_file) and feeding more than 65000 bytes of comment text.

Common situations: Redirecting a large text file or pipe into wrjpgcom's comment input without a size check, or an interactive session pasting a very long comment.

Related errors

AI-assisted analysis of DrKLO/Telegram@45ab8f4308 (2026-08-14). Data as JSON: /api/errors/828bd41061c941ec. Report an issue: GitHub.