What causes "need even-number of ascii chars for byte-stream: (offset: %08lx, %s, %ld)\n" in NationalSecurityAgency/ghidra?

Make the hex string length even by removing the trailing nibble or padding with a leading '0'.

need even-number of ascii chars for byte-stream: (offset: %0

Error message

need even-number of ascii chars for byte-stream: (offset: %08lx, %s, %ld)\n

What it means

gdis validation error: the ascii byte-string has an odd number of characters, so it cannot be decoded into whole bytes (each byte = 2 hex chars). The program prints the offset, the buffer, and its length, then calls exit(-1).

Source

Thrown at GPL/GnuDisassembler/src/gdis/c/disasm_1.c:376

			}
			else {
				fprintf(stderr, "exiting, no ASCII hex found\n");
				return 0; // finished! #TODO
			}
		}
		else {
			if(strlen(byteString) > BYTE_BUFFER_SIZE*2) {
				fprintf(stderr, "Max ascii string size is %d you provided: %lu chars. Exiting.\n", BYTE_BUFFER_SIZE*2,
						strlen(byteString));
				exit(-1);
			}
			strncpy(byteStringBuffer, byteString, BYTE_BUFFER_SIZE*2);
			stdin_mode = 0; // break out of the while loop
		}

		int size = strlen(byteStringBuffer);
		if((size % 2) != 0){
			fprintf(stderr, "need even-number of ascii chars for byte-stream: (offset: %08lx, %s, %ld)\n", offset, byteStringBuffer, strlen(byteStringBuffer));
			exit(-1);
		}

		memset(byteBuffer, 0x00, BYTE_BUFFER_SIZE);

		//
		// TODO:
		// check to make sure chars are only valid HEX.
		//
		int i, j;
		for(i=j=0; (i < size) && (j < BYTE_BUFFER_SIZE); i+=2, j++){
			tmp[0] = byteStringBuffer[i];
			tmp[1] = byteStringBuffer[i+1];
			tmp[2] = 0;
			sscanf(tmp, "%02X", &byteValue);
			byteBuffer[j] = (unsigned char)byteValue;
		}

View on GitHub (pinned to d5f144c24d)

Solutions

Make the hex string length even by removing the trailing nibble or padding with a leading '0'.
Validate `strlen(str) % 2 == 0` in the caller/wrapper before invoking gdis.
Regenerate the byte string from the original binary to avoid partial bytes.
Add a preflight check in your script that rejects odd-length input.

Example fix

// before
$ gdis ... 90901      // 5 chars -> odd

// after
$ gdis ... 9090         // drop stray nibble
// or
$ gdis ... 090901       // pad to even

Defensive patterns

Strategy: validation

Validate before calling

size_t n = strlen(byteString);
if (n % 2 != 0) {
  fprintf(stderr, "odd-length hex string (%zu)\n", n);
  return 2;
}

Prevention

Validate even length in the caller/wrapper.
Generate hex from raw bytes with a %02x formatter to avoid partial bytes.
Reject non-hex characters too (the source marks this as a TODO).

When it happens

Trigger: Passing/piping a hex string whose length is odd, e.g. '9090' is fine but '90901' (5 chars) triggers it.

Common situations: A stray nibble from a copy-paste; a hex-dump tool that emits a partial final byte; truncation of the string at a non-even boundary.

Related errors

AI-assisted analysis of NationalSecurityAgency/ghidra@d5f144c24d (2026-08-14). Data as JSON: /api/errors/4764b6f414483dd1. Report an issue: GitHub.