On Tue, Sep 06, 2022 at 09:38:59AM +0100, Tom Jones wrote:
> +			rc = fprintf(dest, ".\n");
> +			if (rc != DIFF_RC_OK)
> +				return rc;

DIFF_RC_OK is not a valid return code of fprintf.

> +			if (outinfo) {
> +				ARRAYLIST_ADD(offp, outinfo->line_offsets);
> +				if (offp == NULL)
> +					return ENOMEM;
> +				outoff += rc;
> +				*offp = *outoff;
> +			}
> +		}
> +		if (rc)
> +			return rc;

The check for success (rc < 0 means error) should happen earlier, before
the new line offset is computed based on the return code (amount of bytes
written to the file if not < 0).