Tex. Code Crim. Proc. art. 24.18
When a subpoena is returnable forthwith, the officer shall immediately serve the witness with a copy of the same; and it shall be the duty of said witness to immediately make his appearance before the court, magistrate or other authority issuing the same. If said witness makes affidavit of his inability from lack of funds to appear in obedience to said subpoena, the officer executing the same shall provide said witness, if said subpoena be issued as provided in Article 24.16, with the necessary funds or means to appear in obedience to said subpoena, taking his receipt therefor, and showing in his return on said subpoena, under oath, the amount furnished to said witness, together with the amount of his fees for executing said subpoena.
Tex. Code Crim. Proc. § 24.18