int Tcl_GetOpenFile(interp, string, write, checkUsage, filePtr)
Tcl_GetOpenFile takes as argument a file identifier of the form returned by the open command and returns at *filePtr a pointer to the FILE structure for the file. The write argument indicates whether the FILE pointer will be used for reading or writing. In some cases, such as a channel that connects to a pipeline of subprocesses, different FILE pointers will be returned for reading and writing. Tcl_GetOpenFile normally returns TCL_OK. If an error occurs in Tcl_GetOpenFile (e.g. string didn't make any sense or checkUsage was set and the file wasn't opened for the access specified by write) then TCL_ERROR is returned and the interpreter's result will contain an error message. In the current implementation checkUsage is ignored and consistency checks are always performed.
Note that this interface is only supported on the Unix platform.