Proc should have arguments and result that match the type Tcl_FileProc:
typedef void Tcl_FileProc( ClientData clientData, int mask);
The clientData parameter to proc is a copy of the clientData argument given to Tcl_CreateFileHandler when the callback was created. Typically, clientData points to a data structure containing application-specific information about the file. Mask is an integer mask indicating which of the requested conditions actually exists for the file; it will contain a subset of the bits in the mask argument to Tcl_CreateFileHandler.
There may exist only one handler for a given file at a given time. If Tcl_CreateFileHandler is called when a handler already exists for fd, then the new callback replaces the information that was previously recorded.
Tcl_DeleteFileHandler may be called to delete the file handler for fd; if no handler exists for the file given by fd then the procedure has no effect.
The purpose of file handlers is to enable an application to respond to events while waiting for files to become ready for I/O. For this to work correctly, the application may need to use non-blocking I/O operations on the files for which handlers are declared. Otherwise the application may block if it reads or writes too much data; while waiting for the I/O to complete the application will not be able to service other events. Use Tcl_SetChannelOption with -blocking to set the channel into blocking or nonblocking mode as required.
Note that these interfaces are only supported by the Unix implementation of the Tcl notifier.