In general, omitting a function means that any attempt to perform the associated operation on the resulting stream will fail. If the close function is omitted, closing the stream will flush any buffered output and then succeed.
The calling conventions of Fa readfn , Fa writefn , Fa seekfn and Fa closefn must match those, respectively, of read(2), write(2), lseek(2), and close(2) with the single exception that they are passed the Fa cookie argument specified to Fn funopen in place of the traditional file descriptor argument.
Read and write I/O functions are allowed to change the underlying buffer on fully buffered or line buffered streams by calling setvbuf(3). They are also not required to completely fill or empty the buffer. They are not, however, allowed to change streams from unbuffered to buffered or to change the state of the line buffering flag. They must also be prepared to have read or write calls occur on buffers other than the one most recently specified.
All user I/O functions can report an error by returning -1. Additionally, all of the functions should set the external variable errno appropriately if an error occurs.
An error on Fn closefn does not keep the stream open.
On Fx , Ox and Dx the Fn funopen interface erroneously assumes that Vt fpos_t is an integral type, and uses it in the Fa seekfn hook; but because code using a Fa seekfn hook will fail to build on systems where Vt fpos_t is a struct, and it will need to be slightly fixed anyway, the implementation provided by libbsd (in the same way as Nx ) uses the correct Vt off_t types.