Buffered and Unbuffered inputs in C

Question

Is there any way to figure out whether the input is buffered or unbuffered (Apart from manual pages.)? Cant we figure out by looking at the names of the functions? Also for echoing and nonechoing... For quick reference where to find the list which has the details of Buffered, Unbuffered, echoing and nonechoing inputs?

Answer 1

All stdio.h functions for reading from a FILE may exhibit either "buffered" or "unbuffered" behavior, and either "echoing" or "non-echoing" behavior. What controls these things is not which function you use, but settings on the stream and/or its underlying file descriptor.

• The standard library function setvbuf can be used to enable or disable buffering of input (and output) by the C library. This has no effect on buffering by the operating system. There are three possible modes: "fully buffered" (read or write in substantial chunks); "line buffered" (buffer until a '\n' character is read or written, but not beyond that); and "unbuffered" (all reads and writes go to the OS immediately).

• The default buffering for new FILE objects (including stdin and friends) is implementation-defined. Unixy C libraries generally default all FILEs to fully buffered, with two exceptions. stderr defaults to unbuffered. For any other FILE, if setvbuf has not been used on it at the time of the first actual read or write, and isatty is true for the underlying file descriptor, then the FILE becomes line-buffered.

• Some C libraries provide extension functions, e.g. __flbf and friends on Linux and Solaris, for reading back some of the settings controlled by setvbuf. Keep in mind that, as described above, the buffering mode can change upon the first actual read or write, if it hasn't been explicitly set.

If input is from a file, setvbuf is the only knob you have. If input is from some sort of communication channel, there may be other knobs:

• On POSIX-conformant systems (read "everything except Windows"), a program can request any of several different modes for terminal input. Of these, the most important distinction to make is between "canonical" and "noncanonical". A terminal in canonical mode exhibits both buffering and echoing. (This buffering is separate from the buffering the C library may do if setvbuf has not been used to disable it.) Non-canonical mode allows you to toggle buffering and echoing separately. The low-level POSIX terminal interface is extensive, complicated, and allows you to both read and write all of these settings.

• If you think you want to put the terminal in a non-canonical mode, before writing a bunch of code against the low-level POSIX API for doing so, you should first consider whether the readline or ncurses library would make your life easier.

• If you are reading from a pipe, you are at the mercy of whoever is writing to it; you cannot control the size of the chunks you get.

• If you are reading from a socket, you may be able to exercise some control over the size of the chunks you get by careful use of recvmsg, but there are no guarantees.

• I don't know how it is on Windows.