Unix has two similar concepts for reading and writing file-like objects. The first, file descriptors, is used by lower-level functions like read(2). The second, streams, is used by higher-level functions like fprintf(3). So what are the similarities and differences between them?

  • File descriptors are represented by objects of type int. Streams are represented by objects of type FILE *.

  • Streams themselves use file descriptors to access files. Streams are a layer on top of file descriptors to take care of some of the details of the most common kinds of I/O.

  • Both can represent connections to files, devices (e.g terminals), and pipes/sockets (for IPC).

  • Why use streams? Because using streams gives you access to a more powerful set of functions. Namely, formatted I/O functions like printf and scanf. These functions have provide two main benefits:
    • They save you from having to write ad-hoc parsers for reading and writing strings dynamically
    • They save you system calls by using buffered I/O
  • Why use file descriptors? They offer more control what and when you read and write:
    • They allow you to do control operations that are specific to particular devices
    • They let you use special modes of I/O, like nonblocking I/O. By contrast, the only control that streams give you is the choice of styles of buffering (unbuffered, line buffered, fully buffered).
  • You can always change your mind:
    • You can request that a streams give you the underlying file descriptor for low-level operations
    • You can create a stream from an existing a file descriptor

  • In general, try to use streams unless you have a reason to do otherwise

  • File descriptors are a POSIX feature, whereas streams are an ISO C feature. Therefore, streams are more portable than file descriptors. Non-POSIX systems are free to implement streams using something besides file descriptors under the hood. File descriptors live in the kernel, while streams live in C standard library.

  • File position is one of the attributes of a file. Therefore, streams and file descriptors (which both provide communication channels to files) both have file position as one of their attributes. File position on a stream can be changed with fseek(3), and file position on a file descriptor can be changed using lseek(2).
    • File position on a file descriptor is just an integer which is initialized to 0 and incremented with each written character.
    • Not all files support random-access (this is, the seek operation). You will receive the error ESPIPE (“error due to seeking on a pipe [or socket or FIFO]”).