(libc.info.gz) Descriptors and Streams

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 13.4 Descriptors and Streams
 ============================
 
 Given an open file descriptor, you can create a stream for it with the
 'fdopen' function.  You can get the underlying file descriptor for an
 existing stream with the 'fileno' function.  These functions are
 declared in the header file 'stdio.h'.
 
  -- Function: FILE * fdopen (int FILEDES, const char *OPENTYPE)
      Preliminary: | MT-Safe | AS-Unsafe heap lock | AC-Unsafe mem lock |
       POSIX Safety Concepts.
 
      The 'fdopen' function returns a new stream for the file descriptor
      FILEDES.
 
      The OPENTYPE argument is interpreted in the same way as for the
      'fopen' function ( Opening Streams), except that the 'b'
      option is not permitted; this is because GNU systems make no
      distinction between text and binary files.  Also, '"w"' and '"w+"'
      do not cause truncation of the file; these have an effect only when
      opening a file, and in this case the file has already been opened.
      You must make sure that the OPENTYPE argument matches the actual
      mode of the open file descriptor.
 
      The return value is the new stream.  If the stream cannot be
      created (for example, if the modes for the file indicated by the
      file descriptor do not permit the access specified by the OPENTYPE
      argument), a null pointer is returned instead.
 
      In some other systems, 'fdopen' may fail to detect that the modes
      for file descriptor do not permit the access specified by
      'opentype'.  The GNU C Library always checks for this.
 
    For an example showing the use of the 'fdopen' function, see 
 Creating a Pipe.
 
  -- Function: int fileno (FILE *STREAM)
      Preliminary: | MT-Safe | AS-Safe | AC-Safe |  POSIX Safety
      Concepts.
 
      This function returns the file descriptor associated with the
      stream STREAM.  If an error is detected (for example, if the STREAM
      is not valid) or if STREAM does not do I/O to a file, 'fileno'
      returns -1.
 
  -- Function: int fileno_unlocked (FILE *STREAM)
      Preliminary: | MT-Safe | AS-Safe | AC-Safe |  POSIX Safety
      Concepts.
 
      The 'fileno_unlocked' function is equivalent to the 'fileno'
      function except that it does not implicitly lock the stream if the
      state is 'FSETLOCKING_INTERNAL'.
 
      This function is a GNU extension.
 
    There are also symbolic constants defined in 'unistd.h' for the file
 descriptors belonging to the standard streams 'stdin', 'stdout', and
 'stderr'; see  Standard Streams.
 
 'STDIN_FILENO'
      This macro has value '0', which is the file descriptor for standard
      input.
 
 'STDOUT_FILENO'
      This macro has value '1', which is the file descriptor for standard
      output.
 
 'STDERR_FILENO'
      This macro has value '2', which is the file descriptor for standard
      error output.
 

Info Catalog

(libc.info.gz) File Position Primitive

(libc.info.gz) Low-Level I/O

(libc.info.gz) Stream/Descriptor Precautions