(libc.info.gz) File Position

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(libc.info.gz) Streams and File Descriptors

(libc.info.gz) I/O Concepts

 
 11.1.2 File Position
 --------------------
 
 One of the attributes of an open file is its "file position" that keeps
 track of where in the file the next character is to be read or written.
 On GNU systems, and all POSIX.1 systems, the file position is simply an
 integer representing the number of bytes from the beginning of the file.
 
    The file position is normally set to the beginning of the file when
 it is opened, and each time a character is read or written, the file
 position is incremented.  In other words, access to the file is normally
 "sequential".
 
    Ordinary files permit read or write operations at any position within
 the file.  Some other kinds of files may also permit this.  Files which
 do permit this are sometimes referred to as "random-access" files.  You
 can change the file position using the 'fseek' function on a stream
 ( File Positioning) or the 'lseek' function on a file descriptor
 ( I/O Primitives).  If you try to change the file position on a
 file that doesn't support random access, you get the 'ESPIPE' error.
 
    Streams and descriptors that are opened for "append access" are
 treated specially for output: output to such files is _always_ appended
 sequentially to the _end_ of the file, regardless of the file position.
 However, the file position is still used to control where in the file
 reading is done.
 
    If you think about it, you'll realize that several programs can read
 a given file at the same time.  In order for each program to be able to
 read the file at its own pace, each program must have its own file
 pointer, which is not affected by anything the other programs do.
 
    In fact, each opening of a file creates a separate file position.
 Thus, if you open a file twice even in the same program, you get two
 streams or descriptors with independent file positions.
 
    By contrast, if you open a descriptor and then duplicate it to get
 another descriptor, these two descriptors share the same file position:
 changing the file position of one descriptor will affect the other.
 

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(libc.info.gz) Streams and File Descriptors

(libc.info.gz) I/O Concepts