File: libc.info,  Node: Line Input,  Next: Unreading,  Prev: Character Input,  \Up: I/O on Streams

Line-Oriented Input
===================

   Since many programs interpret input on the basis of lines, it's
convenient to have functions to read a line of text from a stream.

   Standard C has functions to do this, but they aren't very safe: null
characters and even (for `gets') long lines can confuse them.  So the
GNU library provides the nonstandard `getline' function that makes it
easy to read lines reliably.

   Another GNU extension, `getdelim', generalizes `getline'.  It reads
a delimited record, defined as everything through the next occurrence
of a specified delimiter character.

   All these functions are declared in `stdio.h'.

 - Function: ssize_t getline (char **LINEPTR, size_t *N, FILE *STREAM)
     This function reads an entire line from STREAM, storing the text
     (including the newline and a terminating null character) in a
     buffer and storing the buffer address in `*LINEPTR'.

     Before calling `getline', you should place in `*LINEPTR' the
     address of a buffer `*N' bytes long, allocated with `malloc'.  If
     this buffer is long enough to hold the line, `getline' stores the
     line in this buffer.  Otherwise, `getline' makes the buffer bigger
     using `realloc', storing the new buffer address back in `*LINEPTR'
     and the increased size back in `*N'.  *Note Unconstrained
     Allocation::.

     If you set `*LINEPTR' to a null pointer, and `*N' to zero, before
     the call, then `getline' allocates the initial buffer for you by
     calling `malloc'.

     In either case, when `getline' returns,  `*LINEPTR' is a `char *'
     which points to the text of the line.

     When `getline' is successful, it returns the number of characters
     read (including the newline, but not including the terminating
     null).  This value enables you to distinguish null characters that
     are part of the line from the null character inserted as a
     terminator.

     This function is a GNU extension, but it is the recommended way to
     read lines from a stream.  The alternative standard functions are
     unreliable.

     If an error occurs or end of file is reached, `getline' returns
     `-1'.

 - Function: ssize_t getdelim (char **LINEPTR, size_t *N, int
          DELIMITER, FILE *STREAM)
     This function is like `getline' except that the character which
     tells it to stop reading is not necessarily newline.  The argument
     DELIMITER specifies the delimiter character; `getdelim' keeps
     reading until it sees that character (or end of file).

     The text is stored in LINEPTR, including the delimiter character
     and a terminating null.  Like `getline', `getdelim' makes LINEPTR
     bigger if it isn't big enough.

     `getline' is in fact implemented in terms of `getdelim', just like
     this:

          ssize_t
          getline (char **lineptr, size_t *n, FILE *stream)
          {
            return getdelim (lineptr, n, '\n', stream);
          }

 - Function: char * fgets (char *S, int COUNT, FILE *STREAM)
     The `fgets' function reads characters from the stream STREAM up to
     and including a newline character and stores them in the string S,
     adding a null character to mark the end of the string.  You must
     supply COUNT characters worth of space in S, but the number of
     characters read is at most COUNT - 1.  The extra character space
     is used to hold the null character at the end of the string.

     If the system is already at end of file when you call `fgets', then
     the contents of the array S are unchanged and a null pointer is
     returned.  A null pointer is also returned if a read error occurs.
     Otherwise, the return value is the pointer S.


     *Warning:*  If the input data has a null character, you can't tell.
     So don't use `fgets' unless you know the data cannot contain a
     null.  Don't use it to read files edited by the user because, if
     the user inserts a null character, you should either handle it
     properly or print a clear error message.  We recommend using
     `getline' instead of `fgets'.

 - Deprecated function: char * gets (char *S)
     The function `gets' reads characters from the stream `stdin' up to
     the next newline character, and stores them in the string S.  The
     newline character is discarded (note that this differs from the
     behavior of `fgets', which copies the newline character into the
     string).  If `gets' encounters a read error or end-of-file, it
     returns a null pointer; otherwise it returns S.

     *Warning:* The `gets' function is *very dangerous* because it
     provides no protection against overflowing the string S.  The GNU
     library includes it for compatibility only.  You should *always*
     use `fgets' or `getline' instead.  To remind you of this, the
     linker (if using GNU `ld') will issue a warning whenever you use
     `gets'.

