GNU Info
(libc.info)Line Input
Line-Oriented Input
===================
Since many programs interpret input on the basis of lines, it is
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 without any bytes
read, `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'.
- Function: wchar_t * fgetws (wchar_t *WS, int COUNT, FILE *STREAM)
The `fgetws' function reads wide characters from the stream STREAM
up to and including a newline character and stores them in the
string WS, adding a null wide character to mark the end of the
string. You must supply COUNT wide characters worth of space in
WS, but the number of characters read is at most COUNT - 1. The
extra character space is used to hold the null wide character at
the end of the string.
If the system is already at end of file when you call `fgetws',
then the contents of the array WS 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 WS.
*Warning:* If the input data has a null wide character (which are
null bytes in the input stream), you can't tell. So don't use
`fgetws' 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.
- Function: char * fgets_unlocked (char *S, int COUNT, FILE *STREAM)
The `fgets_unlocked' function is equivalent to the `fgets'
function except that it does not implicitly lock the stream.
This function is a GNU extension.
- Function: wchar_t * fgetws_unlocked (wchar_t *WS, int COUNT, FILE
*STREAM)
The `fgetws_unlocked' function is equivalent to the `fgetws'
function except that it does not implicitly lock the stream.
This function is a GNU extension.
- 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'.
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