xref: /freebsd/lib/libc/stdio/fgetln.3 (revision dc36d6f9bb1753f3808552f3afd30eda9a7b206a)

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.Dd June 11, 2020
.Dt FGETLN 3
.Os
.Sh NAME
.Nm fgetln
.Nd get a line from a stream
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft char *
.Fn fgetln "FILE *stream" "size_t *len"
.Sh DESCRIPTION
The
.Fn fgetln
function
returns a pointer to the next line from the stream referenced by
.Fa stream .
This line is
.Em not
a C string as it does not end with a terminating
.Dv NUL
character.
The length of the line, including the final newline,
is stored in the memory location to which
.Fa len
points.
(Note, however, that if the line is the last
in a file that does not end in a newline,
the returned text will not contain a newline.)
.Sh RETURN VALUES
Upon successful completion a pointer is returned;
this pointer becomes invalid after the next
.Tn I/O
operation on
.Fa stream
(whether successful or not)
or as soon as the stream is closed.
Otherwise,
.Dv NULL
is returned.
The
.Fn fgetln
function
does not distinguish between end-of-file and error; the routines
.Xr feof 3
and
.Xr ferror 3
must be used
to determine which occurred.
If an error occurs, the global variable
.Va errno
is set to indicate the error.
The end-of-file condition is remembered, even on a terminal, and all
subsequent attempts to read will return
.Dv NULL
until the condition is
cleared with
.Xr clearerr 3 .
.Pp
The text to which the returned pointer points may be modified,
provided that no changes are made beyond the returned size.
These changes are lost as soon as the pointer becomes invalid.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EBADF
The argument
.Fa stream
is not a stream open for reading.
.It Bq Er ENOMEM
The internal line buffer could not be expanded due to lack of available memory,
or because it would need to expand beyond INT_MAX in size.
.El
.Pp
The
.Fn fgetln
function
may also fail and set
.Va errno
for any of the errors specified for the routines
.Xr fflush 3 ,
.Xr malloc 3 ,
.Xr read 2 ,
.Xr stat 2 ,
or
.Xr realloc 3 .
.Sh SEE ALSO
.Xr ferror 3 ,
.Xr fgets 3 ,
.Xr fgetwln 3 ,
.Xr fopen 3 ,
.Xr getline 3 ,
.Xr putc 3
.Sh HISTORY
The
.Fn fgetln
function first appeared in
.Bx 4.4 .
.Sh CAVEATS
Since the returned buffer is not a C string (it is not NUL terminated), a
common practice is to replace the newline character with
.Sq \e0 .
However, if the last line in a file does not contain a newline,
the returned text won't contain a newline either.
The following code demonstrates how to deal with this problem by allocating a
temporary buffer:
.Bd -literal
	char *buf, *lbuf;
	size_t len;

	lbuf = NULL;
	while ((buf = fgetln(fp, &len)) != NULL) {
		if (buf[len - 1] == '\en')
			buf[len - 1] = '\e0';
		else {
			/* EOF without EOL, copy and add the NUL */
			if ((lbuf = malloc(len + 1)) == NULL)
				err(1, NULL);
			memcpy(lbuf, buf, len);
			lbuf[len] = '\e0';
			buf = lbuf;
		}
		printf("%s\en", buf);
	}
	free(lbuf);
	if (ferror(fp))
		err(1, "fgetln");
.Ed