OpenBSD-4.6/lib/libc/stdio/fgets.3

.\"	$OpenBSD: fgets.3,v 1.29 2009/06/02 22:28:18 ray Exp $
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.Dd $Mdocdate: June 2 2009 $
.Dt FGETS 3
.Os
.Sh NAME
.Nm fgets ,
.Nm gets
.Nd get a line from a stream
.Sh SYNOPSIS
.Fd #include <stdio.h>
.Ft char *
.Fn fgets "char *str" "int size" "FILE *stream"
.Ft char *
.Fn gets "char *str"
.Sh DESCRIPTION
The
.Fn fgets
function reads at most
.Ar size Ns \-1
characters from the given
.Fa stream
and stores them in the string
.Fa str .
Reading stops when a newline character is found,
at end-of-file, or on error.
The newline, if any, is retained.
The string will be NUL-terminated if
.Fn fgets
succeeds; otherwise the contents of
.Fa str
are undefined.
.Pp
The
.Fn gets
function is equivalent to
.Fn fgets
with an infinite
.Ar size
and a
.Fa stream
of
.Em stdin ,
except that the newline character (if any) is not stored in the string.
It is the caller's responsibility to ensure that the input line,
if any, is sufficiently short to fit in the string.
.Sh RETURN VALUES
Upon successful completion,
.Fn fgets
and
.Fn gets
return
a pointer to the string.
If end-of-file or an error occurs before any characters are read,
they return
.Dv NULL .
The
.Fn fgets
and
.Fn gets
functions
do not distinguish between end-of-file and error, and callers must use
.Xr feof 3
and
.Xr ferror 3
to determine which occurred.
Whether
.Fn fgets
can possibly fail with a
.Ar size
argument of 1 is implementation-dependent.
On
.Ox ,
.Fn fgets
will never return
.Dv NULL
when
.Ar size
is 1.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EBADF
The given
.Fa stream
is not a readable stream.
.It Bq Er EINVAL
The given
.Fa size
is less than or equal to 0.
.El
.Pp
The function
.Fn fgets
may also fail and set
.Va errno
for any of the errors specified for the routines
.Xr fflush 3 ,
.Xr fstat 2 ,
.Xr read 2 ,
or
.Xr malloc 3 .
.Pp
The function
.Fn gets
may also fail and set
.Va errno
for any of the errors specified for the routine
.Xr getchar 3 .
.Sh SEE ALSO
.Xr feof 3 ,
.Xr ferror 3 ,
.Xr fgetln 3
.Sh STANDARDS
The functions
.Fn fgets
and
.Fn gets
conform to
.St -ansiC .
.Sh CAVEATS
The following bit of code illustrates a case where the programmer assumes a
string is too long if it does not contain a newline:
.Bd -literal -offset indent
char buf[1024], *p;

while (fgets(buf, sizeof(buf), fp) != NULL) {
	if ((p = strchr(buf, '\en')) == NULL) {
		fprintf(stderr, "input line too long.\en");
		exit(1);
	}
	*p = '\e0';
	printf("%s\en", buf);
}
.Ed
.Pp
While the error would be true if a line \*(Gt 1023 characters were read,
it would be false in two other cases:
.Bl -enum -offset indent
.It
If the last line in a file does not contain a newline, the string returned by
.Fn fgets
will not contain a newline either.
Thus
.Fn strchr
will return
.Dv NULL
and the program will terminate, even if the line was valid.
.It
All C string functions, including
.Fn strchr ,
correctly assume the end of the string is represented by a NUL
.Pq Sq \e0
character.
If the first character of a line returned by
.Fn fgets
were NUL,
.Fn strchr
would immediately return without considering the rest of the returned text
which may indeed include a newline.
.El
.Pp
Consider using
.Xr fgetln 3
instead when dealing with untrusted input.
.Pp
It is erroneous to assume that
.Fn fgets
never returns an empty string when successful.
If a line starts with the NUL character, fgets will store the NUL and
continue reading until it encounters a newline or end-of-file.
This will result in an empty string being returned.
The following bit of code illustrates a case where the programmer assumes
the string cannot be zero length.
.Bd -literal -offset indent
char buf[1024];

if (fgets(buf, sizeof(buf), fp) != NULL) {
	/* WRONG */
	if (buf[strlen(buf) - 1] == '\en')
		buf[strlen(buf) - 1] = '\e0';
}
.Ed
.Pp
If
.Fn strlen
returns 0, the index into the buffer becomes \-1.
One way to concisely and correctly trim a newline is shown below.
.Bd -literal -offset indent
char buf[1024];

if (fgets(buf, sizeof(buf), fp) != NULL)
	buf[strcspn(buf, "\en")] = '\e0';
.Ed
.Sh BUGS
Since it is usually impossible to ensure that the next input line
is less than some arbitrary length, and because overflowing the
input buffer is almost invariably a security violation, programs
should
.Em NEVER
use
.Fn gets .
The
.Fn gets
function exists purely to conform to
.St -ansiC .