xref: /freebsd/lib/libc/stdio/fgets.3 (revision 3ad12d2782d19864056a04ab3676ba008003b397)

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.\"     @(#)fgets.3	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd April 3, 2018
.Dt FGETS 3
.Os
.Sh NAME
.Nm fgets ,
.Nm gets ,
.Nm gets_s
.Nd get a line from a stream
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdio.h
.Ft char *
.Fn fgets "char * restrict str" "int size" "FILE * restrict stream"
.Ft char *
.Fn gets_s "char *str" "rsize_t size"
.Ft char *
.Fn gets "char *str"
.Sh DESCRIPTION
The
.Fn fgets
function
reads at most one less than the number of characters specified by
.Fa size
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 error.
The newline, if any, is retained.
If any characters are read and there is no error, a
.Ql \e0
character is appended to end the string.
.Pp
The
.Fn gets_s
function
is equivalent to
.Fn fgets
with a
.Fa stream
of
.Dv stdin ,
except that the newline character (if any) is not stored in the string.
.Pp
The
.Fn gets
function
is equivalent to
.Fn fgets
with an infinite
.Fa size
and a
.Fa stream
of
.Dv 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 ,
.Fn gets_s ,
and
.Fn gets
return
a pointer to the string.
If end-of-file occurs before any characters are read,
they return
.Dv NULL
and the buffer contents remain unchanged.
If an error occurs,
they return
.Dv NULL
and the buffer contents are indeterminate.
The
.Fn fgets ,
.Fn gets_s ,
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.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EBADF
The given
.Fa stream
is not a readable stream.
.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
and
.Fn gets_s
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 ,
.Xr fgetws 3 ,
.Xr getline 3
.Sh STANDARDS
The functions
.Fn fgets
and
.Fn gets
conform to
.St -isoC-99 .
.Fn gets_s
conforms to
.St -isoC-2011
K.3.7.4.1.
.Fn gets
has been removed from
.St -isoC-2011 .
.Sh SECURITY CONSIDERATIONS
The
.Fn gets
function cannot be used securely.
Because of its lack of bounds checking,
and the inability for the calling program
to reliably determine the length of the next incoming line,
the use of this function enables malicious users
to arbitrarily change a running program's functionality through
a buffer overflow attack.
It is strongly suggested that the
.Fn fgets
function be used in all cases.