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1 .\" $NetBSD: fgets.3,v 1.18 2003/10/30 12:59:23 grant Exp $
2 .\"
3 .\" Copyright (c) 1990, 1991, 1993
4 .\" The Regents of the University of California. All rights reserved.
5 .\"
6 .\" This code is derived from software contributed to Berkeley by
7 .\" Chris Torek and the American National Standards Committee X3,
8 .\" on Information Processing Systems.
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34 .\" @(#)fgets.3 8.1 (Berkeley) 6/4/93
35 .\"
36 .Dd June 4, 1993
37 .Dt FGETS 3
38 .Os
39 .Sh NAME
40 .Nm fgets ,
41 .Nm gets
42 .Nd get a line from a stream
43 .Sh LIBRARY
44 .Lb libc
45 .Sh SYNOPSIS
46 .In stdio.h
47 .Ft char *
48 .Fn fgets "char * restrict str" "int size" "FILE * restrict stream"
49 .Ft char *
50 .Fn gets "char *str"
51 .Sh DESCRIPTION
52 The
53 .Fn fgets
54 function
55 reads at most one less than the number of characters specified by
56 .Fa size
57 from the given
58 .Fa stream
59 and stores them in the string
60 .Fa str .
61 Reading stops when a newline character is found,
62 at end-of-file or error.
63 The newline, if any, is retained, and a
64 .Ql \e0
65 character is appended to end the string.
66 .Pp
67 The
68 .Fn gets
69 function
70 is equivalent to
71 .Fn fgets
72 with an infinite
73 .Fa size
74 and a
75 .Fa stream
76 of
77 .Em stdin ,
78 except that the newline character (if any) is not stored in the string.
79 It is the caller's responsibility to ensure that the input line,
80 if any, is sufficiently short to fit in the string.
81 .Sh RETURN VALUES
82 Upon successful completion,
83 .Fn fgets
84 and
85 .Fn gets
86 return
87 a pointer to the string.
88 If end-of-file or an error occurs before any characters are read,
89 they return
90 .Dv NULL .
91 The
92 .Fn fgets
93 and
94 .Fn gets
95 functions
96 do not distinguish between end-of-file and error, and callers must use
97 .Xr feof 3
98 and
99 .Xr ferror 3
100 to determine which occurred.
101 .Sh ERRORS
102 .Bl -tag -width Er
103 .It Bq Er EBADF
104 The given
105 .Fa stream
106 is not a readable stream.
107 .El
108 .Pp
109 The function
110 .Fn fgets
111 may also fail and set
112 .Va errno
113 for any of the errors specified for the routines
114 .Xr fflush 3 ,
115 .Xr fstat 2 ,
116 .Xr read 2 ,
117 or
118 .Xr malloc 3 .
119 .Pp
120 The function
121 .Fn gets
122 may also fail and set
123 .Va errno
124 for any of the errors specified for the routine
125 .Xr getchar 3 .
126 .Sh SEE ALSO
127 .Xr feof 3 ,
128 .Xr ferror 3 ,
129 .Xr fgetln 3
130 .Sh STANDARDS
131 The functions
132 .Fn fgets
133 and
134 .Fn gets
135 conform to
136 .St -ansiC .
137 .Sh CAVEATS
138 The following bit of code illustrates a case where the programmer assumes a
139 string is too long if it does not contain a newline:
140 .Bd -literal
141 char buf[1024], *p;
143 while (fgets(buf, sizeof(buf), fp) != NULL) {
144 if ((p = strchr(buf, '\en')) == NULL) {
145 fprintf(stderr, "input line too long.\en");
146 exit(1);
147 }
148 *p = '\e0';
149 printf("%s\en", buf);
150 }
151 .Ed
152 .Pp
153 While the error would be true if a line > 1023 characters were read, it would
154 be false in two other cases:
155 .Bl -enum -offset indent
156 .It
157 If the last line in a file does not contain a newline, the string returned by
158 .Fn fgets
159 will not contain a newline either.
160 Thus
161 .Fn strchr
162 will return
163 .Dv NULL
164 and the program will terminate, even if the line was valid.
165 .It
166 All C string functions, including
167 .Fn strchr ,
168 correctly assume the end of the string is represented by a null
169 .Pq Sq \e0
170 character.
171 If the first character of a line returned by
172 .Fn fgets
173 were null,
174 .Fn strchr
175 would immediately return without considering the rest of the returned text
176 which may indeed include a newline.
177 .El
178 .Pp
179 Consider using
180 .Xr fgetln 3
181 instead when dealing with untrusted input.
182 .Sh SECURITY CONSIDERATIONS
183 Since it is usually impossible to ensure that the next input line
184 is less than some arbitrary length, and because overflowing the
185 input buffer is almost invariably a security violation, programs
186 should
187 .Em NEVER
188 use
189 .Fn gets .
190 The
191 .Fn gets
192 function
193 exists purely to conform to
194 .St -ansiC .