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1 .\" $NetBSD: fgets.3,v 1.22 2010/05/13 18:38:24 jruoho Exp $
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34 .\" @(#)fgets.3 8.1 (Berkeley) 6/4/93
35 .\"
36 .Dd May 13, 2010
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 and
138 .St -p1003.1-2001 .
139 The
140 .St -p1003.1-2008
141 revision marked
142 .Fn gets
143 as obsolescent.
144 .Sh CAVEATS
145 The following bit of code illustrates a case where the programmer assumes a
146 string is too long if it does not contain a newline:
147 .Bd -literal
148 char buf[1024], *p;
150 while (fgets(buf, sizeof(buf), fp) != NULL) {
151 if ((p = strchr(buf, '\en')) == NULL) {
152 fprintf(stderr, "input line too long.\en");
153 exit(1);
154 }
155 *p = '\e0';
156 printf("%s\en", buf);
157 }
158 .Ed
159 .Pp
160 While the error would be true if a line longer than 1023 characters
161 were read, it would be false in two other cases:
162 .Bl -enum -offset indent
163 .It
164 If the last line in a file does not contain a newline, the string returned by
165 .Fn fgets
166 will not contain a newline either.
167 Thus
168 .Fn strchr
169 will return
170 .Dv NULL
171 and the program will terminate, even if the line was valid.
172 .It
173 All C string functions, including
174 .Fn strchr ,
175 correctly assume the end of the string is represented by a null
176 .Pq Sq \e0
177 character.
178 If the first character of a line returned by
179 .Fn fgets
180 were null,
181 .Fn strchr
182 would immediately return without considering the rest of the returned text
183 which may indeed include a newline.
184 .El
185 .Pp
186 Consider using
187 .Xr fgetln 3
188 instead when dealing with untrusted input.
189 .Sh SECURITY CONSIDERATIONS
190 Since it is usually impossible to ensure that the next input line
191 is less than some arbitrary length, and because overflowing the
192 input buffer is almost invariably a security violation, programs
193 should
194 .Em NEVER
195 use
196 .Fn gets .
197 The
198 .Fn gets
199 function
200 exists purely to conform to
201 .St -ansiC .