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32 .\" @(#)strcat.3 8.1 (Berkeley) 6/4/93

33 .\" $FreeBSD: src/lib/libc/string/strcat.3,v 1.17 2009/12/01 07:28:56 brueffer Exp $

34 .\"

35 .Dd December 1, 2009

36 .Dt STRCAT 3

37 .Os

38 .Sh NAME

39 .Nm strcat ,

40 .Nm strncat

41 .Nd concatenate strings

42 .Sh LIBRARY

43 .Lb libc

44 .Sh SYNOPSIS

45 .In string.h

46 .Ft char *

47 .Fo strcat

48 .Fa "char *restrict s1"

49 .Fa "const char *restrict s2"

50 .Fc

51 .Ft char *

52 .Fo strncat

53 .Fa "char *restrict s1"

54 .Fa "const char *restrict s2"

55 .Fa "size_t n"

56 .Fc

57 .Sh DESCRIPTION

58 The

59 .Fn strcat

60 and

61 .Fn strncat

62 functions

63 append a copy of the null-terminated string

64 .Fa s2

65 to the end of the null-terminated string

66 .Fa s1 ,

67 then add a terminating

68 .Ql \e0 .

69 The string

70 .Fa s1

71 must have sufficient space to hold the result.

72 .Pp

73 The

74 .Fn strncat

75 function

76 appends not more than

77 .Fa n

78 characters from

79 .Fa s2 ,

80 and then adds a terminating

81 .Ql \e0 .

82 .Pp

83 The source and destination strings should not overlap, as the

84 behavior is undefined.

85 .Sh RETURN VALUES

86 The

87 .Fn strcat

88 and

89 .Fn strncat

90 functions

91 return the pointer

92 .Fa s1 .

93 .Sh SECURITY CONSIDERATIONS

94 The

95 .Fn strcat

96 function is easily misused in a manner

97 which enables malicious users to arbitrarily change

98 a running program's functionality through a buffer overflow attack.

99 (See

100 the FSA.)

101 .Pp

102 Avoid using

103 .Fn strcat .

104 Instead, use

105 .Fn strncat

106 or

107 .Fn strlcat

108 and ensure that no more characters are copied to the destination buffer

109 than it can hold.

110 .Pp

111 Note that

112 .Fn strncat

113 can also be problematic.

114 It may be a security concern for a string to be truncated at all.

115 Since the truncated string will not be as long as the original,

116 it may refer to a completely different resource

117 and usage of the truncated resource

118 could result in very incorrect behavior.

119 Example:

120 .Bd -literal

121 void

122 foo(const char *arbitrary_string)

123 {

124 char onstack[8] = "";

125

126 #if defined(BAD)

127 /*

128 * This first strcat is bad behavior. Do not use strcat!

129 */

130 (void)strcat(onstack, arbitrary_string); /* BAD! */

131 #elif defined(BETTER)

132 /*

133 * The following two lines demonstrate better use of

134 * strncat().

135 */

136 (void)strncat(onstack, arbitrary_string,

137 sizeof(onstack) - strlen(onstack) - 1);

138 #elif defined(BEST)

139 /*

140 * These lines are even more robust due to testing for

141 * truncation.

142 */

143 if (strlen(arbitrary_string) + 1 >

144 sizeof(onstack) - strlen(onstack))

145 err(1, "onstack would be truncated");

146 (void)strncat(onstack, arbitrary_string,

147 sizeof(onstack) - strlen(onstack) - 1);

148 #endif

149 }

150 .Ed

151 .Sh SEE ALSO

152 .Xr bcopy 3 ,

153 .Xr memccpy 3 ,

154 .Xr memcpy 3 ,

155 .Xr memmove 3 ,

156 .Xr strcpy 3 ,

157 .Xr strlcat 3 ,

158 .Xr strlcpy 3 ,

159 .Xr wcscat 3

160 .Sh STANDARDS

161 The

162 .Fn strcat

163 and

164 .Fn strncat

165 functions

166 conform to

167 .St -isoC .