1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
3 .\" %%%LICENSE_START(VERBATIM)
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13 .\" Since the Linux kernel and libraries are constantly changing, this
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21 .\" Formatted or processed versions of this manual, if unaccompanied by
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25 .\" References consulted:
26 .\" Linux libc source code
27 .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
29 .\" Modified Sat Jul 24 18:11:47 1993 by Rik Faith (faith@cs.unc.edu)
30 .\" 2007-06-15, Marc Boyer <marc.boyer@enseeiht.fr> + mtk
31 .\" Improve discussion of strncat().
32 .TH STRCAT 3 2019-03-06 "GNU" "Linux Programmer's Manual"
34 strcat, strncat \- concatenate two strings
37 .B #include <string.h>
39 .BI "char *strcat(char *" dest ", const char *" src );
41 .BI "char *strncat(char *" dest ", const char *" src ", size_t " n );
51 overwriting the terminating null byte (\(aq\e0\(aq) at the end of
53 and then adds a terminating null byte.
54 The strings may not overlap, and the
57 enough space for the result.
60 is not large enough, program behavior is unpredictable;
61 .IR "buffer overruns are a favorite avenue for attacking secure programs" .
65 function is similar, except that
74 does not need to be null-terminated if it contains
80 the resulting string in
82 is always null-terminated.
97 plus the terminating null byte).
98 Therefore, the size of
101 .IR "strlen(dest)+n+1" .
103 A simple implementation of
110 strncat(char *dest, const char *src, size_t n)
112 size_t dest_len = strlen(dest);
115 for (i = 0 ; i < n && src[i] != \(aq\e0\(aq ; i++)
116 dest[dest_len + i] = src[i];
117 dest[dest_len + i] = \(aq\e0\(aq;
128 functions return a pointer to the resulting string
131 For an explanation of the terms used in this section, see
137 Interface Attribute Value
141 T} Thread safety MT-Safe
144 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
146 Some systems (the BSDs, Solaris, and others) provide the following function:
148 size_t strlcat(char *dest, const char *src, size_t size);
150 This function appends the null-terminated string
155 .IR "size\-strlen(dest)\-1"
158 and adds a terminating null byte to the result,
163 This function fixes the buffer overrun problem of
165 but the caller must still handle the possibility of data loss if
168 The function returns the length of the string
170 tried to create; if the return value is greater than or equal to
173 If data loss matters, the caller
175 either check the arguments before the call, or test the function return value.
177 is not present in glibc and is not standardized by POSIX,
178 .\" https://lwn.net/Articles/506530/
179 but is available on Linux via the
188 must find the null byte that terminates the string
190 using a search that starts at the beginning of the string,
191 the execution time of these functions
192 scales according to the length of the string
194 This can be demonstrated by running the program below.
195 (If the goal is to concatenate many strings to one target,
196 then manually copying the bytes from each source string
197 while maintaining a pointer to the end of the target string
198 will provide better performance.)
208 main(int argc, char *argv[])
218 for (j = 0; j < LIM; j++) {
219 if ((j % 10000) == 0)
220 printf("%d %ld\en", j, (long) (time(NULL) \- base));