1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
|
'\" t
.\" Copyright 2022 Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH strncat 3 2023-12-05 "Linux man-pages 6.7"
.SH NAME
strncat
\-
append non-null bytes from a source array to a string,
and null-terminate the result
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.P
.BI "char *strncat(char *restrict " dst ", const char " src "[restrict ." ssize ],
.BI " size_t " ssize );
.fi
.SH DESCRIPTION
This function appends at most
.I ssize
non-null bytes from the array pointed to by
.IR src ,
followed by a null character,
to the end of the string pointed to by
.IR dst .
.I dst
must point to a string contained in a buffer that is large enough,
that is, the buffer size must be at least
.IR "strlen(dst) + strnlen(src, ssize) + 1" .
.P
An implementation of this function might be:
.P
.in +4n
.EX
char *
strncat(char *restrict dst, const char *restrict src, size_t ssize)
{
#define strnul(s) (s + strlen(s))
\&
stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), "");
return dst;
}
.EE
.in
.SH RETURN VALUE
.BR strncat ()
returns
.IR dst .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR strncat ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C89, SVr4, 4.3BSD.
.SH CAVEATS
The name of this function is confusing;
it has no relation to
.BR strncpy (3).
.P
If the destination buffer does not already contain a string,
or is not large enough,
the behavior is undefined.
See
.B _FORTIFY_SOURCE
in
.BR feature_test_macros (7).
.SH BUGS
This function can be very inefficient.
Read about
.UR https://www.joelonsoftware.com/\:2001/12/11/\:back\-to\-basics/
Shlemiel the painter
.UE .
.SH EXAMPLES
.\" SRC BEGIN (strncat.c)
.EX
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
\&
#define nitems(arr) (sizeof((arr)) / sizeof((arr)[0]))
\&
int
main(void)
{
size_t n;
\&
// Null-padded fixed-size character sequences
char pre[4] = "pre.";
char new_post[50] = ".foo.bar";
\&
// Strings
char post[] = ".post";
char src[] = "some_long_body.post";
char *dest;
\&
n = nitems(pre) + strlen(src) \- strlen(post) + nitems(new_post) + 1;
dest = malloc(sizeof(*dest) * n);
if (dest == NULL)
err(EXIT_FAILURE, "malloc()");
\&
dest[0] = \[aq]\e0\[aq]; // There's no 'cpy' function to this 'cat'.
strncat(dest, pre, nitems(pre));
strncat(dest, src, strlen(src) \- strlen(post));
strncat(dest, new_post, nitems(new_post));
\&
puts(dest); // "pre.some_long_body.foo.bar"
free(dest);
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.in
.SH SEE ALSO
.BR string (3),
.BR string_copying (7)
|
