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
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
|
'\" t
.\" Copyright 2022 Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH stpncpy 3 2024-02-12 "Linux man-pages 6.06"
.SH NAME
stpncpy, strncpy
\-
fill a fixed-size buffer with non-null bytes from a string,
padding with null bytes as needed
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.P
.BI "char *strncpy(char " dst "[restrict ." dsize "], \
const char *restrict " src ,
.BI " size_t " dsize );
.BI "char *stpncpy(char " dst "[restrict ." dsize "], \
const char *restrict " src ,
.BI " size_t " dsize );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR stpncpy ():
.nf
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
.fi
.SH DESCRIPTION
These functions copy non-null bytes from the string pointed to by
.I src
into the array pointed to by
.IR dst .
If the source has too few non-null bytes to fill the destination,
the functions pad the destination with trailing null bytes.
If the destination buffer,
limited by its size,
isn't large enough to hold the copy,
the resulting character sequence is truncated.
For the difference between the two functions, see RETURN VALUE.
.P
An implementation of these functions might be:
.P
.in +4n
.EX
char *
strncpy(char *restrict dst, const char *restrict src, size_t dsize)
{
stpncpy(dst, src, dsize);
return dst;
}
\&
char *
stpncpy(char *restrict dst, const char *restrict src, size_t dsize)
{
size_t dlen;
\&
dlen = strnlen(src, dsize);
return memset(mempcpy(dst, src, dlen), 0, dsize \- dlen);
}
.EE
.in
.SH RETURN VALUE
.TP
.BR strncpy ()
returns
.IR dst .
.TP
.BR stpncpy ()
returns a pointer to
one after the last character in the destination character sequence.
.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 stpncpy (),
.BR strncpy ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
.TP
.BR strncpy ()
C11, POSIX.1-2008.
.TP
.BR stpncpy ()
POSIX.1-2008.
.SH HISTORY
.TP
.BR strncpy ()
C89, POSIX.1-2001, SVr4, 4.3BSD.
.TP
.BR stpncpy ()
glibc 1.07.
POSIX.1-2008.
.SH CAVEATS
The name of these functions is confusing.
These functions produce a null-padded character sequence,
not a string (see
.BR string_copying (7)).
For example:
.P
.in +4n
.EX
strncpy(buf, "1", 5); // { \[aq]1\[aq], 0, 0, 0, 0 }
strncpy(buf, "1234", 5); // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], 0 }
strncpy(buf, "12345", 5); // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
strncpy(buf, "123456", 5); // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
.EE
.in
.P
It's impossible to distinguish truncation by the result of the call,
from a character sequence that just fits the destination buffer;
truncation should be detected by
comparing the length of the input string
with the size of the destination buffer.
.P
If you're going to use this function in chained calls,
it would be useful to develop a similar function that accepts
a pointer to the end (one after the last element) of the destination buffer
instead of its size.
.SH EXAMPLES
.\" SRC BEGIN (stpncpy.c)
.EX
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
\&
int
main(void)
{
char *p;
char buf1[20];
char buf2[20];
size_t len;
\&
if (sizeof(buf2) < strlen("Hello world!"))
errx("strncpy: truncating character sequence");
strncpy(buf2, "Hello world!", sizeof(buf2));
len = strnlen(buf2, sizeof(buf2));
\&
printf("[len = %zu]: ", len);
fwrite(buf2, 1, len, stdout);
putchar(\[aq]\en\[aq]);
\&
if (sizeof(buf1) < strlen("Hello world!"))
errx("stpncpy: truncating character sequence");
p = stpncpy(buf1, "Hello world!", sizeof(buf1));
len = p \- buf1;
\&
printf("[len = %zu]: ", len);
fwrite(buf1, 1, len, stdout);
putchar(\[aq]\en\[aq]);
\&
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR wcpncpy (3),
.BR string_copying (7)
|
