1STRCPY(3) Linux Programmer's Manual STRCPY(3)
2
6 strcpy, strncpy - copy a string
7
9 #include <string.h>
10 char *strcpy(char *dest, const char *src);
12 char *strncpy(char *dest, const char *src, size_t n);
14
16 The strcpy() function copies the string pointed to by src, including
17 the terminating null byte ('\0'), to the buffer pointed to by dest.
18 The strings may not overlap, and the destination string dest must be
19 large enough to receive the copy. Beware of buffer overruns! (See
20 BUGS.)
21 The strncpy() function is similar, except that at most n bytes of src
23 are copied. Warning: If there is no null byte among the first n bytes
24 of src, the string placed in dest will not be null-terminated.
25 If the length of src is less than n, strncpy() writes additional null
27 bytes to dest to ensure that a total of n bytes are written.
28 A simple implementation of strncpy() might be:
30 char *
32 strncpy(char *dest, const char *src, size_t n)
33 {
34 size_t i;
35 for (i = 0; i < n && src[i] != '\0'; i++)
37 dest[i] = src[i];
38 for ( ; i < n; i++)
39 dest[i] = '\0';
40 return dest;
42 }
43
45 The strcpy() and strncpy() functions return a pointer to the destina‐
46 tion string dest.
47
49 For an explanation of the terms used in this section, see
50 attributes(7).
51 ┌────────────────────┬───────────────┬─────────┐
53 │Interface │ Attribute │ Value │
54 ├────────────────────┼───────────────┼─────────┤
55 │strcpy(), strncpy() │ Thread safety │ MT-Safe │
56 └────────────────────┴───────────────┴─────────┘
58 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
59
61 Some programmers consider strncpy() to be inefficient and error prone.
62 If the programmer knows (i.e., includes code to test!) that the size
63 of dest is greater than the length of src, then strcpy() can be used.
64 One valid (and intended) use of strncpy() is to copy a C string to a
66 fixed-length buffer while ensuring both that the buffer is not over‐
67 flowed and that unused bytes in the destination buffer are zeroed out
68 (perhaps to prevent information leaks if the buffer is to be written to
69 media or transmitted to another process via an interprocess communica‐
70 tion technique).
71 If there is no terminating null byte in the first n bytes of src,
73 strncpy() produces an unterminated string in dest. If buf has length
74 buflen, you can force termination using something like the following:
75 if (buflen > 0) {
77 strncpy(buf, str, buflen - 1);
78 buf[buflen - 1]= '\0';
79 }
80 (Of course, the above technique ignores the fact that, if src contains
82 more than buflen - 1 bytes, information is lost in the copying to
83 dest.)
84 strlcpy()
86 Some systems (the BSDs, Solaris, and others) provide the following
87 function:
88 size_t strlcpy(char *dest, const char *src, size_t size);
90 This function is similar to strncpy(), but it copies at most size-1
92 bytes to dest, always adds a terminating null byte, and does not pad
93 the destination with (further) null bytes. This function fixes some of
94 the problems of strcpy() and strncpy(), but the caller must still han‐
95 dle the possibility of data loss if size is too small. The return
96 value of the function is the length of src, which allows truncation to
97 be easily detected: if the return value is greater than or equal to
98 size, truncation occurred. If loss of data matters, the caller must
99 either check the arguments before the call, or test the function return
100 value. strlcpy() is not present in glibc and is not standardized by
101 POSIX, but is available on Linux via the libbsd library.
102
104 If the destination string of a strcpy() is not large enough, then any‐
105 thing might happen. Overflowing fixed-length string buffers is a
106 favorite cracker technique for taking complete control of the machine.
107 Any time a program reads or copies data into a buffer, the program
108 first needs to check that there's enough space. This may be unneces‐
109 sary if you can show that overflow is impossible, but be careful: pro‐
110 grams can get changed over time, in ways that may make the impossible
111 possible.
112
114 bcopy(3), memccpy(3), memcpy(3), memmove(3), stpcpy(3), stpncpy(3),
115 strdup(3), string(3), wcscpy(3), wcsncpy(3)
116
118 This page is part of release 5.04 of the Linux man-pages project. A
119 description of the project, information about reporting bugs, and the
120 latest version of this page, can be found at
121 https://www.kernel.org/doc/man-pages/.
122GNU 2019-03-06 STRCPY(3)