strncpy(3)
NAME
strcpy, strncpy - copy a string
SYNOPSIS
#include <string.h> char *strcpy(char *dest, const char *src); char *strncpy(char *dest, const char *src, size_t n);
DESCRIPTION
The strcpy() function copies the string pointed to by src, including
the terminating null byte ('\0'), to the buffer pointed to by dest.
The strings may not overlap, and the destination string dest must be
large enough to receive the copy. Beware of buffer overruns! (See
BUGS.)
The strncpy() function is similar, except that at most n bytes of src
are copied. Warning: If there is no null byte among the first n bytes
of src, the string placed in dest will not be null-terminated.
If the length of src is less than n, strncpy() writes additional null
bytes to dest to ensure that a total of n bytes are written.
A simple implementation of strncpy() might be:
char *
strncpy(char *dest, const char *src, size_t n)
{
size_t i;
for (i = 0; i < n && src[i] != '\0'; i++)
dest[i] = src[i];
for ( ; i < n; i++)
dest[i] = '\0';
return dest;
}
RETURN VALUE
The strcpy() and strncpy() functions return a pointer to the destination string dest.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7). ┌────────────────────┬───────────────┬─────────┐ │Interface │ Attribute │ Value │ ├────────────────────┼───────────────┼─────────┤ │strcpy(), strncpy() │ Thread safety │ MT-Safe │ └────────────────────┴───────────────┴─────────┘
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
NOTES
Some programmers consider strncpy() to be inefficient and error prone.
If the programmer knows (i.e., includes code to test!) that the size
of dest is greater than the length of src, then strcpy() can be used.
One valid (and intended) use of strncpy() is to copy a C string to a
fixed-length buffer while ensuring both that the buffer is not
overflowed and that unused bytes in the target buffer are zeroed out
(perhaps to prevent information leaks if the buffer is to be written to
media or transmitted to another process via an interprocess
communication technique).
If there is no terminating null byte in the first n bytes of src,
strncpy() produces an unterminated string in dest. If buf has length
buflen, you can force termination using something like the following:
strncpy(buf, str, buflen - 1);
if (buflen > 0)
buf[buflen - 1]= '\0';
(Of course, the above technique ignores the fact that, if src contains
more than buflen - 1 bytes, information is lost in the copying to
dest.)
strlcpy()
Some systems (the BSDs, Solaris, and others) provide the following
function:
size_t strlcpy(char *dest, const char *src, size_t size);
This function is similar to strncpy(), but it copies at most size-1
bytes to dest, always adds a terminating null byte, and does not pad
the target with (further) null bytes. This function fixes some of the
problems of strcpy() and strncpy(), but the caller must still handle
the possibility of data loss if size is too small. The return value of
the function is the length of src, which allows truncation to be easily
detected: if the return value is greater than or equal to size,
truncation occurred. If loss of data matters, the caller must either
check the arguments before the call, or test the function return value.
strlcpy() is not present in glibc and is not standardized by POSIX, but
is available on Linux via the libbsd library.
BUGS
If the destination string of a strcpy() is not large enough, then anything might happen. Overflowing fixed-length string buffers is a favorite cracker technique for taking complete control of the machine. Any time a program reads or copies data into a buffer, the program first needs to check that there's enough space. This may be unnecessary if you can show that overflow is impossible, but be careful: programs can get changed over time, in ways that may make the impossible possible.
SEE ALSO
bcopy(3), memccpy(3), memcpy(3), memmove(3), stpcpy(3), stpncpy(3), strdup(3), string(3), wcscpy(3), wcsncpy(3)
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
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