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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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STRCPY(3) Linux Programmer's Manual STRCPY(3)
strcpy, strncpy - copy a string
#include <string.h>
char *strcpy(char *restrict dest, const char *src);
char *strncpy(char *restrict dest, const char *restrict src, size_t n);
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;
}
The strcpy() and strncpy() functions return a pointer to the
destination string dest.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│strcpy(), strncpy() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
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 destination 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:
if (buflen > 0) {
strncpy(buf, str, buflen - 1);
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 destination 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.
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.
bcopy(3), memccpy(3), memcpy(3), memmove(3), stpcpy(3),
stpncpy(3), strdup(3), string(3), wcscpy(3), wcsncpy(3)
This page is part of release 5.13 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/.
GNU 2021-03-22 STRCPY(3)
Pages that refer to this page: bcopy(3), memccpy(3), memcpy(3), memmove(3), pmstrncpy(3), stpcpy(3), stpncpy(3), strcat(3), string(3), wcpcpy(3), wcscpy(3), wcsncpy(3), feature_test_macros(7), signal-safety(7)
Copyright and license for this manual page
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