#include <string.h> char *strcpy(char *dest, const char *src); char *strncpy(char *dest, const char *src, size_t n);
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;
}
| Interface | Attribute | Value |
| strcpy(), strncpy() | Thread safety | MT-Safe |
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.)
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.