Описание тега strncpy

Used to copy a maximum of n characters of non-overlapping, fixed-width or null-terminated strings. It is defined in the <string.h> C standard header or the <cstring> C++ standard header.

This function is not recommended to use for any purpose, neither in C nor C++. It was never intended to be a "safe version of strcpy" but is often misused for such purposes. It is in fact considered to be much more dangerous than strcpy, since the null termination mechanism of strncpy is not intuitive and therefore often misunderstood. This is because of the following behavior specified by ISO 9899:2011 7.24.2.4:

char *strncpy(char * restrict s1, 
     const char * restrict s2, 
     size_t n);

/--/

3 If the array pointed to by s2 is a string that is shorter than n characters, null characters are appended to the copy in the array pointed to by s1, until n characters in all have been written.

A very common mistake is to pass an s2 which is exactly as many characters as the n parameter, in which case s1 will not get null terminated. That is: strncpy(dst, src, strlen(src));

/* MCVE of incorrect use of strncpy */
#include <string.h>
#include <stdio.h>

int main (void)
{
  const char* STR = "hello";
  char buf[] = "halt and catch fire";
  strncpy(buf, STR, strlen(STR));
  puts(buf); // prints "helloand catch fire"
  return 0;
}

Recommended practice in C is to check the buffer size in advance and then use strcpy(), alternatively memcpy(). Recommended practice in C++ is to use std::string instead.

References:
- ISO/IEC 9899:2011 Information technology — Programming languages — C. Chapter 7.24.2.4.
- Why are strlcpy and strlcat considered insecure?
- /questions/15942948/kakih-funktsij-iz-standartnoj-biblioteki-sleduet-sleduet-izbegat

Documentation:
C strncpy documentation.
C++ std::strncpy documentation.