# stpncpy

## NAME



           stpncpy, strncpy - fill a fixed-size buffer with non-null bytes
           from a string, padding with null bytes as needed
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <string.h>**
    
           **char *strncpy(** size_t dsize;
                         **char** _dst_**[restrict** _dsize_**], const char *restrict** _src_**,**
                         **size_t** _dsize_**);**
           **char *stpncpy(** size_t dsize;
                         **char** _dst_**[restrict** _dsize_**], const char *restrict** _src_**,**
                         **size_t** _dsize_**);**
    
       Feature Test Macro Requirements for glibc (see
       feature_test_macros(7)):
    
           **stpncpy**():
               Since glibc 2.10:
                   _POSIX_C_SOURCE >= 200809L
               Before glibc 2.10:
                   _GNU_SOURCE
    


## DESCRIPTION



           These functions copy non-null bytes from the string pointed to by
           _src_ into the array pointed to by _dst_.  If the source has too few
           non-null bytes to fill the destination, the functions pad the
           destination with trailing null bytes.  If the destination buffer,
           limited by its size, isn't large enough to hold the copy, the
           resulting character sequence is truncated.  For the difference
           between the two functions, see RETURN VALUE.
    
           An implementation of these functions might be:
    
               char *
               strncpy(char *restrict dst, const char *restrict src, size_t dsize)
               {
                   stpncpy(dst, src, dsize);
                   return dst;
               }
    
               char *
               stpncpy(char *restrict dst, const char *restrict src, size_t dsize)
               {
                   size_t  dlen;
    
                   dlen = strnlen(src, dsize);
                   return memset(mempcpy(dst, src, dlen), 0, dsize - dlen);
               }
    


## RETURN VALUE



           **strncpy**()
                  returns _dst_.
    
           **stpncpy**()
                  returns a pointer to one past the last non-null wide
                  character written, that is, _dest + strnlen(src, n)_.
    


## ATTRIBUTES



           For an explanation of the terms used in this section, see
           attributes(7).
           ┌──────────────────────────────────────┬───────────────┬─────────┐
           │ **Interface** │ **Attribute** │ **Value** │
           ├──────────────────────────────────────┼───────────────┼─────────┤
           │ **stpncpy**(), **strncpy**()                 │ Thread safety │ MT-Safe │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## STANDARDS



           **strncpy**()
                  C11, POSIX.1-2008.
    
           **stpncpy**()
                  POSIX.1-2008.
    


## CAVEATS



           The name of these functions is confusing.  These functions produce
           a null-padded character sequence, not a string (see
           string_copying(7)).  For example:
    
               strncpy(buf, "1", 5);       // { '1',   0,   0,   0,   0 }
               strncpy(buf, "1234", 5);    // { '1', '2', '3', '4',   0 }
               strncpy(buf, "12345", 5);   // { '1', '2', '3', '4', '5' }
               strncpy(buf, "123456", 5);  // { '1', '2', '3', '4', '5' }
    
           It's impossible to distinguish truncation by the result of the
           call, from a character sequence that just fits the destination
           buffer; truncation should be detected by comparing the length of
           the input string with the size of the destination buffer.
    
           If you're going to use this function in chained calls, it would be
           useful to develop a similar function that accepts a pointer to the
           end (one after the last element) of the destination buffer instead
           of its size.
    


## EXAMPLES



           #include <err.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
    
           int
           main(void)
           {
               char    *p;
               char    buf1[20];
               char    buf2[20];
               size_t  len;
    
               if (sizeof(buf2) < strlen("Hello world!"))
                   errx("strncpy: truncating character sequence");
               strncpy(buf2, "Hello world!", sizeof(buf2));
               len = strnlen(buf2, sizeof(buf2));
    
               printf("[len = %zu]: ", len);
               fwrite(buf2, 1, len, stdout);
               putchar('\n');
    
               if (sizeof(buf1) < strlen("Hello world!"))
                   errx("stpncpy: truncating character sequence");
               p = stpncpy(buf1, "Hello world!", sizeof(buf1));
               len = p - buf1;
    
               printf("[len = %zu]: ", len);
               fwrite(buf1, 1, len, stdout);
               putchar('\n');
    
               exit(EXIT_SUCCESS);
           }
    


* * *