NAME

       stpncpy,  strncpy  - zero a fixed-width buffer and copy a string into a character sequence
       with truncation and zero the rest of it

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <string.h>

       char *stpncpy(char dst[restrict .sz], const char *restrict src,
                      size_t sz);
       char *strncpy(char dst[restrict .sz], const char *restrict src,
                      size_t sz);

   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 the string pointed to by src into a null-padded character sequence at
       the fixed-width buffer pointed to by dst.  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 *
           stpncpy(char *restrict dst, const char *restrict src, size_t sz)
           {
               bzero(dst, sz);
               return mempcpy(dst, src, strnlen(src, sz));
           }

           char *
           strncpy(char *restrict dst, const char *restrict src, size_t sz)
           {
               stpncpy(dst, src, sz);
               return dst;
           }

RETURN VALUE

       stpncpy()
              returns  a  pointer  to  one  after the last character in the destination character
              sequence.

       strncpy()
              returns dst.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                      │ Attribute     │ Value   │
       ├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │stpncpy(), strncpy()                                           │ Thread safety │ MT-Safe │
       └───────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       stpncpy()
              POSIX.1-2008.

       strncpy()
              POSIX.1-2001, POSIX.1-2008, C99, SVr4, 4.3BSD.

CAVEATS

       The name of these functions is confusing.  These functions produce a null-padded character
       sequence, not a string (see string_copying(7)).

       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(buf1) < strlen("Hello world!"))
               warnx("stpncpy: truncating character sequence");
           p = stpncpy(buf1, "Hello world!", sizeof(buf1));
           len = p - buf1;

           printf("[len = %zu]: ", len);
           printf("%.*s\n", (int) len, buf1);  // "Hello world!"

           if (sizeof(buf2) < strlen("Hello world!"))
               warnx("strncpy: truncating character sequence");
           strncpy(buf2, "Hello world!", sizeof(buf2));
           len = strnlen(buf2, sizeof(buf2));

           printf("[len = %zu]: ", len);
           printf("%.*s\n", (int) len, buf2);  // "Hello world!"

           exit(EXIT_SUCCESS);
       }

SEE ALSO

       wcpncpy(3), string_copying(7)