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.

HISTORY

       strncpy()
              C89, POSIX.1-2001, SVr4, 4.3BSD.

       stpncpy()
              glibc 1.07.  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);
       }

SEE ALSO

       wcpncpy(3), string_copying(7)

Linux man-pages 6.15                               2025-07-11                                         stpncpy(3)