stpncpy(3)                 Library Functions Manual                 stpncpy(3)

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(char dst[restrict .dsize], const char *restrict src,
                     size_t dsize);
       char *stpncpy(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 after the last character in the
              destination character sequence.

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 6.7               2024-02-12                        stpncpy(3)