stpncpy(3) — Linux manual page

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)

COLOPHON

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Linux man-pages 6.9.1          2024-06-15                     stpncpy(3)

Pages that refer to this page: bcopy(3), memccpy(3), memcpy(3), memmove(3), pmstrncpy(3), string(3), strncat(3), wcpncpy(3), wcsncpy(3), feature_test_macros(7), signal-safety(7), string_copying(7)

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