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stpncpy(3) — Linux manual page

NAME \|LIBRARY \|SYNOPSIS \|DESCRIPTION \|RETURN VALUE \|ATTRIBUTES \|STANDARDS \|HISTORY \|CAVEATS \|EXAMPLES \|SEE ALSO \|COLOPHON

stpncpy(3)               Library Functions Manualstpncpy(3)

NAME top

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

LIBRARY top

       Standard C library (libc,-lc)

SYNOPSIS top

#include <string.h>char *strncpy(size_t dsize;chardst[restrictdsize], const char *restrictsrc,size_tdsize);char *stpncpy(size_t dsize;chardst[restrictdsize], const char *restrictsrc,size_tdsize);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):stpncpy():           Since glibc 2.10:               _POSIX_C_SOURCE >= 200809L           Before glibc 2.10:               _GNU_SOURCE

DESCRIPTION top

       These functions copy non-null bytes from the string pointed to bysrc into the array pointed to bydst.  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 top

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

ATTRIBUTES top

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

STANDARDS top

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

HISTORY top

strncpy()              C89, POSIX.1-2001, SVr4, 4.3BSD.stpncpy()              glibc 1.07.  POSIX.1-2008.

CAVEATS top

       The name of these functions is confusing.  These functions produce       a null-padded character sequence, not a string (seestring_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 top

       #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);       }

COLOPHON top

       This page is part of theman-pages (Linux kernel and C library       user-space interface documentation) project.  Information about       the project can be found at        ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report       for this manual page, see       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.       This page was obtained from the tarball man-pages-6.15.tar.gz       fetched from       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on       2025-08-11.  If you discover any rendering problems in this HTML       version of the page, or you believe there is a better or more up-       to-date source for the page, or you have corrections or       improvements to the information in this COLOPHON (which isnot       part of the original manual page), send a mail to       man-pages@man7.orgLinux man-pages 6.15            2025-07-11stpncpy(3)

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