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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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strncat(3) Library Functions Manual strncat(3)
strncat - append non-null bytes from a source array to a string,
and null-terminate the result
Standard C library (libc, -lc)
#include <string.h>
char *strncat(size_t ssize;
char *restrict dst, const char src[restrict ssize],
size_t ssize);
This function appends at most ssize non-null bytes from the array
pointed to by src, followed by a null character, to the end of the
string pointed to by dst. dst must point to a string contained in
a buffer that is large enough, that is, the buffer size must be at
least strlen(dst) + strnlen(src, ssize) + 1.
An implementation of this function might be:
char *
strncat(char *restrict dst, const char *restrict src, size_t ssize)
{
#define strnul(s) (s + strlen(s))
stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), "");
return dst;
}
strncat() returns dst.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ strncat() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C11, POSIX.1-2008.
POSIX.1-2001, C89, SVr4, 4.3BSD.
The name of this function is confusing; it has no relation to
strncpy(3).
If the destination buffer does not already contain a string, or is
not large enough, the behavior is undefined. See _FORTIFY_SOURCE
in feature_test_macros(7).
This function can be very inefficient. Read about Shlemiel the
painter
⟨https://www.joelonsoftware.com/2001/12/11/back-to-basics/⟩.
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define nitems(arr) (sizeof((arr)) / sizeof((arr)[0]))
int
main(void)
{
size_t n;
// Null-padded fixed-size character sequences
char pre[4] = "pre.";
char new_post[50] = ".foo.bar";
// Strings
char post[] = ".post";
char src[] = "some_long_body.post";
char *dest;
n = nitems(pre) + strlen(src) - strlen(post) + nitems(new_post) + 1;
dest = malloc(sizeof(*dest) * n);
if (dest == NULL)
err(EXIT_FAILURE, "malloc()");
dest[0] = '\0'; // There's no 'cpy' function to this 'cat'.
strncat(dest, pre, nitems(pre));
strncat(dest, src, strlen(src) - strlen(post));
strncat(dest, new_post, nitems(new_post));
puts(dest); // "pre.some_long_body.foo.bar"
free(dest);
exit(EXIT_SUCCESS);
}
string(3), string_copying(7)
This page is part of the man-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 is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 strncat(3)
Pages that refer to this page: pmstrncat(3), pmstrncpy(3), string(3), wcsncat(3), feature_test_macros(7), signal-safety(7), string_copying(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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