strncat(3) — Linux manual page

strncat(3)               Library Functions Manual              strncat(3)

NAME         top

       strncat - append non-null bytes from a source array to a string,
       and null-terminate the result

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <string.h>

       char *strncat(size_t ssize;
                     char *restrict dst, const char src[restrict ssize],
                     size_t ssize);

DESCRIPTION         top

       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.

       It is equivalent to

           stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), ""), dst

RETURN VALUE         top

       strncat() returns dst.

ATTRIBUTES         top

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

STANDARDS         top

       C11, POSIX.1-2008.

HISTORY         top

       POSIX.1-2001, C89, SVr4, 4.3BSD.

CAVEATS         top

       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).

BUGS         top

       This function can be very inefficient.  Read about Shlemiel the
       painter
       ⟨https://www.joelonsoftware.com/2001/12/11/back-to-basics/⟩.

EXAMPLES         top

       #include <stdcountof.h>
       #include <stdio.h>
       #include <string.h>
       #include <utmp.h>

       void print_ut_user(struct utmp *ut);

       void
       print_ut_user(struct utmp *ut)
       {
            char  buf[countof(ut->ut_user) + 1];

            strcpy(buf, "");
            strncat(buf, ut->ut_user, countof(ut->ut_user));
            puts(buf);
       }

SEE ALSO         top

       string(3), string_copying(7)

COLOPHON         top

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Linux man-pages 6.18            2026-02-25                     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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