strncat(3)

💖 strncat(3) — the "nonstring catenate" bestie 💖

okay so we're in the Library Functions Manual section, page 3, and today's main character is strncat — yes it's in the library, yes it's the C one, and bb it is not the function you think it is. buckle up 🎀

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✨ NAME ✨

strncat — nonstring catenate 🦋

📚 LIBRARY 📚

Standard C library (libc, -lc) 💅

🌸 SYNOPSIS 🌸

#include <string.h>  // see memory.h(3head)

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

👑 DESCRIPTION 👑

so here's the vibe: strncat() appends at most ssize non-null bytes from the array that src points at, and then it slaps a null character on the end, appending all of that to the end of the string that dst points to. ✨

and dst must point to a string sitting inside a buffer that's big enough — like, the buffer size has to be at least strlen(dst) + strnlen(src, ssize) + 1. no notes, that's the requirement, fr fr. 😭

it's literally equivalent to this:

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

🫶 RETURN VALUE 🫶

strncat() returns dst. that's it. that's the whole return value. iconic behavior honestly 💅

💀 ATTRIBUTES 💀

for the tea on all the terms used in this section, go peek at attributes(7).

• Interface: strncat()
• Attribute: Thread safety
• Value: MT-Safe

(translation: she's safe to run from multiple threads, no cap 🔥)

⭐ STANDARDS ⭐

C11, POSIX.1-2008.

🌈 HISTORY 🌈

POSIX.1-2001, C89, SVr4, 4.3BSD. she's been around, she's got lore 👑

🙈 CAVEATS 🙈

the name of this function is, and I say this with love, deeply confusing:

• it has no relation to strncpy(3). zero. none. different girl entirely.
• it does not truncate the input nonstring. nope.

⚠️ and listen — if the destination buffer doesn't already contain a string, or it isn't large enough, the behavior is undefined. undefined!! we do not do that here. see _FORTIFY_SOURCE in feature_test_macros(7) for the safety rails. 🚨

🍓 EXAMPLES 🍓

#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);
}

like… look at her. countof for the buffer, empty string first, then catenate exactly as many bytes as fit, then puts. chef's kiss 🧚

💜 SEE ALSO 💜

memory.h(3head), string_copying(7)

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and that's the whole page bestie 💖 this was Linux man-pages 6.20, dated 2026-08-05, and honestly? slay. go forth and never overflow a buffer, ily, bye 👋✨🎀