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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ATTRIBUTES |STANDARDS |HISTORY |CAVEATS |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
strncat(3) Library Functions Manualstrncat(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 *restrictdst, const charsrc[restrictssize],size_tssize);
This function appends at mostssize non-null bytes from the array pointed to bysrc, followed by a null character, to the end of the string pointed to bydst.dst must point to a string contained in a buffer that is large enough, that is, the buffer size must be at leaststrlen(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() returnsdst.
For an explanation of the terms used in this section, seeattributes(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 tostrncpy(3). If the destination buffer does not already contain a string, or is not large enough, the behavior is undefined. See_FORTIFY_SOURCE infeature_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 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 2025-06-28strncat(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)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |