strncat(3) Library Functions Manual strncat(3) NAME strncat - concatenate a null-padded character sequence into a string LIBRARY Standard C library (libc, -lc) SYNOPSIS #include <string.h> char *strncat(char *restrict dst, const char src[restrict .sz], size_t sz); DESCRIPTION This function catenates the input character sequence contained in a null-padded fixed-width buffer, into a string at the buffer pointed to by dst. The programmer is responsible for allocating a destination buffer large enough, that is, strlen(dst) + strnlen(src, sz) + 1. An implementation of this function might be: char * strncat(char *restrict dst, const char *restrict src, size_t sz) { int len; char *p; len = strnlen(src, sz); p = dst + strlen(dst); p = mempcpy(p, src, len); *p = '\0'; return dst; } RETURN VALUE strncat() returns dst. ATTRIBUTES For an explanation of the terms used in this section, see at- tributes(7). ┌────────────────────────────────────────────┬───────────────┬─────────┐ │Interface │ Attribute │ Value │ ├────────────────────────────────────────────┼───────────────┼─────────┤ │strncat() │ Thread safety │ MT-Safe │ └────────────────────────────────────────────┴───────────────┴─────────┘ STANDARDS POSIX.1-2001, POSIX.1-2008, C99, SVr4, 4.3BSD. CAVEATS The name of this function is confusing. This function has no relation to strncpy(3). If the destination buffer is not large enough, the behavior is unde- fined. See _FORTIFY_SOURCE in feature_test_macros(7). BUGS This function can be very inefficient. Read about Shlemiel the painter ⟨https://www.joelonsoftware.com/2001/12/11/back-to-basics/⟩. EXAMPLES #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 maxsize; // Null-padded fixed-width character sequences char pre[4] = "pre."; char new_post[50] = ".foo.bar"; // Strings char post[] = ".post"; char src[] = "some_long_body.post"; char *dest; maxsize = nitems(pre) + strlen(src) - strlen(post) + nitems(new_post) + 1; dest = malloc(sizeof(*dest) * maxsize); 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); } SEE ALSO string(3), string_copying(3) Linux man-pages 6.03 2023-02-05 strncat(3)
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