On 12/14/22 23:45, Alejandro Colomar wrote:
Hi,I was rewriting the strncat(3) manual page, and when I tried to compile the example program, I got a surprise from the compiler.Here goes the page: 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 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 *end; len = strnlen(src, sz); end = dst + strlen(dst); end = mempcpy(end, src, len); *end = '\0'; return dst; } RETURN VALUE strncat() returns dest. ATTRIBUTES [...] STANDARDS POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD. CAVEATS The name of this function is confusing. This function has no re‐ lation with strncpy(3). If the destination buffer is not large enough, the behavior is un‐ defined. 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 <stdio.h> #include <stdlib.h> #include <string.h> int main(void) { char buf[BUFSIZ]; size_t len; buf[0] = '\0'; // There’s no ’cpy’ function to this ’cat’. strncat(buf, "Hello ", 6); strncat(buf, "world", 42); // Padding null bytes ignored. strncat(buf, "!", 1); len = strlen(buf); printf("[len = %zu]: <%s>\n", len, buf); exit(EXIT_SUCCESS); } SEE ALSO string(3), string_copy(3) Linux man‐pages (unreleased) (date) strncat(3) And when you compile that, you get: $ cc -Wall -Wextra ./strncat.c ./strncat.c: In function ‘main’:./strncat.c:12:12: warning: ‘strncat’ specified bound 6 equals source length [-Wstringop-overflow=]12 | strncat(buf, "Hello ", 6); | ^~~~~~~~~~~~~~~~~~~~~~~~~./strncat.c:14:12: warning: ‘strncat’ specified bound 1 equals source length [-Wstringop-overflow=]14 | strncat(buf, "!", 1); | ^~~~~~~~~~~~~~~~~~~~So, what? Where's the problem? This function does exactly that: "take an unterminated character sequence and catenate it to an existing string". Clang seems to be fine with the code.
Maybe it's saying that I should be using strncat(buf, "!"); because the length is useless?
Cheers, Alex
-- <http://www.alejandro-colomar.es/>
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