using c11 standard with clang for use of strcpy_s

问题

I'm running OS X Sierra and trying to compile a c program that uses strcpy_s, but my installed clang compiler is using the c99 standard, but from what I've read strcpy_s requires c11.

Here's the code I'm trying to compile

#include <stdio.h>
#include <stdlib.h>
#include <string.h> 

int main(void)
{
    char source[] = "Test string";
    char destination[50];
    if(strcpy_s(destination, sizeof(destination), source))
        printf("string copied - %s",destination);

    return 0;
}

And here's the command I'm using to compile

$ clang copytest.c -o copytest
copytest.c:11:5: warning: implicit declaration of function 'strcpy_s' is invalid in C99 [-Wimplicit-function-declaration]
        if(strcpy_s(copied_string, sizeof(copied_string), source))
           ^
1 warning generated.
Undefined symbols for architecture x86_64:
  "_strcpy_s", referenced from:
      _main in copytest-e1e05a.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I've tried compiling with the standard flag...

clang -std=c11 copytest.c -o copytest

but I get the same exact "invalid in c99" warning. I've also tried compiling with gcc instead, and I still get the same c99 warning.

I tried upgrading via homebrew which shows the following

Warning: gcc 9.2.0 is already installed and up-to-date

I have clang version 9.0.0

$ clang -v
Apple LLVM version 9.0.0 (clang-900.0.39.2)

My xcode version is Xcode 9.2, which from everything I've read should come with c11 support.

Am I doing something wrong with the compiling, is my code itself incorrect? This is the only similar question I found on here, but it didn't even have an answer. Thanks

回答1:

The _s functions are an optional component of the 2011 C standard (Annex K), and, to the best of my knowledge, they have never been implemented as an integrated part of any C library. Portable code cannot rely on their availability. (Microsoft's C compilers for Windows implement an overlapping set of functions with the same names but different semantics (and sometimes even a different argument list), and at least one bolt-on implementation does exist. See this old answer, and the much longer question and answer it links to, for more detail.)

Also, the _s functions do not solve the problem that they were intended to solve (unsafe string handling); it is necessary to put actual thought into a proper fix for each use of strcpy, instead of globally search-and-replacing strcpy with strcpy_s, etc., as was the hope of the authors of Annex K. If you do put appropriate amounts of thought into a proper fix, you won't need any of the _s functions to implement it. For instance, here's a fixed version of your example program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h> 

int main(void)
{
    char source[] = "Test string";
    char destination[50];

    size_t srclen = strlen(source);
    if (srclen + 1 > sizeof destination) {
        fprintf(stderr, "string too long to copy - %zu bytes, need %zu\n",
                sizeof destination, srclen + 1);
        return 1;
    } else {
        memcpy(destination, source, srclen + 1);
        printf("string copied - %s\n", destination);
        return 0;
    }
}

And here's an even better version:

#define _XOPEN_SOURCE 700
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    if (argc != 2) {
        fprintf(stderr, "usage: ./test 'message of arbitrary length'\n");
        return 1;
    }
    char *destination = strdup(argv[1]);
    if (!destination) {
        perror("strdup");
        return 1;
    }
    printf("string copied - '%s'\n", destination);
    free(destination);
    return 0;
}

Therefore: Never use any of the _s functions. If you need to write a program that compiles on Windows with no warnings, put #define _CRT_SECURE_NO_WARNINGS 1 at the top of each file to make MSVC stop giving you bad advice.

来源：https://stackoverflow.com/questions/57915149/using-c11-standard-with-clang-for-use-of-strcpy-s

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