访问string::c_str()结果时的内存访问错误

Question

因此，当我遇到最奇怪的内存问题时，我正在编写一些网络代码，我无法正确地理清这里可能发生的事情。我想知道，在c_str()中是否有某种含义，我没有正确地观察到。

这是包含错误的代码。(也有一个释放错误，但我把这个函数作为一个宠物项目)。

#include <netdb.h>       // for AF_UNSPEC, AF_INET, AF_INET6
#include <stdint.h>      // for uint16_t, etc.
#include <sys/types.h>   // for AF_UNSPEC, AF_INET, AF_INET6
#include <sys/socket.h>  // for AF_UNSPEC, AF_INET, AF_INET6
#include <string>        // for std::strin
#include <stdio.h> 
#include <string.h>
int main() {
    uint16_t port = 2098;


    struct addrinfo hints;
    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_INET6;       // IPv6 (also handles IPv4 clients)
    hints.ai_socktype = SOCK_STREAM;  // stream
    hints.ai_flags = AI_PASSIVE;      // use wildcard "INADDR_ANY"
    hints.ai_protocol = IPPROTO_TCP;  // tcp protocol
    hints.ai_canonname = nullptr;
    hints.ai_addr = nullptr;
    hints.ai_next = nullptr;

    const char* port_num = (std::to_string(port)).c_str();

    struct addrinfo *result;
    int res = getaddrinfo(nullptr, port_num, &hints, &result);

    printf("HI\n");
}

如果您对生成的二进制文件进行了精打细算，将得到：

==45919== Invalid read of size 1
==45919==    at 0x573BA5C: getaddrinfo (in /usr/lib64/libc-2.17.so)
==45919==    by 0x400C95: main (in /homes/iws/kieruc/Coding/a.out)
==45919==  Address 0x5a22058 is 24 bytes inside a block of size 29 free'd
==45919==    at 0x4C2B16D: operator delete(void*) (vg_replace_malloc.c:576)
==45919==    by 0x4EF3B62: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() (in /usr/lib64/libstdc++.so.6.0.19)
==45919==    by 0x400C7C: main (in /homes/iws/kieruc/Coding/a.out)
==45919==  Block was alloc'd at
==45919==    at 0x4C2A1E3: operator new(unsigned long) (vg_replace_malloc.c:334)
==45919==    by 0x4EF3A18: std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) (in /usr/lib64/libstdc++.so.6.0.19)
==45919==    by 0x400FFE: char* std::string::_S_construct<char*>(char*, char*, std::allocator<char> const&, std::forward_iterator_tag) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400F14: char* std::string::_S_construct_aux<char*>(char*, char*, std::allocator<char> const&, std::__false_type) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400EDD: char* std::string::_S_construct<char*>(char*, char*, std::allocator<char> const&) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400E8F: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string<char*>(char*, char*, std::allocator<char> const&) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400E20: std::string __gnu_cxx::__to_xstring<std::string, char>(int (*)(char*, unsigned long, char const*, __va_list_tag*), unsigned long, char const*, ...) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400CDE: std::to_string(int) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400C60: main (in /homes/iws/kieruc/Coding/a.out)
==45919== 
==45919== Invalid read of size 1
==45919==    at 0x573BACA: getaddrinfo (in /usr/lib64/libc-2.17.so)
==45919==    by 0x400C95: main (in /homes/iws/kieruc/Coding/a.out)
==45919==  Address 0x5a22058 is 24 bytes inside a block of size 29 free'd
==45919==    at 0x4C2B16D: operator delete(void*) (vg_replace_malloc.c:576)
==45919==    by 0x4EF3B62: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::~basic_string() (in /usr/lib64/libstdc++.so.6.0.19)
==45919==    by 0x400C7C: main (in /homes/iws/kieruc/Coding/a.out)
==45919==  Block was alloc'd at
==45919==    at 0x4C2A1E3: operator new(unsigned long) (vg_replace_malloc.c:334)
==45919==    by 0x4EF3A18: std::string::_Rep::_S_create(unsigned long, unsigned long, std::allocator<char> const&) (in /usr/lib64/libstdc++.so.6.0.19)
==45919==    by 0x400FFE: char* std::string::_S_construct<char*>(char*, char*, std::allocator<char> const&, std::forward_iterator_tag) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400F14: char* std::string::_S_construct_aux<char*>(char*, char*, std::allocator<char> const&, std::__false_type) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400EDD: char* std::string::_S_construct<char*>(char*, char*, std::allocator<char> const&) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400E8F: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string<char*>(char*, char*, std::allocator<char> const&) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400E20: std::string __gnu_cxx::__to_xstring<std::string, char>(int (*)(char*, unsigned long, char const*, __va_list_tag*), unsigned long, char const*, ...) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400CDE: std::to_string(int) (in /homes/iws/kieruc/Coding/a.out)
==45919==    by 0x400C60: main (in /homes/iws/kieruc/Coding/a.out)

但是，如果我将代码更改为

std::string portstr = std::to_string(port);

struct addrinfo *result;
int res = getaddrinfo(nullptr, portstr.c_str(), &hints, &result);

那么就没有内存错误。我不太明白为什么。

另外要注意的是，如果我尝试用const char*变量编译char*版本，您会得到一个警告，即变量应该是常量。

这里发生什么事情？

Answer 1

c_str返回的缓冲区只有在关联的std::string对象存在时才有效。还有这里

const char* port_num = (std::to_string(port)).c_str();

创建一个临时字符串对象，获取它的缓冲区地址，然后在完整表达式的末尾死亡。使用悬空指针时会出现内存错误。

如果希望使用临时std::string，则必须在使用缓冲区的完整表达式期间创建它：

int res = getaddrinfo(nullptr, std::to_string(port).c_str(), &hints, &result)

Answer 2

使用const char* port_num = (std::to_string(port)).c_str();，您将创建一个类型为std::string的临时对象，该对象的生存期与使用它的表达式相同。因此，.c_str()将指向在语句之后立即释放的内存。

用它画两条线：

auto portStr = std::to_string(port);
const char* port_num = portStr.c_str();

因此，portStr-object将一直保存到函数的末尾，并且您可以将.c_str-call的结果一直使用到函数的末尾(除非在函数之间更改protStr )。