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Showing content from https://en.cppreference.com/w/cpp/language/../keywords/../numeric/../string/basic_string/c_str.html below:

std::basic_string<CharT,Traits,Allocator>::c_str - cppreference.com

const CharT* c_str() const;

Returns a pointer to a null-terminated character array with data equivalent to those stored in the string.

The pointer is such that the range [c_str(), c_str() + size()] is valid and the values in it correspond to the values stored in the string with an additional null character after the last position.

The pointer obtained from c_str() may be invalidated by:

• Passing a non-const reference to the string to any standard library function, or
• Calling non-const member functions on the string, excluding operator[], at(), front(), back(), begin(), rbegin(), end() and rend()(since C++11).

Writing to the character array accessed through c_str() is undefined behavior.

c_str() and data() perform the same function.

(none)

Pointer to the underlying character storage.

c_str()[i] == operator[](i) for every i in [​0​, size()).

c_str() + i == std::addressof(operator[](i)) for every i in [​0​, size()].

Constant.

The pointer obtained from c_str() may only be treated as a pointer to a null-terminated character string if the string object does not contain other null characters.

#include <algorithm>
#include <cassert>
#include <cstring>
#include <string>
 
extern "C" void c_func(const char* c_str)
{
    printf("c_func called with '%s'\n", c_str);
}
 
int main()
{
    std::string const s("Emplary");
    const char* p = s.c_str();
    assert(s.size() == std::strlen(p));
    assert(std::equal(s.begin(), s.end(), p));
    assert(std::equal(p, p + s.size(), s.begin()));
    assert('\0' == *(p + s.size()));
 
    c_func(s.c_str());
}

Output:

c_func called with 'Emplary'

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