Pass raw pointer return by unique_ptr()::get() to a function taking double pointer as function parameter

Question

Code edit: https://gcc.godbolt.org/z/x3hnj46WY

Scenario -1 Trying to pass raw pointer by reference (or pointer) can't pass value of get() using & to setBuffer() // Compilation error : lvalue required as unary '&' operand

Scenario -2 Assign value return by get to a raw char pointer Pass char pointer to setBuffer() using &

NOTE: setBuffer() is present in a C library I am using this API https://www.libssh2.org/libssh2_session_last_error.html to get errmsg and instead of using a char array buffer I want to use smart pointer of char array.

#include <string.h>
#include <iostream>
#include <memory>

int setBuffer(char ** ptr) {
    // ASSUMPTION: *ptr has sufficient memory to store copied data
    strcpy(*ptr, "sample string"); 
    return 0;
}
 
int main()
{
    std::unique_ptr<char []> lup = std::make_unique<char []>(1024);
    memset(lup.get(), 0 , 1024);
    
    strcpy(lup.get(), "sample string - 1");
    std::cout << lup << '\n';                   // c++20
   
    // SCENARIO - 1 | Compilation error
    setBuffer(&lup.get());  // CE: lvalue required as unary '&' operand
    
    // SCENARIO - 2 | Works fine
    char * lp = lup.get();
    setBuffer(&lp);
    
    std::cout << lup << '\n';                   // c++20
    
    return 0;
}

Answer 1

According to the docs:

errmsg - If not NULL, is populated by reference with the human readable form of the most recent error message.

My reading is that it will not copy any error message to the given buffer, only set the pointer to a buffer (that contains the error message), stored in the session object.

Therefore usage would look like this:

char* errmsg = nullptr;
int errmsg_len = 0;
int ec = libssh2_session_last_error(session, &errmsg, &errmsg_len, /*want_buf=*/ 0);

printf("%s\n", errmsg);

On the other hand, if want_buf is set to a non-zero value, a new buffer will be allocated, and the caller is responsible to call free on it later.