Linking custom library with libcurl functions

Question

I have a custom static library called "libcurlwrapper", which just wrapps the execution of libcurl functions, and a main app which calls the library's function.

Compiling the library works, but when it comes to linking the main app I get these errors:

/usr/bin/ld: .//libcurlwrapper.a(curl_wrapper.o): in function `http3_demo()':
curl_wrapper.cpp:(.text+0xd): undefined reference to `curl_easy_init'
/usr/bin/ld: curl_wrapper.cpp:(.text+0x39): undefined reference to `curl_easy_setopt'
/usr/bin/ld: curl_wrapper.cpp:(.text+0x54): undefined reference to `curl_easy_setopt'
/usr/bin/ld: curl_wrapper.cpp:(.text+0x60): undefined reference to `curl_easy_perform'
/usr/bin/ld: curl_wrapper.cpp:(.text+0x73): undefined reference to `curl_easy_strerror'
/usr/bin/ld: curl_wrapper.cpp:(.text+0x9d): undefined reference to `curl_easy_cleanup'
collect2: error: ld returned 1 exit status
make: *** [Makefile:5: all] Error 1

What exactly causes these linker errors?

Ideas how to get rid of them?

Any Help is appreciated!

Thanks

libcurlwrapper > curl_wrapper.h:

#pragma once

#include <curl/curl.h>

void http3_demo();

libcurlwrapper > curl_wrapper.cpp:

#include "curl_wrapper.h"

void http3_demo() {

    // Source taken from here: https://curl.se/libcurl/c/http3.html

    CURL *curl;
    CURLcode res;

    curl = curl_easy_init();

    if(curl) {

        curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
        curl_easy_setopt(curl, CURLOPT_HTTP_VERSION, (long)CURL_HTTP_VERSION_3);

        // Perform the request, res will get the return code
        res = curl_easy_perform(curl);

        // Check for errors
        if(res != CURLE_OK) {
            fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
        }

        // Always cleanup
        curl_easy_cleanup(curl);

    }

    printf("http3_demo: endmarker");

}

libcurlwrapper > Makefile:

CXX = g++
CXXFLAGS = -lcurl

all:
    @$(CXX) -c ./curl_wrapper.cpp $(CXXFLAGS)
    @ar -rc libcurlwrapper.a ./curl_wrapper.o

main_app > main.cpp:

#include "curl_wrapper.h"

int main() {

    http3_demo();

    return 0;

}

main_app > Makefile:

CXX = g++
CXXFLAGS = -L ./ -lcurlwrapper

all:
    @$(CXX) main.cpp $(CXXFLAGS) -o main_app

Answer 1

A static library is compiled into the final executable. External functions used by a static library are just references. The main executable that uses the static library will need to resolve the references to all of the external libraries that the static library refers to. In this case, that means the main executable project needs to link to the libcurl library.