Question

I need link cURL in Ubuntu 11.04 after installed cURL by source code.

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Correction of the PROBLEM

First I discovered that the -l must come before the -L and then discovered that I was not entering a variable in the makefile.

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Get cURL Configs:

On my termial:

# curl-config --libs
-L/usr/local/lib -lcurl

# curl-config --cflags
-I/usr/local/include

It’s all right, where this directory there are files cURL.

---

My Makefile:

# Testing cURL
# MAKEFILE

# C++ Compiler (Default: g++)
CXX = g++
CFLAGS = -Wall -Werror

# Librarys
INCLUDE = -Iusr/local/include
LDFLAGS = -Lusr/local/lib 
LDLIBS = -lcurl

# Details
SOURCES = src/main.cpp
OUT = test

all: build

build: $(SOURCES)
    $(CXX) -o $(OUT) $(INCLUDE) $(CFLAGS) $(LDFLAGS) $(SOURCES)

---

My C++ Source Code:

#include <iostream>
#include <curl/curl.h>
 
int main( void )
{
    CURL *curl;
    CURLcode res;
 
    curl = curl_easy_init();
    if(curl) {
        curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
        res = curl_easy_perform(curl);
        curl_easy_cleanup(curl);
    }
    
    return 0;
}

---

And the ERROR:

# make
g++ -o test -Iusr/local/include -Wall -Werror -Lusr/local/lib  src/main.cpp 
/tmp/ccli90i2.o: In function `main':
main.cpp:(.text+0xa): undefined reference to `curl_easy_init'
main.cpp:(.text+0x31): undefined reference to `curl_easy_setopt'
main.cpp:(.text+0x3d): undefined reference to `curl_easy_perform'
main.cpp:(.text+0x4d): undefined reference to `curl_easy_cleanup'
collect2: ld returned 1 exit status
make: ** [build] Erro 1

---

I know this is a error of not finding the library, but for me everything is correct

Answer 1

This should do the job. You didn’t really link to cURL before.

build: $(SOURCES)
    $(CXX) -o $(OUT) $(INCLUDE) $(CFLAGS) $(LDFLAGS) $(LDLIBS) $(SOURCES)

Notice the added $(LDLIBS).

Oh, I should add that basically what happens is that you throw overboard the built-in rules of GNU make (see output of make -np) and define your own. I would suggest that you either use the built-in ones if you want to rely on the respective variables to be sufficient to control the build or that you still split it up into compilation and link step for the sake of brevity.

Brief explanation: GNU make comes with a rule that states how to make a .o file from a .cpp (or .c) file. So your make file could perhaps be rewritten to (approx.)

# Testing cURL
# MAKEFILE

# C++ Compiler (Default: g++)
CXX = g++
CFLAGS = -Wall -Werror

# Librarys
INCLUDE = -I/usr/local/include
LDFLAGS = -L/usr/local/lib 
LDLIBS = -lcurl

# Details
SOURCES = src/main.cpp
OUT = test

.PHONY: all

all: build

$(OUT): $(patsubst %.cpp,%.o,$(SOURCES))

This should generate the binary with the name test (contents of OUT) and makes otherwise use of the built-in rules. Make infers from the use of .o files that there must be source files, will look for them and compile them. So implicitly this build will run one invocation for each .cpp file and one for the linking step.