在makefile中链接库

Question

i have problem with linking libraries boost and curl in my simple makefile. When i use single command:

g++ -I /usr/lib/boost/include -L /usr/lib/boost/lib website.cpp website.h main.cpp data.cpp data.h -lboost_regex -lcurl

everything is ok but when i use makefile

a.out: website.o data.o main.o
    g++ -L /usr/lib/boost/lib -lboost_regex website.o data.o main.o -o a.out -lcurl 
data.o: data.cpp data.h
    g++ -c data.cpp -o data.o
website.o: website.cpp website.h
    g++ -I /usr/lib/boost/include -c website.cpp -o website.o 
main.o: main.cpp
    g++ -c main.cpp -o main.o

i have a lot of errors:

g++ -c main.cpp -o main.o 
g++ -L /usr/lib/boost/lib -lboost_regex website.o data.o main.o -o a.out -lcurl
website.o: In function boost::basic_regex<char, boost::regex_traits<char, boost::cpp_regex_traits<char> > >::assign(char const*, char const*, unsigned int)':
website.cpp:(.text._ZN5boost11basic_regexIcNS_12regex_traitsIcNS_16cpp_regex_traitsIcEEEEE6assignEPKcS7_j[boost::basic_regex<char, boost::regex_traits<char, boost::cpp_regex_traits<char> > >::assign(char const*, char const*, unsigned int)]+0x22): undefined reference to `boost::basic_

I use boost and curl in website.cpp (of course headlines are in website.h).

Answer 1

The problem is that -lboost_regex is being placed before .o s in the compiler command, unlike your single command which has -lboost_regex at the end.

Change:

g++ -L /usr/lib/boost/lib -lboost_regex website.o data.o main.o -o a.out -lcurl

to:

g++ -L/usr/lib/boost/lib website.o data.o main.o -o a.out -lcurl -lboost_regex

See GCC Link Options for explanantion of why the order matters.