编译libnftnl示例代码出现链接错误，如何解决？

问题 #

我需要编译nftables归档中的libnftnl示例源码，操作步骤如下：

• 执行命令sudo apt-get source libnftnl11获取源码包
• 进入libnftnl-1.2.1/examples/目录，该目录下有多个包含main函数的小型示例源码文件
• 尝试使用gcc nft-chain-add.c -o main -lnftables编译nft-chain-add.c，却出现以下链接错误：

gcc nft-chain-add.c -o main -lnftables 
/usr/bin/ld: /tmp/cc8sxDLv.o: in function `chain_add_parse':
nft-chain-add.c:(.text+0x133): undefined reference to `nftnl_chain_alloc'
/usr/bin/ld: nft-chain-add.c:(.text+0x170): undefined reference to `nftnl_chain_set_str'
/usr/bin/ld: nft-chain-add.c:(.text+0x18c): undefined reference to `nftnl_chain_set_str'
/usr/bin/ld: nft-chain-add.c:(.text+0x1a6): undefined reference to `nftnl_chain_set_u32'
/usr/bin/ld: nft-chain-add.c:(.text+0x1cc): undefined reference to `nftnl_chain_set_u32'
/usr/bin/ld: /tmp/cc8sxDLv.o: in function `main':
nft-chain-add.c:(.text+0x454): undefined reference to `mnl_nlmsg_batch_start'
/usr/bin/ld: nft-chain-add.c:(.text+0x470): undefined reference to `mnl_nlmsg_batch_current'
/usr/bin/ld: nft-chain-add.c:(.text+0x47b): undefined reference to `nftnl_batch_begin'
/usr/bin/ld: nft-chain-add.c:(.text+0x487): undefined reference to `mnl_nlmsg_batch_next'
/usr/bin/ld: nft-chain-add.c:(.text+0x4ac): undefined reference to `mnl_nlmsg_batch_current'
/usr/bin/ld: nft-chain-add.c:(.text+0x4c4): undefined reference to `nftnl_nlmsg_build_hdr'
/usr/bin/ld: nft-chain-add.c:(.text+0x4db): undefined reference to `nftnl_chain_nlmsg_build_payload'
/usr/bin/ld: nft-chain-add.c:(.text+0x4e7): undefined reference to `nftnl_chain_free'
/usr/bin/ld: nft-chain-add.c:(.text+0x4f3): undefined reference to `mnl_nlmsg_batch_next'
/usr/bin/ld: nft-chain-add.c:(.text+0x50b): undefined reference to `mnl_nlmsg_batch_current'
/usr/bin/ld: nft-chain-add.c:(.text+0x516): undefined reference to `nftnl_batch_end'
/usr/bin/ld: nft-chain-add.c:(.text+0x522): undefined reference to `mnl_nlmsg_batch_next'
/usr/bin/ld: nft-chain-add.c:(.text+0x52c): undefined reference to `mnl_socket_open'
/usr/bin/ld: nft-chain-add.c:(.text+0x566): undefined reference to `mnl_socket_bind'
/usr/bin/ld: nft-chain-add.c:(.text+0x58f): undefined reference to `mnl_socket_get_portid'
/usr/bin/ld: nft-chain-add.c:(.text+0x59e): undefined reference to `mnl_nlmsg_batch_size'
/usr/bin/ld: nft-chain-add.c:(.text+0x5ad): undefined reference to `mnl_nlmsg_batch_head'
/usr/bin/ld: nft-chain-add.c:(.text+0x5c2): undefined reference to `mnl_socket_sendto'
/usr/bin/ld: nft-chain-add.c:(.text+0x5ec): undefined reference to `mnl_nlmsg_batch_stop'
/usr/bin/ld: nft-chain-add.c:(.text+0x602): undefined reference to `mnl_socket_recvfrom'
/usr/bin/ld: nft-chain-add.c:(.text+0x62b): undefined reference to `mnl_cb_run'
/usr/bin/ld: nft-chain-add.c:(.text+0x64a): undefined reference to `mnl_socket_recvfrom'
/usr/bin/ld: nft-chain-add.c:(.text+0x681): undefined reference to `mnl_socket_close'
collect2: error: ld returned 1 exit status

解决方法 #

1. 链接正确的库文件 #

你当前用的-lnftables是nftables主程序的库，而示例依赖的是libnftnl和libmnl两个库，编译命令需要改成：

gcc nft-chain-add.c -o main -lnftnl -lmnl

• -lnftnl：对应nftnl_*开头的函数（比如nftnl_chain_alloc）
• -lmnl：对应mnl_*开头的函数（比如mnl_nlmsg_batch_start）

2. 确保依赖库已安装开发包 #

如果执行上述命令仍报错，说明缺少库的开发文件，先安装：

sudo apt-get install libnftnl-dev libmnl-dev

3. 直接使用源码自带的Makefile编译（推荐） #

libnftnl源码包的examples目录下已有现成的Makefile，直接执行make就能自动编译所有示例，无需手动编写gcc命令：

cd libnftnl-1.2.1/examples/
make

编译完成后，当前目录会生成对应示例的可执行文件，直接运行即可。

内容的提问来源于stack exchange，提问作者mah454