即使 nm 指示此符号存在于共享库中，对符号的未定义引用 [英] undefined reference to symbol even when nm indicates that this symbol is present in the shared library

问题描述

这里可能有什么问题?我有以下简单的类:

What could be wrong here? I have the following simple class:

#include  "libmnl/libmnl.h"

int main() {
    struct mnl_socket *a = mnl_socket_open(12);
}

在运行一个简单的 gcc 编译 (gcc -lmnl main.c) 后，我收到以下错误:

And after running a simple gcc compile (gcc -lmnl main.c) I get the following errors:

/tmp/cch3GjuS.o: In function `main':
main.c:(.text+0xe): undefined reference to `mnl_socket_open'
collect2: ld returned 1 exit status

在共享库上运行 nm 表明确实找到了:

Running nm on the shared library shows that it's actually found:

aatteka@aatteka-Dell1:/tmp$ nm -D /usr/lib/libmnl.so | grep mnl_socket_open
0000000000001810 T mnl_socket_open

这发生在 Ubuntu 12.04 上.libmnl-dev 和 libmnl0 软件包已安装.gcc 的 strace 输出表明 ld 正在使用该 *.so 文件:

This is happening on Ubuntu 12.04. The libmnl-dev and libmnl0 packages are installed. The strace output of gcc indicates that ld is using exactly that *.so file:

[pid 10988] stat("/usr/lib/gcc/x86_64-linux-gnu/4.6/libmnl.so", 0x7fff2a39b470) = -1 ENOENT (No such file or directory)
[pid 10988] open("/usr/lib/gcc/x86_64-linux-gnu/4.6/libmnl.so", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid 10988] stat("/usr/lib/gcc/x86_64-linux-gnu/4.6/libmnl.a", 0x7fff2a39b4d0) = -1 ENOENT (No such file or directory)
[pid 10988] open("/usr/lib/gcc/x86_64-linux-gnu/4.6/libmnl.a", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid 10988] stat("/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/libmnl.so", 0x7fff2a39b470) = -1 ENOENT (No such file or directory)
[pid 10988] open("/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/libmnl.so", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid 10988] stat("/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/libmnl.a", 0x7fff2a39b4d0) = -1 ENOENT (No such file or directory)
[pid 10988] open("/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/libmnl.a", O_RDONLY) = -1 ENOENT (No such file or directory)
[pid 10988] stat("/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib/libmnl.so", {st_mode=S_IFREG|0644, st_size=18608, ...}) = 0
[pid 10988] open("/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib/libmnl.so", O_RDONLY) = 7

推荐答案

库必须列在使用它们的对象之后(更准确地说，只有当库包含满足已知的未定义引用的符号时，才会使用库遇到的时间).将 -lmnl 移动到命令的末尾.

Libraries must be listed after the objects that use them (more precisely, a library will be used only if it contains a symbol that satisfies an undefined reference known at the time it is encountered). Move the -lmnl to the end of the command.

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