图书馆的未定义参考,那么我怎样才能找到正确的路径？

Question

我想在Ubuntu中编译一个v4l2示例,但是我收到以下错误:

guilherme@notedev01:~/Downloads/V4l2_samples-0.4.1$ make
gcc -O2  -L/usr/include -lX11 -lXext -o viewer viewer.c
/tmp/ccUjnjWQ.o: In function `image_destroy':
viewer.c:(.text+0x234): undefined reference to `XDestroyImage'
viewer.c:(.text+0x256): undefined reference to `XFreeGC'
viewer.c:(.text+0x277): undefined reference to `XShmDetach'
viewer.c:(.text+0x2ac): undefined reference to `XFreePixmap'
/tmp/ccUjnjWQ.o: In function `image_create':
viewer.c:(.text+0x305): undefined reference to `XCreateGC'
viewer.c:(.text+0x31d): undefined reference to `XGetWindowAttributes'
viewer.c:(.text+0x39e): undefined reference to `XShmCreateImage'
viewer.c:(.text+0x3f5): undefined reference to `XShmAttach'
viewer.c:(.text+0x44e): undefined reference to `XCreateImage'
viewer.c:(.text+0x494): undefined reference to `XShmQueryExtension'
viewer.c:(.text+0x4b4): undefined reference to `XShmPixmapFormat'
viewer.c:(.text+0x4dc): undefined reference to `XShmCreatePixmap'
/tmp/ccUjnjWQ.o: In function `image_put':
viewer.c:(.text+0x54c): undefined reference to `XPutImage'
viewer.c:(.text+0x586): undefined reference to `XShmPutImage'
/tmp/ccUjnjWQ.o: In function `main':
viewer.c:(.text.startup+0x18b): undefined reference to `XOpenDisplay'
viewer.c:(.text.startup+0x1b1): undefined reference to `XScreenOfDisplay'
viewer.c:(.text.startup+0x1ee): undefined reference to `XCreateSimpleWindow'
viewer.c:(.text.startup+0x249): undefined reference to `XMapRaised'
viewer.c:(.text.startup+0x263): undefined reference to `XStoreName'
viewer.c:(.text.startup+0x280): undefined reference to `XGetWindowAttributes'
viewer.c:(.text.startup+0x92f): undefined reference to `XPending'
viewer.c:(.text.startup+0x94c): undefined reference to `XNextEvent'
viewer.c:(.text.startup+0xaee): undefined reference to `XPending'
viewer.c:(.text.startup+0xb0b): undefined reference to `XNextEvent'
viewer.c:(.text.startup+0xf39): undefined reference to `XPending'
viewer.c:(.text.startup+0xf56): undefined reference to `XNextEvent'
collect2: error: ld returned 1 exit status
make: *** [viewer] Error 1

我能看到的是-lx11和-lXext的路径不是-L/usr/include.如何为这些库找到正确的路径？

谢谢.

Answer 1

正如克里斯指出的那样,顺序是错误的,你需要把-lX11 -lXext源代码/对象文件放在后面.

这是因为现代编译器试图优化最终结果而不是链接未使用的库.他们通过保持未解决的符号列表对象中,并使用来任何二进制文件做aferwards在连接参数来解决这些符号.

例

你的程序测试使用功能do_foo()从libfoo的和函数do_bar_do()从libbar的.

你用它链接它:

$ gcc -o test test.o -lfoo -lbar

链接器首先搜索test.o并注意到某些符号(do_foo和do_bar_do)未在任何地方定义.然后它进入libfoo(后面指定test.o)并发现它提供do_foo,因此它创建代码以从程序中使用它.do_bar_do仍未解决,直到链接器检查libbar.

考虑以错误的方式做到这一点:

$ gcc -o test -lfoo test.o -lbar

链接器将首先检查libfoo它是否包含任何未解析的符号.凉.那么它会继续test.o和通知do_bar_do和do_foo.do_bar_do由右手解决,libbar但do_foo根本没有解决,你得到一个错误:

对`do_foo'的未定义引用

"但代码是一个教程..."

那为什么不起作用？

较旧的编译器,它们对依赖关系的顺序有点松懈(它们会检查所有二进制文件/库/对象是否可以解析给定的符号); 这就是为什么你仍然可以找到那些使库在目标文件之前链接的代码.