参考文章：http://www.linuxquestions.org/questions/programming-9/undefined-reference-to-%60__isoc99_sscanf%27-873058/
原文关键内容：
You have glibc version prior to 2.7 installed, and are using libraries (ffmpeg?) which were compiled against glibc 2.7. The __isoc99_sscanf was added to glibc in version 2.7.
You'll need to check which static library file (*.a) refers to that symbol, and recompile it. You can use command
Code:
objdump -t path-to-library.a | grep -e __isoc99_sscanf
to find which library refers to that symbol.
Alternatively, you can upgrade your c library to version 2.7 or newer. If you're using Debian or Ubuntu, upgrading package
libc6-dev
to at least 2.7 should be enough.
If the error message referred to an unversioned symbol (i.e. the error message did not refer to __isoc99_sscanf@@GLIBC...), then you could write a __isoc99_sscanf wrapper using stdargs and GNU sscanf (their differences being usually insignificant) and include that in the linking.
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错误：
undefined reference to `__isoc99_sscanf'
原因是我们的程序中使用的某个库，如xxx.a, xxx.so是在高版本的glibc环境里面进行编译的。
有2种解决方法：
1. 升级我们的glibc到2.7版本
2. 找到xxx.a 或 xxx.so，在我们的系统里面重新编译，然后拿来使用即可。
