I wrote a code in C in Ubuntu which checks for balanced brackets in the input given.I compiled it using gcc compiler and I am getting the correct output.This is actually a part of an online course and they are asking me to use the compiler flag 

```
gcc -pipe -O2 -std=c11 filename -lm

```

I don't think I understand what I am supposed to do so I tried compiling using this flag and my code is not compiling. My question is if my code compiles when I do 

```
gcc filename.c

```

why isn't it compiling when I do 

```
gcc -pipe -O2 -std=c11 filename -lm

```

The error message I am getting is :

```
cc1plus: warning: command line option ‘-std=c11’ is valid for C/ObjC but not for C++

```