View Compilation Error

0_0_20414772_22616.cpp: In function 'int main()':
0_0_20414772_22616.cpp:14:19: error: 'strlen' was not declared in this scope
     for(i=strlen(a)-1;i>=0;i--)
                   ^
0_0_20414772_22616.cpp:20:19: error: 'strlen' was not declared in this scope
     for(i=strlen(b)-1;i>=0;i--)
                   ^
0_0_20414772_22616.cpp:25:15: error: 'strlen' was not declared in this scope
     m=strlen(a)>strlen(b)?strlen(a):strlen(b);
               ^
0_0_20414772_22616.cpp:40:12: error: unable to find string literal operator 'operator""a'
     printf("%s+%s="a,b);
            ^