View Compilation Error

0_0_34907718_1655.c: In function 'main':
0_0_34907718_1655.c:8:9: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
         for (int i=0;i<strlen(a);i++)
         ^
0_0_34907718_1655.c:8:9: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code