View Compilation Error

0_0_20767719_20857.c: In function 'main':
0_0_20767719_20857.c:13:3: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
   for(int i=0; i<length1; i++){
   ^
0_0_20767719_20857.c:13:3: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
0_0_20767719_20857.c:21:11: error: redefinition of 'i'
   for(int i=0; i<length2; i++){
           ^
0_0_20767719_20857.c:13:11: note: previous definition of 'i' was here
   for(int i=0; i<length1; i++){
           ^
0_0_20767719_20857.c:21:3: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
   for(int i=0; i<length2; i++){
   ^