View Compilation Error

0_0_19451596_14602.c: In function 'main':
0_0_19451596_14602.c:24:5: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
     for (int i = 0; i < len; ++i) {
     ^
0_0_19451596_14602.c:24:5: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code