View Compilation Error

0_0_21813412_26114.cpp: In function 'Matrix operator*(Matrix, Matrix)':
0_0_21813412_26114.cpp:11:27: error: 'memset' was not declared in this scope
   memset(c.a,0,sizeof(c.a));
                           ^
0_0_21813412_26114.cpp: In function 'void build(int, int, int)':
0_0_21813412_26114.cpp:27:46: error: 'memset' was not declared in this scope
    memset(tree[p][i].a,0,sizeof(tree[p][i].a));
                                              ^