foo.cc: In function 'void mul(int*, int*, int, int)':
foo.cc:18:33: warning: 'len' may be used uninitialized [-Wmaybe-uninitialized]
   18 |         for (int i = 0; i < len + nx + 1; i++)
      |                             ~~~~^~~~
foo.cc:7:13: note: 'len' was declared here
    7 |         int len;
      |             ^~~

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