foo.c: In function 'main':
foo.c:11:9: error: 'len' undeclared (first use in this function)
   11 |         len = strlen(a);
      |         ^~~
foo.c:11:9: note: each undeclared identifier is reported only once for each function it appears in
foo.c:21:1: error: expected declaration or statement at end of input
   21 | }
      | ^

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