/in/foo.c: In function 'main':
/in/foo.c:8:11: error: 'b' undeclared (first use in this function)
  j=strlen(b)-1;
           ^
/in/foo.c:8:11: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:16:9: error: 'all' undeclared (first use in this function)
  while(*all*b)
         ^~~
/in/foo.c:18:3: error: 'c' undeclared (first use in this function)
  *c=*a;
   ^
/in/foo.c:18:6: error: 'a' undeclared (first use in this function)
  *c=*a;
      ^
/in/foo.c: At top level:
/in/foo.c:29:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main()
      ^~~~
/in/foo.c:29:6: error: conflicting types for 'main'
/in/foo.c:4:5: note: previous definition of 'main' was here
 int main()
     ^~~~
/in/foo.c: In function 'main':
/in/foo.c:33:2: error: 'fp' undeclared (first use in this function)
  fp=forpen("TIXT1","W");
  ^~
/in/foo.c:33:5: warning: implicit declaration of function 'forpen' [-Wimplicit-function-declaration]
  fp=forpen("TIXT1","W");
     ^~~~~~
/in/foo.c:35:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s1);
  ^~~~
In file included from /in/foo.c:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.c:37:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s2);
  ^~~~
In file included from /in/foo.c:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.c:32:7: warning: unused variable 'FP' [-Wunused-variable]
  FILE*FP;
       ^~