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int main(void) { 
        FILE *f; 
        f = fopen("file","wb"); 
        printf("%d",f != NULL); 
        fclose(f); 
        return 0; 
    }

prog.c: In function ‘main’:
prog.c:2:9: error: unknown type name ‘FILE’
         FILE *f;
         ^~~~
prog.c:3:13: warning: implicit declaration of function ‘fopen’ [-Wimplicit-function-declaration]
         f = fopen("file","wb");
             ^~~~~
prog.c:3:11: warning: assignment makes pointer from integer without a cast [-Wint-conversion]
         f = fopen("file","wb");
           ^
prog.c:4:9: warning: implicit declaration of function ‘printf’ [-Wimplicit-function-declaration]
         printf("%d",f != NULL);
         ^~~~~~
prog.c:4:9: warning: incompatible implicit declaration of built-in function ‘printf’
prog.c:4:9: note: include ‘<stdio.h>’ or provide a declaration of ‘printf’
prog.c:4:26: error: ‘NULL’ undeclared (first use in this function)
         printf("%d",f != NULL);
                          ^~~~
prog.c:4:26: note: each undeclared identifier is reported only once for each function it appears in
prog.c:5:9: warning: implicit declaration of function ‘fclose’ [-Wimplicit-function-declaration]
         fclose(f);
         ^~~~~~

Standard output is empty