You can use a returned stucts in c to pass both an err code and a value

    typedef struct
    {
       int err;
       int value;
    } rtn_t;

    rtn_t rtn = foo();
    if(rtn.err < 0)
       printf( "err=%i", rtn.err);
    else
       printf( "value = %i\n", rtn.value);