Lecture 33 – May 4th, 2020


  1. Log in to clyde.
  2. Create a new directory.


  1. Write a short program containing a function with the prototype
    double varsum(int num, ...)

    where the num parameter controls how many additional double parameters there are. varsum should add up all of the double parameters and return the result. If num <= 0, return 0.0.

  2. The printf family of functions includes (among others) printf(3), fprintf(3), vprintf(3), and vfprintf(3). The first three can easily be written in terms of the fourth.

    Write four similar functions

    void msg(char const *type, char const *fmt, ...);
    void vmsg(char const *type, char const *fmt, va_list ap);
    void fmsg(FILE *fp, char const *type, char const *fmt, ...);
    void vfmsg(FILE *fp, char const *type, char const *fmt, va_list ap);

    where the intended behavior is

    msg("Info", "0x%x %d", 32, 5);

    prints [Info] 0x20 5.

    Implement the first three in terms of the fourth. Feel free to use fprintf(3), vfprintf(3), or other standard I/O functions to implement vfmsg.

    Write a short program that tests them.

  3. The execl(3) function takes a path and a variable number of arguments. The end of the argument list is denoted by an explicit (char *)0.

    Using the same strategy of marking the end of the list with (char *)0, write a function

    char *join(char const *sep, ...)

    That allocates and returns a string consisting of the variable number of strings joined together but separated the string sep. For example,

    join(" ", "This", "is", "a", "sentence.", (char *)0);`

    should return the string "This is a sentence.";

    join("foo", (char *)0);

    should return the empty string ""; and

    join("+-+" "a", "b", "c", (char *)0);

    should return the string "a+-+b+-+c".

    In all cases, the string should be allocated via malloc(3) or realloc(3). Make sure you don’t leak memory.

    Write a short program to test your functions. The strings returned from join should ultimately be passed to free(3).