Operating Systems 2014F: Tutorial 4

In this tutorial you will be learning how address spaces and segments are present on modern Linux systems.

For hello.c, syscall-hello.c, and csimpleshell.c (from Tutorial 2) do the following (substituting the appropriate source file for prog.c):

1. Compile the program prog.c using gcc -O2 prog.c -o prog-dyn and run prog-dyn. What does it do?
2. Statically compile and optimize prog.c by running gcc -O2 -static prog.c -o prog-static. How does the size compare with prog?
3. See what system calls prog-static produces by running strace -o static.log ./prog-static. Do the same for prog-dyn. Which version generates more system calls?
4. Using the nm command, see what symbols are defined in prog-static and prog-dyn.
5. Run the command gcc -c -O2 prog.c to produce an object file. What file was produced? What symbols does it define?
6. Look at the assembly code of the program by running gcc -S -O2 prog.c. What file was produced? Identify the following:
   • A function call
   • A system call (if any)
   • A global/static variable (if any)
   • A local variable
7. Disassemble the object file using objdump -d. How does this disassembly compare with the output from gcc -S?
8. Examine the headers of object file, dynamically linked executable, and the statically linked executable using objdump -h
9. Examine the contents of object file, dynamically linked executable, and the statically linked executable using objdump -s
10. Re-run all of the previous gcc commands adding the "-v" flag. What is all of that output?
11. (optional) Look up the documentation for each of the system calls made by the static versions of the programs. You may need to append a 2 to the manpage invocation, e.g. "man 2 write" gets you the write system call documentation.

Code

hello.c

#include <stdio.h>

int main(int argc, char *argv[]) {

        printf("Hello world!\n");

        return 0;
}

syscall-hello.c

#include <unistd.h>
#include <sys/syscall.h>

char *buf = "Hello world!\n";

int main(int argc, char *argv) {
        size_t result;

        /* "man 2 write" to see arguments to write syscall */
        result = syscall(SYS_write, 1, buf, 13);

        return (int) result;
}