Assignment 3, Solutions

Part of the homework for 22C:112, Fall 2012
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

Background: When you call a subroutine in a civilized programming language, for example f(a,b,c,d) it is perfectly reasonable to set things up so that parameter a is a file variable that you have already opened and partially modified, parameter b is a character string, parameter c is an integer, and parameter d is the handle on an object.
Now suppose you have decided to make f into a program so you can execute it as an application from within shell scripts as well as from other application programs.
a) Which of the 4 parameters could you pass through to the new version of f() via execve() in essentially the way they were passed in the original. (0.5 point)

Parameter b, the character string.

b) Suggest appropriate compromizes that would allow you to pass the information required for the other parameters despite the barriers created by the execve() mechanism. (0.5 point)
Hint: The key information is all contained in the notes for Friday, Aug. 31.

Parameter a, the open file, can be passed as an open file by making sure that it is opened with the correct "unit number", making sure that the "close on exec" bit is not marked "close-on-exec" and writing the exec'd application to use that "unit number" as its file argument.
Parameter c, the integer, can be passed as the character representation of an integer. For example, the caller can use the sprintf(s,"%d",c) conversion routine to convert c to the string s. On receipt, c=itoa(s) will reverse the conversion.
Parameter d, the object, poses the biggest problems. We could convert the entire representation of the object to textual form and pass it as a string, or we could store the object in a file and pass the open file, thereby allowing the exec'd application to make changes that persist after the application terminates.
Background: Some computers have multiple CPUs that execute entirely different instructions sets. For example, the Pr1me computers the University of Iowa had in the 1980s had 16-bit and 32-bit instruction sets with distinctly different memory models. Today, it is possible to imagine a multiprocessor with, perhaps, a mix of Intel x86 and ARM processors with access to a common shared memory.
A problem: Explain how adding a Unix-style magic number at the head of each object file can be useful in such a system. Your explanation must both identify the system functions that check the magic numbers and what use is made of them. (1 point)

The execve() service can examine the magic number and use it to determine which type of CPU should be set to work executing that file.
Background Consider this C source file:
```
    int a;
    static int b;
    int c( int d );
    static int e( int f ) {
            return c( f );
    }
    int g( int h ) {
            return e( h );
    }
```
A problem: Which of the identifiers above (a through h) are visible to the linker? The others are private symbols known only to the compiler. (1 point)

a is visible.
b is invisible because it was declared to be static.
c is visible.
d is invisible because it is a local parameter name.
e is invisible because it was declared to be static.
f is invisible because it is a local parameter name.
g is visible.
h is invisible because it is a local parameter name.