Write a C program (time_shm.c and time_pipe.c) that determines the amount of time necessary to run a command from the command line. This program will be run as

./time

and will report the amount of elapsed time to run the specified command. This will involve using fork() and execvp() functions, as well as the gettimeofday() function to determine the elapsed time. It will also require the use of two different IPC mechanisms.

The general strategy is to fork a child process that will execute the specified command. However, before the child executes the command, it will record a timestamp of the current time (which we term starting time). The parent process will wait for the child process to terminate. Once the child terminates, the parent will record the current timestamp for the ending time. The difference between the starting and ending times represents the elapsed time to execute the command. The example output below reports the amount of time to run the command ls:

./time ls -l

total 156

-rwxr-xr-x 1 thomas users 25616 Feb 4 15:59 a.out

-rw-r--r-- 1 thomas users 252 Feb 4 16:00 output.txt

-rwxr-xr-x 1 thomas users 86024 Feb 2 20:58 project.exe -rwxr-xr-x 1 thomas users 25616 Feb 2 21:00 time

-rw-r--r-- 1 thomas users 5144 Feb 2 20:58 time.c

Elapsed time: 0.001448 seconds

As the parent and child are separate processes, they will need to arrange how the starting time will be shared between them. You will write two versions of this program, each representing a different method of IPC.

2a) (10 points) The first version, time_shm.c, will have the child process write the starting time to a region of shared memory before it calls execvp(). After the child process terminates, the parent will read the starting time from shared memory. The region of shared memory should be established before the child process is forked, allowing both the parent and child processes access to the region of shared memory.

2b) (10 points) The second, time_pipe.c, version will use a pipe. The child will write the starting time to the pipe, and the parent will read from it following the termination of the child process.

You will use the gettimeofday() function to record the current timestamp. This function is passed a pointer to a struct timeval object, which contains two members: tv_sec and tv_usec. These represent the number of elapsed seconds and microseconds since January 1, 1970 (known as the UNIX EPOCH). The following code sample illustrates how this function can be used:

// Get current time

timeval_t startTime; gettimeofday( &startTime, 0 );

// get the end time timeval_t end_time; gettimeofday( &end_time, 0 );

// calculate elapsed time timeval_t elapsed_time; timersub( &end_time, startTime, &elapsed_time );

// print elapsed time (microseconds right justified zero filled) printf( " Elapsed time: %d.%06d seconds ", elapsed_time.tv_sec, elapsed_time.tv_usec );

Hint:

1. For IPC between the child and parent processes, the contents of the shared memory pointer can be assigned the struct timeval representing the starting time. When pipes are used, a pointer to a struct timeval can be written to - and read from - the pipe.

2. gcc time_shm.c -otime -lrt is an example command to compile and link your program

3. ./time ls -l | tee time_shm_output.txt is an example command to run your program while collecting your programs output to a text file and seeing the output on the console.