CITS2002 - Lecture 18

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CITS2002 - Lecture 17
Command-line Arguments & Inter-Process Communication - Slides

Command-line Arguments

• main(int arc, char *argv[])
  • argc $-$ number of command-line arguments
  • argv $-$ an array of strings (char pointers) holding the arguments
  • argv[0] $-$ always the program’s name

Parsing Command-line Arguments

• Most applications support command switches that appear between a program’s name and the ‘true’ arguments
  • They all start with a hyphen (-) to indicate they are a switch
• Command switches are accepted in any order:
  • ls -l -t -r is equivalent to ls -ltr
• Switches are interpreted by the program, not the shell or OS

Here is an example of a switch implementation:

while (argc > 0 && (*argv)[0] == '-') {
    if ((*argv)[1] == 'd') dflag = true;
    --argc; ++argv;
}

-d → dflag switch

Parsing with getopt()

• As programs become more complicated, they often accept many command switches to define and constrain their execution
• getopt() simplifies handling complex command-line options
  • From POSIX standard (not standard C library) but widely used
    Defined in <unistd.h> and <getopt.h>

int getopt(int argc, char * const argv[], const char *OPTLIST);

• OPTLIST defines valid switches, e.g. {C#}"df:n:" means:
  • -d → no argument
  • -f → expects a string argument (optarg)
  • -n → expects a numeric argument (optarg)

#define OPTLIST "df:n:"
while ((opt = getopt(argc, argv, OPTLIST)) != -1) {
    if (opt == 'd') dflag = true;
    else if (opt == 'f') filename = strdup(optarg);
    else if (opt == 'n') value = atoi(optarg);
}

• getopt() automatically:
  • Sets optarg to the switch’s argument
  • Sets optind to the index of the next unprocessed argument

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Inter-Process Communication (IPC)

• Programs can cooperate and exchange data using IPC mechanisms provided by the OS
  • This is useful as we want to always try employ functions and programs that already work well from 3rd parties
• OS’s provide many IPC mechanisms
  • e.g. signals, pipes, shared memory blocks, etc.

Using Pipes in C

• Pipes enable shells (or other programs) to connect the output of one program to the input of another
• The system call pipe() is used to create a unidirectional communication buffer
• A pipe is represented as an array of two integer file-descriptors
  • Writing data to array[0] adds the data to the pipe, and reading from array[1] removes that data

Creating a pipe:

#include <unistd.h>
int thepipe[2];
pipe(thepipe);
write(thepipe[1], data, size); // write end
read(thepipe[0], buffer, size); // read end

• Pipes have a finite size, typically 4096 bytes long
• A process trying to read an empty pipe will block until data arrives
• A process writing to a full pipe will block until space frees up

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CITS2002 - Lecture 19