What are the three algorithm constructs?
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Answer:
/ to access the input scanf()
// and output printf()
#include <stdio.h>
// to access the functions like
// pipe(), fork(), execvp(), dup2()
#include <unistd.h>
// to access the string functions like
// strtok()
#include <string.h>
// to access function wait()
#include <sys/wait.h>
#include <stdlib.h>
int main()
{
// declare a variable to hold the process id
pid_t p_id;
// declare a variable to hold the index value
int array_index;
// declare the string to hold the user input
// as command
char userIn_Command[128];
// use a continuous loop
while (1)
{
// display the prompt for the user
printf("minor5> ");
// read the input from the user
scanf("%[^\n]", userIn_Command);
// check the condition that whether the user
// inputs a command called "quit"
// If the user inputs quit command then exit from
// the script
if (strcmp(userIn_Command, "quit") == 0)
{
printf("\n");
break;
}
// if there are any usage of pipelining or redirection
// display the error message and exit from the script
if (strchr(userIn_Command, '|') != NULL || strchr(userIn_Command, '>') != NULL ||
strchr(userIn_Command, '<') != NULL)
{
printf("Error: Cannot perform the multiple command operations or directions\n");
break;
}
// declare the variables to hold the process
int p_pids[2];
// create the system call to pipe() from kernal
// and display the error message
if (pipe(p_pids) < 0)
{
printf("Error: Pipe creation failed!\n");
}
// create the child process
p_id = fork();
// condition to check whether the fork() is
// created. If so, display an error message
if (p_pids < 0)
{
printf("Error: fork() failed!\n");
break;
}
// if the child process is created
if (p_id == 0)
{
// close pipe in child process
close(p_pids[0]);
// create duplicate of the standard output
dup2(p_pids[1], STDOUT_FILENO);
// close the pipe in child process
close(p_pids[1]);
// declare a variable to store path
// to execute the command
char *command_path;
// declare an array of string to hold the options
char * args[32];
// tokenize the command by using the delimiter at " "(single space)
char *cmd_token = strtok(userIn_Command, " ");
// store the token value
command_path = cmd_token;
args[0] = cmd_token;
array_index = 1;
// loop until all the options in the command are
// tokenized
while (1)
{
// get the next token
cmd_token = strtok(NULL, " ");
// condition to check whether the token is null
// or not
if (cmd_token == NULL)
{
break;
}
// store the token if it is not null
args[array_index] = cmd_token;
// increment the index
array_index++;
}
// last parameter to the command should be NULL */
args[array_index] = NULL;
/* calling exec function with command path and parameters */
if (strcmp(args[0], "cd") == 0 || strcmp(args[0], "history") == 0 ||
strcmp(args[0], "exit") == 0)
{
printf("%s: Command not found\n", args[0]);
break;
}
if (execvp(command_path, args) < 0 )
{
printf("%s: Command not found\n", args[0]);
break;
}
}
else
{
/* closing writing end of pipe in parent process */
close(p_pids[1]);
/* reading ouput written to pipe in child process and
* writing to console */
while (1)
{
char output[1024];
int n = read(p_pids[0], output, 1024);
if (n <= 0)
{
break;
}
output[n] = '\0';
printf("%s", output);
}
/* closing read end of pipe1 */
close(p_pids[0]);
/* waiting until child process complete its execution */
wait(NULL);
}
/* skipping newline character read while scanf() */
getchar();
}
exit(0);
}
Explanation:
