using execvp/fork to catenate a file

[ohaqqi]

Hi guys,
I'm still working on my shell. I'm trying to implement a function typefile that will take a command line input as follows:

> type <file1>

This command will implement a catenation of file1, equal to the command cat <file1>

I need to use execvp() and fork() system calls to create a new process that will type/cat any text file. In my code below, the execvp() call in the function at the bottom gets me the following:

o-shell.c:128: warning: passing arg 2 of `execvp' from incompatible pointer type

I know for sure i'm calling execvp incorrectly, any tips/pointers would be great. I want to use /bin/cat followed by filename as the argument.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
 
int main(int argc, char *argv[])
{
    //VARIABLE DECLARATIONS
 
    char command[50];                //Command line input
 
    //tokens for strtok() funtion
    char * com;                        //Primary command
    char * arg1;                    //First argument of command (usually a file name)
    char * arg2;                    //2nd argument of command (usually a file name)
 
    int args;                        //Number of arguments entered
 
    //FUNCTION DECLARATIONS
 
    void typefile(char * filename);
 
    printf("***************************************************\n");
    printf("Welcome to O-Shell! A UNIX shell by O Haqqi\n\n");
    printf("Type 'help' for list of commands, 'exit' to exit\n");
    printf("***************************************************\n");
 
    for(;;)
    {
 
    printf("o-shell>");                        //user prompts for o-shell
 
    fgets(command, 1024, stdin);        //command line input from user
 
    com = strtok(command, " \r\n\t");
 
    args = 0;
    arg1 = strtok(NULL, " \r\n\t");
    if(arg1) args++;                        //to see if arguments being tracked
    arg2 = strtok(NULL, " \r\n\t");
    if(arg2) args++;                        //to see if arguments being tracked
 
    if(com != NULL)            //Can only execute commands if a command is entered
                            //Otherwise it keeps looping and printing prompt
    {
    printf("command entered: %s\n", com);
    printf("There are %d arguments\n", args);
 
    if ((strcmp(com,"help")) == 0 || (strcmp(com,"Help")) == 0)
    {
        printf("***************************************************\n");
        printf("\nO-Shell Help Menu:\n");
        printf("\nhelp - Print help menu\n");
        printf("\nexit - Exit O-Shell\n");
        printf("\ntype <file> - Print contents of <file> to terminal\n");
        printf("\ncopy <file1> <file2> - Copy contents of <file1> to <file2>.");
        printf("\n     <file2> must be a non-existent file and will be created\n");
        printf("\ndelete <file> - Delete <file> from system\n");
        printf("***************************************************\n");
    }
 
    else if ((strcmp(com,"exit")) == 0 || (strcmp(com,"Exit")) == 0)
    {
        printf("Exiting O-Shell...Goodbye!\n");
 
        //if exit command is entered, exit() system call is performed
        exit(1);
    }
 
    else if ((strcmp(com,"type")) == 0 || (strcmp(com,"Type")) == 0)
    {
        if((arg1 == NULL) || (arg2 != NULL))        //arg1 must exist, arg2 must be empty
        {
            printf("Incorrect number of arguments! Type 'help' for correct format.\n");
        }
 
        else
        printf("Printing contents of file: %s\n", arg1);
        printf("***************************************************\n");
 
        typefile(arg1);            //call typefile function (see below)
    }
 
    else
    printf("Invalid command!!! Type 'help' for list of valid commands.\n");
 
    }
 
    }
 
    return 0;
}
 
/************************************************************************************************
Function typefile() prints contents of a file to the terminal
Format: type <file1>
************************************************************************************************/
 
void typefile(char * filename)
{
    int pid, status;
 
    const char cat[] = "/bin/cat";
 
    pid = fork();
 
    if(pid < 0)
    printf("ERROR: Child pid = %i FORK FAILED! Please try again.\n", pid);
 
    else if(pid > 0)
    printf("ERROR: Child pid = %i Parent process executing, FORK FAILED!\n", pid);
 
    else if(pid == 0)                //If the process id is zero, fork() was successful
    {
        printf("Child pid = %i FORK SUCCESSFUL...\n");
 
        execvp(cat, filename);
    }
 
}

[ohaqqi]

I finally got the cat command to work..I'm having a problem with the child process creation now

I have testfile.txt to verify that the cat command works. It does work, but the fork() command is assigning a pid other than 0 to the child, or it just looks that way:

o-shell>type testfile.txt
command entered: type
There are 1 arguments
Printing contents of file: testfile.txt
************************************************** *
ERROR: Child pid = 15966 FORK FAILED! Please try again.

Child pid = 0 FORK SUCCESSFUL...

o-shell>This is a test file for the o-shell terminal. Hit <ENTER> to continue.

void typefile(char * filename)
{
    int status;
    char *arg[] = {"cat", filename, NULL};
 
    pid_t pid = fork();
 
 
    if(pid == 0)            //pid=0 indicates child process created, fork() was successful
    {
        printf("Child pid = %i FORK SUCCESSFUL...\n\n", pid);
 
        status = execvp("cat", arg);
 
        while (wait(&status) != pid)        //parent process waits for child process to complete
        ;
 
    }
 
    else
    printf("ERROR: Child pid = %i FORK FAILED! Please try again.\n\n", pid);
 
}

Does something glare out and say "this is why the non-zero pid is showing up!!" ?