Thread: segmentation fault... first time with unix...

I have never programmed in C on UNIX earlier and the first time I did returned some surprising results. I first coded the programmed in Turbo C in Windows and it compiled and executed perfectly well. Then I compiled the program in GCC in UNIX and some surprise I got when I learned a new thing called "Segmentation Fault". The programmed compiled with just a warning against using "gets" function and produced an executable. Running it, I encounter "Segmentation Fault" and the program terminates whenever (as I discovered through running the program couple of times) either the character array is tokenized or the pointer token's string is copied into an empty character array: don't know exactly which one of them is causing the trouble! I have never used UNIX before and have absolutely no idea what to do! Here's my code and also the exact output:-

Code:

//declaring preprocessor directives
#include <stdio.h>
#include <string.h>

//method to validate user's input as  a legal Mynix command
int parseInput(char checkCommand[])
{
	int counter;
	char *inputToken, tempString[30];

	//loop to verify any character in the input is either a small alphabet or a hyphen or a space
	for (counter= 0; counter<strlen(checkCommand); counter++)
	{
		if (checkCommand[counter]<'a' || checkCommand[counter]>'z')
		{
			if (checkCommand[counter]!='-' && checkCommand[counter]!=' ')
			{
				printf("mynix> - No such thing!");
				return 0;
			}
		}
	}//character verification loop ends here

	//checking if input is an exit command
	if (strcmp(checkCommand, "exit")==0)
	{
		return 1;	//returning a non-zero value to exit the do-while loop in main
	}

	//checking for commands that can work without (or do not need) any switch or parameter like 'cd', 'ls', 'pwd' and 'cat'
	else if(strcmp(checkCommand, "cd")==0 || strcmp(checkCommand, "ls")==0 || 
	        strcmp(checkCommand, "pwd")==0 || strcmp(checkCommand, "cat")==0)
	{
		printf("mynix> - Done!");
		return 0;
	}

	//checking for commands that can have (or need) switches or parameters like 'cd', 'ls', 'cat', 'man', 'mkdir', 'whereis'
	else
	{
		inputToken= strtok(checkCommand, " ");	//tokenizing by space for first part
		
		//checking first token for a valid command
		if (strcmp(inputToken, "man")==0 || strcmp(inputToken, "ls")==0 || 
		    strcmp(inputToken, "cd")==0 || strcmp(inputToken, "mkdir")==0 || 
		    strcmp(inputToken, "cat")==0 || strcmp(inputToken, "whereis")==0)
		{
			inputToken= strtok(NULL, " ");	//tokenizing by space for second part
			strcpy(tempString, inputToken);	//storing token's value in a string

			//checking that second part does not start with a hyphen and that its a valid parameter
			if (tempString[0]!='-' && strlen(tempString)>0)
			{

				inputToken= strtok(NULL, " ");	//tokenizing by space for third part
				strcpy(tempString, inputToken); //storing token's value in a string

				//checking that there is no third part after a parameter
				if (strcmp(tempString, NULL)==0)
				{
					printf("mynix> - Done!");
					return 0;
				}

				else
				{
					printf("mynix> - No such thing!");
					return 0;
				}
			}//third part verification ends here

			//checking validity if second part starts with a hyphen
			else
			{
				//checking that there is only one character after a hyphen in the second part
				if (strlen(tempString)==2)
				{
					inputToken= strtok(NULL, " ");	//tokenizing by space for third part
					strcpy(tempString, inputToken);	//storing token's value in a string

					//checking that after a valid switch the third part does not start with a hyphen
					if (tempString[0]!='-' && strcmp(tempString, NULL)!=0)
					{
						inputToken= strtok(NULL, " ");	//tokenizing by space for fourth part
						strcpy(tempString, inputToken);	//storing token's value in a string

						//checking that there is no fourth part
						if (strcmp(tempString, NULL)==0)
						{
							printf("mynix> - Done!");
							return 0;
						}

						//if more than three parts are there in the command
						else
						{
							printf("mynix> - No such thing!");
							return 0;
						}
					}
					
					//if parameter is incorrectly declared
					else
					{
						printf("mynix> - No such thing!");
						return 0;
					}
				}
				
				//if switch is incorrectly delcared
				else
				{
					printf("mynix> - No such thing!");
					return 0;
				}
			}//second part verification if-else ladder ends here
		}
		
		//if first part not valid then
		else
		{
			printf("mynix> - No such thing!");
			return 0;
		}//first part verification if-else ladder ends here
	}//total command verification if-else ladder ends here
}//method parseInput ends here

//main method begins here
int main()
{
	char userInput[30];
	int exitVariable= 0;

	//do-while loop to prompt user for next command till exit command is executed
	do
	{
		printf("\nmynix> ");

		gets(userInput);

		exitVariable= parseInput(userInput);
	}
	while(exitVariable==0);
	//do-while loop ends here
	
	return 0;	//no value required to be returned by main method
}//main method ends here
//program ends here

The output is:-

Code:

mynix> hey
mynix> - No such thing!
mynix> ls
mynix> - Done!
mynix> ls -a
Segmentation fault

Please help! Thanks a lot for taking out some time!