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!
