Getting a seg fault

This is driving me nuts! I dont know much about C so I cant find what I am doing wrong but I have debugged it to the point that I know where I am getting a Seg. Fault but dont know why:

Code:

void parse_string(char input_string[])  
{
  char *token;
  char *delim = " ";

  token = strtok(input_string,delim);
  while (token != NULL)
   {
    token = strtok(NULL, delim);
   }

  return 0;
}                                                                      
//------------------------
int main() {
  int x = 0;                       //holds answer from calculations
  char *buf;                       //pointer to where the directory is
  long size;                       //size of the array, which is the directory name
  char *dir;                       //this is the variable that stores the direcory name and then prints it
  char *path;                      //this is the array that will hold the changed directory's name
  char input_string[maxbuf];       //this is the array which will hold the input
  int flag = 1;
  int pipecount = 0;
  
  signal(SIGTSTP, signal_handler);                        //called when the user hit ctrl-z
      printf("mysh$>");
      if ((buf = (char *)malloc((size_t)size)) != NULL)
        dir = getcwd(buf, size);                          //this is the call to get the directory
      printf("%s >", dir);                                //will print the directory
      fgets (input_string, maxbuf, stdin);                //this will get the command the user enters
      parse_string(input_string);                         //tokenizes the command the user enters

        if(strcmp(command[0], "cd") == 0);  {              //SEG FAULT HERE!!!!!!
          dir = chdir(path);
          printf("%s%", dir);  }                            //SEG FAULT HERE!!!!!!
        if (strcmp(command[0], "quit" == 0))
          exit(0);
      // else if (strcmp(command[0], "list" == 0)
      //   command[0] = "ls";
      // else if (strcmp(command[0], "type" == 0)
      //   command[0] = "cat";
      // else if (strcmp(command[0], "cls" == 0)
      //   command[0] = "clear";
      // else if (strcmp(command[0], "move" == 0)
      //   command[0] = "mv";
      // else if (strcmp(command[0], "copy" == 0)
      //   command[0] = "cp";
      // else if (strcmp(command[0], "del" == 0)
      //   command[0] = "rm";
      // else if (strcmp(command[0], "=" == 0){
        // x = arith(command);
        // printf("%s", x); }
      // else
      // printf("Bad Command.  Please try again.");
                                                                         
      execvp(command[0], command);                              //execute the commands

  return 0;
}

Basically I have other things commented because I am trying to just get at least one or two things to work! Ahhhh! ok so this is am implementation of a Shell program. I want to type cd in and have it go back one directory, naturally. Or quit when I type quit in. I can type something in and once I hit enter it seg faults. In the code in my comments it shows where the seg faults occur. Please help! Thanks!

I don't see where command is defined. Is it defined as

Code:

char *command[N];

for some N? If so, why do you not allocate memory for these poor pointers to point to?

Code:

if(strcmp(command[0], "cd") == 0);  {              //SEG FAULT HERE!!!!!!

wild guess: did you check the content of command[0]?

Code:

printf("%s%", dir);  }                            //SEG FAULT HERE!!!!!!

I'm not sure the trailing % is a good idea...

Where did "command" come from? What is it?

Why the ; after the if in the first line here, and the extra % in the printf?

Code:

if(strcmp(command[0], "cd") == 0);  {              //SEG FAULT HERE!!!!!!
dir = chdir(path);
printf("%s%", dir);  }                            //SEG FAULT HERE!!!!!!

Code:

        if(strcmp(command[0], "cd") == 0);  {              //SEG FAULT HERE!!!!!!
          dir = chdir(path);
          printf("%s%", dir);  }                            //SEG FAULT HERE!!!!!!

Guess what that ; does.

And who initialises path?

Code:

void parse_string(char input_string[])  
{
  char *token;
  char *delim = " ";

  token = strtok(input_string,delim);
  while (token != NULL)
   {
    token = strtok(NULL, delim);
   }

  return 0;
}

I'm pretty sure you want this to fill in your "command" array.

--
Mats

Ok thanks for your help....cant believe I couldnt see the semi-colon! And I did have command in there...dont know why its not in there in this code...I had it defined somewhere else...which was dumb!

So I tried to fix what you guys were sayin, but now its messed up even more! It doesnt print my directory anymore! it prints crazy letters. Here is my new code, and the output. Thanks!

Code:

void parse_string(char input_string[])  //***********************
{
  char *token;
  char *delim = " ";

  token = strtok(input_string,delim);
  while (token != NULL)
   {
    token = strtok(NULL, delim);
   }

  return 0;
}                                                                      //***********************
//------------------------
int main() {
  int x = 0;                       //holds answer from calculations
  char *buf;                       //pointer to where the directory is
  long size;                       //size of the array, which is the directory name
  char *dir;                       //this is the variable that stores the direcory name and then prints it
  char *path;                      //this is the array that will hold the changed directory's name
  char input_string[maxbuf];       //this is the array which will hold the input
  char *command[maxbuf];         //holds the parsed string
  int flag = 1;
  int pipecount = 0;

  signal(SIGTSTP, signal_handler);                        //called when the user hit ctrl-z
      printf("mysh$>");
      if ((buf = (char *)malloc((size_t)size)) != NULL)
        dir = getcwd(buf, size);                          //this is the call to get the directory
      printf("%s >", dir);                                //will print the directory
      fgets (input_string, maxbuf, stdin);                //this will get the command the user enters
      parse_string(input_string);                         //this will tokenize the command the user enters

        if(strcmp(command[0], "cd") == 0)  { //SEG FAULT HERE!!!
          dir = chdir(path);
          printf("%s", dir);  }              //SEG FAULT HERE!!!
        if (strcmp(command[0], "quit" == 0))
          exit(0);
      // else if (strcmp(command[0], "list" == 0)
      //   command[0] = "ls";
      // else if (strcmp(command[0], "type" == 0)
      //   command[0] = "cat";
      // else if (strcmp(command[0], "cls" == 0)
      //   command[0] = "clear";
      // else if (strcmp(command[0], "move" == 0)
      //   command[0] = "mv";
      // else if (strcmp(command[0], "copy" == 0)
      //   command[0] = "cp";
      // else if (strcmp(command[0], "del" == 0)
//   command[0] = "rm";
      // else if (strcmp(command[0], "=" == 0){
        // x = arith(command);
        // printf("%s", x); }
      // else
      // printf("Bad Command.  Please try again.");

      execvp(command[0], command);                                            //will execute the commands

  return 0;
}

Output:

Code:

-bash-3.1$ gcc project2a.c
project2a.c: In function âarithâ:
project2a.c:27: warning: comparison between pointer and integer
project2a.c:34: warning: comparison between pointer and integer
project2a.c:39: warning: comparison between pointer and integer
project2a.c:44: warning: comparison between pointer and integer
project2a.c:58:18: warning: character constant too long for its type
project2a.c: In function âsignal_handlerâ:
project2a.c:58: warning: comparison between pointer and integer
project2a.c: In function âparse_stringâ:
project2a.c:69: warning: assignment makes pointer from integer without a cast
project2a.c:72: warning: assignment makes pointer from integer without a cast
project2a.c:75: warning: âreturnâ with a value, in function returning void
project2a.c: In function âmainâ:
projec¶ >.c: ÿÿÿ)EðÁ}ðUð assignment makes pointer from integer without a cast
Òt+1ÿÆ ÿÿÿEð/a.out
mysh$>

I know I have a lot of warnings, but I am just trying to fix one thing at a time!


****Ok well I am seeing that I am not saving the parsed strings in command so I am working on fixing that now. Anything else you see that I dont?? Thanks!!!

Post some of the lines mentioned in the error messages.

And you shouldn't be running the code while you've still got warnings and errors.

I will post the error messages in a second but I noticed that when I put the ; back in after the if statement:

Code:

if(strcmp(command[0], "cd") == 0);  { //SEG FAULT HERE!!!
          dir = chdir(path);
          printf("%s", dir);  }

it prints nice and neat but when I take it out it prints this:
output:

Code:

projec¶ >.c: ÿÿÿ)EðÁ}ðUð assignment makes pointer from integer without a cast
Òt+1ÿÆ ÿÿÿEð/a.out
mysh$>

so basically since it is not right to have that semi colon there...something else is majorly wrong!

Warnings 1,2,3,4: warning: comparison between pointer and integer

Code:

  if (command[2] == '/') {

Code:

 else if (command[2] == '*') {

Code:

 else if (command[2] == '+'){

Code:

else if (command[2] == '-'){

Warning: warning: assignment makes pointer from integer without a cast

Code:

token = strtok(input_string,delim);                ***in my parse function

Code:

 token = strtok(NULL, delim);                         ***in my parse function

So command[2] is a string, not a character -- you cannot compare them directly. The strtok warnings mean you forgot to include a header file <string.h>.

First, I shall point out to you that these warnings:
project2a.c:27: warning: comparison between pointer and integer
project2a.c:58:18: warning: character constant too long for its type
project2a.c:69: warning: assignment makes pointer from integer without a cast
...should really be treated as errors because they will not make your code run as you want.
So what do they say, then? Let's examine them.

>>project2a.c:27: warning: comparison between pointer and integer
Means you are comparing a pointer to an integer.
Example:

Code:

if (command[2] == '/')

command in the example is pointer to char (char*), and '/' is a char, a string literal.
That will never yield any good results. It's like comparing apples and oranges.

>>project2a.c:58:18: warning: character constant too long for its type
Example:

Code:

char c = 300;

A char cannot hold the value 300; it is too big and will be truncated in some way. Thus it will not store "300", but something else. It will not give you the results you want.

>>project2a.c:69: warning: assignment makes pointer from integer without a cast
Example:

Code:

int x = 50;
void* p = x;

You are assigning the contents of an integer to a pointer. This will never work, since the address 50 is 99% likely NOT to be writable or readable, thus getting you a crash.
This is undefined. You must store addresses in pointers, either from malloc or by taking the address of a variable.

And this warning...
>>project2a.c:75: warning: âreturnâ with a value, in function returning void
While not critical, it is still bad.
It means you are returning a value from a function that you have specified does not return anything!
Example:

Code:

void foo() { return 10; }

Return the appropriate type or return nothing at all!

There's also another bug in the code:

Code:

	if (strcmp(command[0], "quit" == 0))

Should be:

Code:

	if (strcmp(command[0], "quit") == 0)