Segmentation fault

Question

#include<stdio.h>
#include<zlib.h>
#include<unistd.h>
#include<string.h>


int main(int argc, char *argv[])
{
   char *path=NULL;
   size_t size;
   int index ;
   printf("\nArgument count is = %d", argc);
   printf ("\nThe 0th argument to the file is %s", argv[0]);
   path = getcwd(path, size);
   printf("\nThe current working directory is = %s", path);
   if (argc <= 1)
   {
      printf("\nUsage: ./output filename1 filename2 ...");
   }
   else if (argc > 1)
   {
      for (index = 1; index <= argc;index++)
      {
            printf("\n File name entered is = %s", argv[index]);
            strcat(path,argv[index]);
            printf("\n The complete path of the file name is = %s", path);
      }
   }
   return 0;
}

In the above code, here is the output that I get while running the code:

$ ./output test.txt

Argument count is = 2
The 0th argument to the file is ./output
The current working directory is = /home/welcomeuser
 File name entered is = test.txt
 The complete path of the file name is = /home/welcomeusertest.txt
Segmentation fault (core dumped)

Can anyone please me understand why I am getting a core dumped error?

Regards, darkie

Answer 1

getcwd() allocates a buffer for path of the size equal to size. You did not initialize the size variable. Set it large enough to hold the whole path AND the name, and this should work. If the buffer is not large enough, the strcat() will write past the end of the buffer, overwriting other values on the stack (possibly including the function return pointer, which would cause a segfault on return).

Also, getcwd() uses malloc() to allocate the buffer you assign to path. It would be nice to free() this buffer when you're done with it. Though this is not strictly necessary at the end of the program - as the system will reclaim the memory anyway.

There are also some logic errors in your code. Firstly, the argv array indices range from 0 to argc-1. Your for loop exit condition makes you read one element past the end of the argv array.

Note that the strcat() will append a new parameter each iteration to the result of the previous iteration. This means that invoking /home$ ./output foo bar baz would end up with:

The complete path of the file name is = /home/foo
The complete path of the file name is = /home/foobar
The complete path of the file name is = /home/foobarbaz

Which is probably not what you want :). (omitted irrelevant lines of the output).

Answer 2

The strcat is not valid. It is attempting to concatenate data to a buffer returned by the C runtime lib call. You need to use your own buffer. And you need to use free() on the buffer returned by getcwd() in the way that you are using it (passing in NULL causes it to allocate memory).

Answer 3

strcat(path,argv[index]) is adding data to a buffer that's not large enough to hold the additional data.

You should pass in a size value that will ensure the buffer is large enough. You're also not initializing size, so you really don't know what size buffer will be returned (all this is assuming that you're using the GNU libc version of getcwd() which will allocate a buffer if you pass in NULL).

Answer 4

You are going off the end of argv by saying index <= argc. That should be index < argc. Remember, array indexes go from zero to one less than the length of the array.

(You're correct to start at 1, because argv[0] is the program name.)

Answer 5

You're reading off the end of argv. Don't do that. Stop at the argc-1'th argument.

Answer 6

Although the answers about strcat are valid, given the point at which your program is crashing the issue is a NULL pointer deference because you use <= argc and not < argc.

In C, argv[argc] is a NULL pointer.