string value disappearing after strcat

Trying to add a slash to the end of a string that is pulled from an argument (linux). I read in the argument just fine, then add the slash and poof - no more variable. I trimmed it down to bare bones and it's still doing it. Here's the code:

Code:

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

int main( int argc, char *argv[] ) {
  int i;
  char * first_arg = argv[1];
  char * second_arg = argv[2];
  char * third_arg = argv[3];
  char fullcommandline[255] = "";

  for( i = 0 ; i < argc ; i++ ) {
      strcat(fullcommandline, argv[i]);
      strcat(fullcommandline, " ");
  }
  i = strlen(second_arg);
  printf("last character in second_arg is:    %c\n",second_arg[i-1]);
  if (second_arg[i-1] != '/') {
      strcat(second_arg, "/");  // Also have tried "/\0" here w/ same results
  }
  i = strlen(second_arg);
  printf("last character in second_arg is now: %c\n",second_arg[i-1]);
  printf("After change:\n");
  printf("\tCommand Line : %s\n",fullcommandline);
  printf("\tfirst_arg    : %s\n", first_arg);
  printf("\tsecond_arg  : %s\n", second_arg);
  printf("\tthird_arg    : %s\n\n", third_arg);
  return 0;
}

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Here's the output:

Code:

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user@localhost: ~$ ./test -something ./a_directory ./anotherdir
last character in second_arg is:    y
last character in second_arg is now: /
After change:
    Command Line : ./test -something ./a_directory ./anotherdir
    first_arg    : -something
    second_arg  : ./a_directory/
    third_arg    :

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thoughts?
Thanks

The argv arguments seem to be stored contiguously (this isn't a guarantee, but it's how it's done in this implementation):

Code:

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arguments:  ./test\0-something\0./a_directory\0./anotherdir
argv index: 0      1          2              3

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After you append a slash to argv[2], the null terminator is one character later, argv[3][0] is now that same null terminator (the first character was overwritten):

Code:

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arguments:  ./test\0-something\0./a_directory/\0/anotherdir
argv index: 0      1          2            3

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A string with a null terminator in the first character is an empty string.

Quote:

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Originally Posted by christop

The argv arguments seem to be stored contiguously (this isn't a guarantee, but it's how it's done in this implementation):

Code:

---

arguments:  ./test\0-something\0./a_directory\0./anotherdir
argv index: 0      1          2              3

---

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The light bulb just snapped on, thanks! So I need to get that into another string (via strcpy) before I start walking over everything else. I tested it out and it works fine now.
Thank you!