using arguments in main

Hello,

What I'm trying to do is call a program with 3 parameters.
eg. myProgram ABCD input.txt output.txt

then I want to add to ABCD argument the extension .test

As you can see in my code after the functionCall the argv[2] changes value. I believe that the strcat(filename, ".test") causes the problem as the argv[2] takes the value "est" but why is this happening and how can I correct it?

Thanks
/m

Code:

#include <stdio.h>
#include <strings.h>


void functionCall (char* filename) {


    strcat(filename, ".test");
    FILE *file = fopen(filename, "r");


    if ( file != NULL ){
        //code
    }
}


int main (int argc, char * argv[]){


   int i;


    //DEBUG: PARAMETERS BEFORE
    printf("\n Program %s called with %d parameters:\n", argv[0], argc-1);
    for (i = 1; i < argc; i++)
      printf("\t%s \n", argv[i]);


    functionCall(argv[1]);


    //DEBUG: PARAMETERS AFTER
    printf("\n Program %s called with %d parameters:\n", argv[0], argc-1);
    for (i = 1; i < argc; i++)
    printf("\t%s \n", argv[i]);


   return 0;
}

You're running into a buffer overflow when you strcat something to the values pointed by argv. The program arguments are guaranteed only to have enough memory to hold each of the arguments passed to the program, and no more.

Thanks now I understand Why this is happening, any suggestions to fix it? I tried to concatenate in another function but since it's a pointer nothing changes. Is there a way to assign the argv[1] value (the one i want to alter by adding the extension) to a char[] (not pointer)?

It's not a good idea to alter the arguments. Copy the argv[n] into some other char array and concatenate there.

OK. Always you have to try before you ask I used malloc and it works.

Thanks once more King Mir.

Originally Posted by nonoob

It's not a good idea to alter the arguments.

I don't see any good reason for that, as long as you're careful not to overflow.

If marlaFC resolved this it how I think, then there's a memory leak, but under the circumstances an acceptable one.

Originally Posted by King Mir

I don't see any good reason for that, as long as you're careful not to overflow.

In fact, the C standard specifically allows that. An implementation is required to allow the strings pointed to by the argv array to be modifiable by the program.

Originally Posted by King Mir

If marlaFC resolved this it how I think, then there's a memory leak, but under the circumstances an acceptable one.

This is what I did. So I still have a memory leak (though I don't understand it) but it's an acceptable one? Any other better ideas?

This is what I did and it works now. So are you saying that I still have a memory leak? Any other better ideas or it's fine like this?

Code:

#include <stdio.h>
#include <strings.h>


void functionCall (char* filename) {

  char *dst;
  char ext[] = ".test";

  dst = (char *)malloc(strlen(filename)+st​rlen(ext));
   strcpy(dst, filename);
  strcat(dst, ext);
  FILE *file = fopen(dst, "r");


   if ( file != NULL ){
       //code
   }
}


int main (int argc, char * argv[]){


  int i;


   //DEBUG: PARAMETERS BEFORE
   printf("\n Program %s called with %d parameters:\n", argv[0], argc-1);
   for (i = 1; i < argc; i++)
     printf("\t%s \n", argv[i]);


   functionCall(argv[1]);


   //DEBUG: PARAMETERS AFTER
   printf("\n Program %s called with %d parameters:\n", argv[0], argc-1);
   for (i = 1; i < argc; i++)
   printf("\t%s \n", argv[i]);


  return 0;
}

1) you need to malloc space for the '\0'.

2) you should free what you allocate.

Implemented! Thanks!

Theres a couple (really pedantic) corrections I have, take them as you want:

You don't need to typecast malloc(), as it returns a void pointer which can be used for any type:

Code:

char *dst;
dst = malloc(strlen(filename + strlen(ext));

Also:

Code:

if (file != NULL) {} /* is the same as : */

if (file) {}