Pointers in function parameter

Hello,

I'm having a problem with my code where it crashes when trying to copy and append the parameter data to the variables used by the program. This problem is unrelated to libCurl which is what I'm using for the uploading procedure. This code is part of a DLL file.

Code:

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DLLIMPORT void UploadFile(char *filename, char *newfilename, char *urlDir, char *username, char *password);
{
  char *UPLOAD_FILE_AS;
                           
  CURL *curl;
  CURLcode res;
  FILE *hd_src;
 
  strcpy(UPLOAD_FILE_AS, filename); // CRASHES HERE
 
  strcat(urlDir, newfilename); // SAME PROBLEM HERE

  struct curl_slist *headerlist=NULL;
  char buf_1 [] = "RNFR ";
  char buf_2 [] = "RNTO ";
 
  strcat(buf_1, filename); // AND POSSIBLY THESE TWO AS WELL
  strcat(buf_2, newfilename);

  /* get a FILE * of the same file */
  hd_src = fopen(filename, "rb");

  /* In windows, this will init the winsock stuff */
  curl_global_init(CURL_GLOBAL_ALL);

  /* get a curl handle */
  curl = curl_easy_init();
  if(curl) {
    /* build a list of commands to pass to libcurl */
    headerlist = curl_slist_append(headerlist, buf_1);
    headerlist = curl_slist_append(headerlist, buf_2);

    /* we want to use our own read function */
    curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_callback);
 
    /* enable uploading */
    curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
 
    /* specify target */
    curl_easy_setopt(curl,CURLOPT_URL, urlDir);
 
    /* pass in that last of FTP commands to run after the transfer */
    curl_easy_setopt(curl, CURLOPT_POSTQUOTE, headerlist);
 
    /* now specify which file to upload */
    curl_easy_setopt(curl, CURLOPT_READDATA, hd_src);
   
    /* specify username and password for ftp */
    curl_easy_setopt(curl, CURLOPT_USERNAME, username);
    curl_easy_setopt(curl, CURLOPT_PASSWORD, password);
 
    /* Now run off and do what you've been told! */
    res = curl_easy_perform(curl);
   
    /* clean up the FTP commands list */
    curl_slist_free_all (headerlist);
 
    /* always cleanup */
    curl_easy_cleanup(curl);
  }
  fclose(hd_src); /* close the local file */
  curl_global_cleanup();
}

---

Thanks for the help in advance.

It's not allocated. It's a dangling pointer pointing to anywhere in memory.

strcpy does not allocate memory as it copies. You have to do that yourself. Since you don't use UPLOAD_FILE_AS at all during this function, I recommend you remove it and don't bother to copy anything to it.

Code:

---

strcpy(UPLOAD_FILE_AS, filename); // CRASHES HERE
 
strcat(urlDir, newfilename); // SAME PROBLEM HERE
 
struct curl_slist *headerlist=NULL;
char buf_1 [] = "RNFR ";
char buf_2 [] = "RNTO ";

strcat(buf_1, filename); // AND POSSIBLY THESE TWO AS WELL
strcat(buf_2, newfilename);

---

You're right about all of these being a problem:

• UPLOAD_FILE_AS - this is a pointer, but it doesn't point anywhere useful, just to random memory you don't own. Make it a sufficiently large char array.
• urlDir - Can't say for sure without knowing how you call UploadFile, but I would guess something similar. It's either not pointing to valid memory, or it's pointing to an array that is too small.
• buf_1 and buf_2 - These are char arrays, but their size is 6 chars (the five you initialize it with plus a null terminator). Strings do not automatically grow in C. Try making it a larger buffer, but only initializing the first few bytes (see below).

Code:

---

char buf_1[BUFSIZ] = "RNFR ";  // declare it to be BUFSIZ (a standard #define in stdio.h), which is probably 512 chars or bigger, but only initialize the first 6 bytes
strcat(buf_1, filename);  // now there is space to concatenate filename onto the end

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Ok, I'm making these changes as I type.

Here is the way I call UploadFile.

Code:

---

UploadFile("filename.txt", "newfilename.txt", "ftp://website.com/public_html/upload/", "usernameForServer", "passwordForServer");

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Quote:

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Originally Posted by binks

Ok, I'm making these changes as I type.

Here is the way I call UploadFile.

Code:

---

UploadFile("filename.txt", "newfilename.txt", "ftp://website.com/public_html/upload/", "usernameForServer", "passwordForServer");

---

---

Not really helpful. Are you actually passing literals and and these are merely parameter representation only?

If they are literals, the memory cannot be written to, thus newfilename.txt will generate a fault if you attempt to change it. If you're passing variables, it would be helpful to show that and what specifically the variables are (arrays or pointers to dynamically allocated memory).

libCurl gives the option to rename a file once it's uploaded. "newfilename.txt" is just that.

I don't understand what you mean by literals? this is an example of how I would call the function in a program (this is a DLL).

A literal is a literal. It is a explicitly stated string.

strcyp( str_ptr, "literal" );


the word "literal" is a literal. str_ptr is a pointer to a block of allocated memory (though it might actually point to any point in an static or locally defined array).

I personal will use literals in the program that uses the function. Won't it work the same as a variable would?

Are you suggesting I won't be able to append to char * urlDir since it's a parameter variable?

Quote:

---

Originally Posted by binks

Are you suggesting I won't be able to append to char * urlDir since it's a parameter variable?

---

No, I'm saying you won't be able to change it because it's a literal with no associated holding variable.

Think on this statement:

strcpy( "literal", str_ptr );

This would or should generate a compiler error, but it's for demo purposes.

ohh! but if I use this for example:

char urlDir[BUFSIZ] = "ftp://website.com/public_html/upload/";
UploadFile("filename.txt", "newfilename.txt", urlDir, "usernameForServer", "passwordForServer");

It should work then, shouldn't it?

Yes, provided you don't overwrite the bounds of the array.

Can a DLL write to a variable local to the executable, or must it be global or something similar?

Generally, yes. Of course, it can't magically know the name of a variable in some other code. The functions in the DLL must be able to "see" the variable in the executable. Declaring them in the executable and passing them in to the DLL functions, as you did in post #10, is a good way to achieve this.

It's working great, thanks. (forgot to link at first :))