Thread: Problem with pointers/strings/memory

Originally Posted by Exile

I doubt it. You don't have any constant strings by the sounds of things. The write protected/constant strings are those written in code as "String", for example:

Code:

char String[] = "Constant string";

Except that isn't a constant string. This is:

Code:

char * stringliteral = "Don't modify this string. Can change the pointer, but not the string.";
char notconstant[] = "Can edit this string. Can't change what it points to. Because it's not a pointer.";

That is to say, as long as you don't run past the end of the second one, you can change all the values inside it.

Quzah.

OK here's the actual code, I did simplify it a bit initially, but I think this may help if I paste the code proper. (There's a few things I've tried and failed that are commented out)

Code:

#define CHARS_IN_ADVERT 714
struct MemoryStruct {
  char *memory;
  size_t size;
};
void stripads(char *s2)
{
    if (strlen(s2) >CHARS_IN_ADVERT)
    {//chunk.memory is bigger than CHARS_IN_ADVERT characters
        s2[strlen(s2) - CHARS_IN_ADVERT] = '\0';
    }
    else
    {//Not enough characters, so cut them all off I guess
        s2[0] = '\0';
    }
    return *s2;
}

void *myrealloc(void *ptr, size_t size)
{
  // There might be a realloc() out there that doesn't like reallocing NULL pointers, so we take care of it here 
  if(ptr)
    return realloc(ptr, size);
  else
    return malloc(size);
}

size_t WriteMemoryCallback(void *ptr, size_t size, size_t nmemb, void *data)
{
  register int realsize = size * nmemb;
  struct MemoryStruct *mem = (struct MemoryStruct *)data;

  mem->memory = (char *)myrealloc(mem->memory, mem->size + realsize + 1);
  if (mem->memory) {
    memcpy(&(mem->memory[mem->size]), ptr, realsize);
    mem->size += realsize;
    mem->memory[mem->size] = 0;
  }
  return realsize;
}

struct message getstatus(void)
{
    char tmp[4],url[255]="",fileline[9000]="",*thedataPtr,thedata[9000]="";
    struct MemoryStruct chunk;
    struct message messages[100];
    int i=0,nlpos=0,error;    
    
    chunk.memory=NULL; // we expect realloc(NULL, size) to work
    chunk.size = 0;    // no data at this point
    
    if(curlhttp) {
        sprintf(url,"%s/messages.php?user=%s&hash=%s",SERVERADDR,usrname,hash);
#if DEBUG
        printf("Sending data...\n");
#endif
        curl_easy_setopt(curlhttp, CURLOPT_URL, url);
#if DEBUG
        printf("Opened %s\nSetting up memory...\n",url);
#endif
        // send all data to this function
        curl_easy_setopt(curlhttp, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);

        // we pass our 'chunk' struct to the callback function
        curl_easy_setopt(curlhttp, CURLOPT_WRITEDATA, (void *)&chunk);

#if DEBUG
        printf("Reading into memory...\n",url);
#endif
        // get it!
        if(0!=(error=curl_easy_perform(curlhttp))) {
            fprintf(stderr,"Error: Getstatus: curl: %s.\n",curl_easy_strerror(error));
            return *messages;
        }

        if(strlen(chunk.memory)<6) {
            fprintf(stderr,"Error: Getstatus: Too little, or no data returned.\n");
            return *messages;
        }
        memmove((char *)&chunk.memory[0],(char *)&chunk.memory[1],chunk.size-1); // strip initial newline char.
        stripads(chunk.memory);
#if DEBUG
        printf("Getstatus: Data Received:\n\"%s\"\n",chunk.memory);
        printf("Getstatus: Checking for errors\n");
#endif
        sscanf(chunk.memory,"%3s",&tmp);
        printf("tmp: \"%s\"\n",tmp);
        if(0!=strcmp(tmp,"ERR")) {
#if DEBUG
            printf("Getstatus: We have results!\n");

#endif
            // parse the data
            // tokenize it into individual lines and then into the different parts and create the array of filedetails.
            
//            nlpos=chrpos(chunk.memory,10,0);
            printf("Getstatus: %p fileline: %s\n",&fileline,fileline);
//            filelinePtr=fileline;
//            substr(filelinePtr,chunk.memory,0,nlpos);
        printf("Getstatus: Data Received:\n\"%s\"\n",chunk.memory); 
//            thedata=malloc(sizeof(*chunk.memory));
            memcpy(thedata,chunk.memory,chunk.size);
            strcat(thedata,'\0');
            thedataPtr = thedata;
//            sscanf(chunk.memory,"%s",thedata);
/ *************** segfault now after output of string here, but before "here" is printed. *************/
            printf("Thedata: \"%s\"\n",thedata);
            printf("here");
            *fileline=strtok(thedataPtr,10);

HTH

details on the curl library are available here: http://curl.haxx.se/libcurl/c/