Thread: Help with malloc and pointers.

Will it not store in in_name?

Yes it will. I don't even know how I missed that there, but I did, so apologies. Yes that should be fine.

Ok, thank you so much. So the big main problem is still occurring. I've figured out my function "filesize" is returning -1.

Code:

int filesize(char * filename){
   struct stat status;
   if (stat(filename, &status) == -1)
   {
      return -1;
   }
   else
   {
      return (int) status.st_size;
    }
}

Unfortunately, this is the piece of code I was given. I am nowhere near understanding what's going on except that "stat(filename, &status)" apparently is equal to -1. I don't even know what stat does. I'm stuck.

That's why they gave us a manual. As it says, -1 is returned on error, and errno is set appropriately. You can use perror to show the system error message:

Code:

int filesize(char * filename){
 struct stat status;
 if (stat(filename, &status) == -1)
   {
 perror("stat");
 return -1;
   }
 else
   {
 return (int) status.st_size;
    }
}

although I'd be willing to be a dollar to a doughnut that you've passed in a filename that doesn't exist.

Read the man page. From your terminal, you can type "man 3 stat", or search for that on the web. That 3 ensures you use section 3 of the man pages, which are for C library functions. You may occassionally want to check section 2 as well, which is system calls.

So, the code you were given calls the stat function. If stat returns -1 (error), then your function returns -1. Otherwise, your function returns the size as reported by stat.

Well, the filesize function was being passed "argv[1]" because that's what the professor told me to do but I changed it to in_name (the name of the file) and it began to work. I really hope changing that won't cause me trouble later on.

Originally Posted by RElainaJ

Well, the filesize function was being passed "argv[1]" because that's what the professor told me to do but I changed it to in_name (the name of the file) and it began to work. I really hope changing that won't cause me trouble later on.

Someone (anduril) mentioned that earlier: are the specs to pass the filename in on the command line (in which case argv[1] is the way to go) or to type it in in response to a prompt, in which case your scanf is the way to go? You'll have to check the specs and follow them.

The filename is prompted for in the command line.

I've gotten much further, actually starting the sorting process before running into another problem. Here's what I have at the moment.

Code:

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>


int filesize(char * filename)
{
   struct stat status;
   if (stat(filename, &status) == -1)
   {
      return -1;
   }
   else
   {
     return (int) status.st_size;
   }
}


int compare(const void *elem1, const void *elem2)
{
    char **e1 = (char **) elem1;
    char **e2 = (char **) elem2;
    if (strcmp(*e1,*e2)<0)
        return -1;
    else if (strcmp(*e1,*e2)>0)
        return 1;
    else
        return 0;
}


void sort(char array[], int size)
{
    qsort(array, size, sizeof(array[0]), &compare);
}


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

    FILE *read;
    char in_name[255];
    int n = 0;
    int j = 0;
    int linecount = 1;

   printf("What file should be read?\n");
    if (scanf("%s", in_name) != 1)
    {
      printf("Invalid input.\n");
      return 1;
   }

    read = fopen(in_name,"r");

    if (read == NULL)
    {
      printf("Cannot open input file.\n");
      return 1;
    }


    char * data = malloc(filesize(in_name));

    data[n] = fgetc(read);
    while (data[n] != EOF)
   {
      if (data[n] == '\n')
      {
         data[n] = '\0';
         linecount++;
      }
      data[++n] = fgetc(read);
   }
     if (data[n] != '\0')
      data[++n] = '\0';

    char ** lines = malloc(sizeof(lines[0]) * linecount);

    int i = 0;
    while (j < linecount)
    {
      lines[j] = &data[i];
      while(data[i] != '\0')
         i++;
      i++;
      j++;
    }

    sort(*lines,linecount);

    for (int k = 0; k < linecount; k++)
    {
      printf("%s\n", lines[k]);
    }

    fclose(read);
    free(data);
    free(lines);
    return 0;
}

The problem is "if (strcmp(*e1,*e2)<0)" and, it seems I'm going about accessing the arrays to sort the wrong way.

Barney- Honestly, I don't know how to use a growing array. I imagine figuring out how to do so, messing up, figuring out what went wrong, and fixing it would tack on more time. I've got a lot of work besides this to do before tomorrow. So... I suppose it's trying to take the easy way out, even if the program won't be as solid. Yes, I'm ashamed of this, but... well... not enough to give up on sleeping at all.

man sort gives me the parameters but not an example. I wasn't bright enough to pick up on discrepancies there.

Thank you for the link! Going through it... I think I'm doing something wrong regarding whether I'm using pointers to things or the things themselves? Am I on the right track?