Thread: Pointer to struct

Not very explaining title maybe, but don't know how to explain shortly.

I'm trying to create an array of structs, but as I'm not very good at structs so I'm getting some problems. What I want to do is to have an array (using pointers as I don't know what the size of the array will be) of a struct containing two fields.

So it's supposed to be an array containing a number of structs, where the struct contains a char* and an in.

I tried it with the below testprogram, but I get segmentation-fault, testing with the printf I discovered that the temp-structure works, but it seems that I'm accessing or populating the processes the wrong way.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

struct process {
  char* name;
  int pid;
} *processes;
  
main()
{

  processes=malloc(2*sizeof(*processes));
  char* string;
  //  int temp;
  int x;
  struct process temp;
  temp.name="Testing";
  temp.pid=25;
  processes[0]=temp;
  struct process temp2;
  temp2.name="structs";
  temp2.pid=35;
  
  processes[1]=temp2;
  printf("Name: %s\n",temp.name);

  for (x=0;x<2;x++)
    {
      printf("Pid: %d Name: %s\n",processes[x].pid,processes[x].pid); // This fails (or maybe the assigning)
    }
  free(processes);
  exit(0);
}

Thanks, but it doesn't work here for some reason. Now I'm doing (in my program I will use variables for the string and the size of it). I've tried both with the number of chars, and with number of chars +1 to make room for null-termination, but I'm still getting segmentation fault in the same place (I also tried assigning directly):

Code:

  processes[0].pid=25;
  processes[0].name=malloc(8*sizeof(char));
  strcpy(processes[0].name,"Testing");
  processes[1].pid=35;
  processes[1].name=malloc(9*sizeof(char));
  strcpy(processes[1].name," structs");

If it matters, I'm compiling with cc in Ubuntu 8.04 (it seems to be linked to gcc).

Thanks to quzah's example I found the bug. I don't know if I did things right from the start, but I discovered something that it seems everyone missed. I try to read processes[x].pid as a string in the for-loop which is most likely what's causing the segmentation fault :-/ When quzah's program ran on my computer with the same functionality I discovered this since I knew then that my program "should" work.

Thanks for the help.

Hello again!

It seems that I have received a problem when I added functionality using the above. I now created a function for adding a new "processes" to the array, but I'm getting segmentation fault, but I don't understand why. Can anyone see a problem? Here is the testprogram I'm using:

Code:

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

struct process {
  char* name;
  int pid;
} *processes,*temp;
int number;

int addProcesses(int pid, char* newname)
{
  int x;
  if (number==0)
    {
      number++;
      temp=malloc(number*sizeof(*processes));
      temp[0].name=newname;
      temp[0].pid=pid;
      return(temp);
    }
  number++;
  printf("Adding number %d\n",number);
  temp=malloc(number*sizeof(*processes));
  for (x=0;x<(number-1);x++)
    {
      temp[x]=processes[x];
    }
  temp[x].name=newname;
  temp[x].pid=pid;
  free(processes);
  processes=temp;
  free(temp);
  return(0);
}

main()
{
  int x=0;
  number=0;
  processes=addProcesses(35,"Testing");
  processes=addProcesses(25," structs");
  processes=addProcesses(15," again!");
  for (x=0;x<number;x++)
    printf("Pid: %d, Name: %s\n",processes[x].pid,processes[x].name);
  exit(0);
}

By using printf-statements around the function (removed most of them before publishing here), I discovered that the segmentation fault occurs when I add the third string for some reason. As before I will use variables/pointers in stead of fixed strings in the end program, but I assume the functionality will be the same. I've tried without freeing also, but it didn't seem to make a difference.

Also, I put temp as another global variable as I couldn't figure out how to create a temp-array inside of the function and the returning it, but it wouldn't compile. Is that not possible (not as important as the main problem)?

Maybe I should have written the explanation above the code instead of below, so I'll write it again, on it's own:

As I wrote above, I get a segmentation fault when I try to add the third string. And it occurs on the assignment in the addProcesses-function.