gcc -fno-stack-protector <file> -o <output-file> 

*** stack smashing detected ***: ./bufferOverflow terminated
Aborted (core dumped)

LIbreria customfunctions.h

// A function to print an error message and exit

void fatal(char *message) {
  char error_message[100];

  strcpy(error_message, "[!!] FATAL ERROR ");
  strncat(error_message, message, 83);
  perror(error_message);
  exit(-1);

}

void *ec_malloc(unsigned int size){
  void *ptr;

  ptr = malloc(size);

  if(ptr == NULL)
    fatal("in ec_malloc() on memory allocation.\n");

  return ptr;
}

void usage(char *prog_name, char *filename){
  printf("Usage: %s <text to add to %s>\n", prog_name, filename);
  exit(0);
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/stat.h>
#include "customfunctions.h"


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

  int userid, fd;
  char *databuffer, *notefile;

  databuffer = (char *) ec_malloc(sizeof(argv[1]));
  strcpy(databuffer, argv[1]);

  notefile = (char *) ec_malloc(sizeof("/tmp/note"));
  strcpy(notefile, "/tmp/note");

  if(argc < 2)
    usage(argv[0], notefile);

  printf("[DEBUG] buffer @ %p: \'%s\'\n", databuffer, databuffer);
  printf("[DEBUG] datafile @ %p: \'%s\'\n", notefile, notefile);
  strncat(databuffer, "\n", 1); //Add a newline at the end

  fd = open(notefile, O_WRONLY|O_CREAT|O_APPEND, S_IRUSR|S_IWUSR);
  if(fd == -1 )
    fatal("in main() when opening file");

  printf("[DEBUG] file descriptor is %d\n", fd);

  userid = getuid(); //Obtiene el id del usuario que lo ejecuta

  if(write(fd, &userid, sizeof(int)) == -1 )
    fatal("in main() while writing userid to file");

  printf("[DEBUG] Address: %p - User id: %i \n", &userid, userid);

  if(write(fd, "\n", 1) == -1)
    fatal("in main() when writing blank line");

  if(write(fd, databuffer, strlen(databuffer)) == -1 )
    fatal("in main() when writing in file");

  if(write(fd, "\n", 1) == -1)
    fatal("in main() when writing blank line");

  if(close(fd) == -1)
    fatal("in main() when closing file");

  free(databuffer);
  free(notefile);

  return 0;
}

(gdb) backtrace full
#0  main (argc=2, argv=0xbffff0c4) at ./notes.c:35
        userid = 1000
        fd = 3
        databuffer = 0x804b008 "test\n"
        notefile = 0x804b018 "/tmp/note"
(gdb) 

superdog@ubuntu:~/home/superdog$ cat /tmp/note
�
message

message2

#include <stdio.h&gt;
#include <stdlib.h&gt;

int global_var; //.bss
int global_initialized_var = 5; //.data

void function(){
int stack_var; //Se metera en la pila.
printf("The function's stack_var is at address %p\n", &stack_var);
}

int main(){
int stack_var; // en la pila
static int static_initialized_var = 5; //en .data
static int static_var; //.bss
int *heap_var_ptr; //.heap
heap_var_ptr = (int *) malloc(4);

//.data segment
printf(".data segment\n");
printf("global_initialized_var is at address %p\n", &global_initialized_var);
printf("static_initialized_var is at address %p\n", &static_initialized_var);

//.bss segment
printf(".bss segment\n");
printf("static_var is at address %p\n", &static_var);
printf("global_var is at address %p\n", &global_var);

//.heap segment
printf(".heap segment\n");
printf("heap_var_ptr is at addres %p\n",heap_var_ptr);

//.stack segment
printf(".stack segment\n");
printf("stack_var is at address %p\n", &stack_var);
function();
}

.data segment
global_initialized_var is at address 0x804a028
static_initialized_var is at address 0x804a02c
.bss segment
static_var is at address 0x804a034
global_var is at address 0x804a038
.heap segment
heap_var_ptr is at addres 0xbfe204e8
.stack segment
stack_var is at address 0xbfe204e4
The function's stack_var is at address 0xbfe204c8