I tried to compile and run the following bubble sort program using the gcc but it shows the error

Question

{

int *v;
int i=0; 
int n;
int j=0;
int aux=0;
File *fp;

fp = fopen("Inteiros.txt", "r"); /*opening a file and read it*/

if(fp == NULL)

    printf("Erro, ficheiro nao encontrado!\n");/*portuguese sentence*/

else

    while(!feof(fp))

    {
        fscanf(fp, "%d", &v[i]);
        i++;
    }

    for(i=1; i<n; i++)
    {   
        for(j=0; j< n-i-1; j++)
        {
            if(v[j] > v[j+1])
            {   
                aux = v[j];
                v[j] = v[j+1];
                v[j+1] = aux;
            }
        }   
    }

than gave me the "segmentation fault" error and I don't know why. I know its a piece of the memory that I don't have access, but I don't know where is the error.

You need to show us the rest of the code, where you declare your variables and such. — cleblanc, May 10 '18 at 17:54
[Why is “while ( !feof (file) )” always wrong?](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) — Eugene Sh., May 10 '18 at 17:56
So? You are still overwriting the `i` count of inputs, by using `i` to control the `for` loop. — Weather Vane, May 10 '18 at 18:19
@PedroRocha you need to allocate some memory for `int *v` it's just a pointer — cleblanc, May 10 '18 at 18:20
try changing `int *v`->`int v[10]` and `int n;`->`int n=10;` — Tormund Giantsbane, May 10 '18 at 18:27
@PedroRocha why initialize to 0? Don't you want to enter the `for(i=1; i — Tormund Giantsbane, May 10 '18 at 18:29
@TormundGiantsbane it's already working because initialized the n in 61 because the folder "Inteiros.txt" has 61 lines with 61 number, but imagine if I don't know the number of lines how do I do? — Pedro Rocha, May 10 '18 at 18:46
@PedroRocha if you don't know the number of lines create some upper bound where you'd stop at. Say 1024.. let me edit answer below — Tormund Giantsbane, May 10 '18 at 18:47
regarding: `printf("Erro, ficheiro nao encontrado!\n");` This does not say 'why' the function failed. Strongly suggest: `perror("Erro, ficheiro nao encontrado!\n");` which outputs your error message AND the text reason the OS thinks the function failed to `stderr` NOT `stdout` — user3629249, May 12 '18 at 13:27
suggest the loop reading the data stop when the returned value (not the parameter values) from the call to `fscanf()` is not 1. suggest the loop make use of `realloc()` to (as needed) allocate more memory for the read in numbers. — user3629249, May 12 '18 at 13:42

Tormund Giantsbane · Accepted Answer · 2018-05-10T18:52:49.657

You're likely getting a seg fault because you didn't allocate any memory for you pointer int *v and then you try to assign values to it like it's an array. Also int n; was never initialized so your getting into undefined behavior. Also File is not a type unless you made your own that you're not showing, should be FILE.

Try something like this:

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

#define MAX_NUMS  1024

int main()
{

   int *v;
   int i=0, j=0, aux=0, n = 0;
   FILE *fp;

   fp = fopen("Inteiros.txt", "r");

   if(fp == NULL) {
      printf("Erro, ficheiro nao encontrado!\n");
      return 1;
   }
   else {
      //allocate memory for v
      if ((v = malloc(sizeof (int) * MAX_NUMS)) == NULL) {
         printf("Error in malloc\n"); 
         return 1;
      }

      while(!feof(fp)) {
         fscanf(fp, "%d", &v[i]);
         i++;
      }
      //number of lines read
      n = i;
      for(i = 0; i < n; i++) {
         for(j = 0; j < n-i-1; j++) {
            if(v[j] > v[j+1]) {
               aux = v[j];
               v[j] = v[j+1];
               v[j+1] = aux;
            }
         }
      }
      for (i = 0; i < MAX_NUMS; i++)
         printf("v[%d] is %d\n", i, v[i]);
   }
   return 0;
}

problems with this answer: 1) this answer is still outputting error messages to `stdout` rather than `stderr` 2) still using `while( !feof(fp)` when it should be using the returned value from the call to `fscanf()` to control the loop. so not a very good answer — user3629249, May 12 '18 at 13:30
unless the number of data items read is `MAX_NUMS`, this loop: ` for (i = 0; i < MAX_NUMS; i++)` will be printing uninitialized garbage. — user3629249, May 12 '18 at 13:35
Regarding: `while(!feof(fp)) { fscanf(fp, "%d", &v[i]);` This loop fails to stop when MAX_NUMS numbers have been read, so can be setting values past the end of the allocated memory .. Which would result in undefined behavior. — user3629249, May 12 '18 at 13:38

I tried to compile and run the following bubble sort program using the gcc but it shows the error

1 Answers1