Seg Fault on scanf

**jimmymax** · 03-19-2008

Hi everyone,

I'm trying to implement Eratosthenes Sieve and I am getting a seg fault on my first scanf function. I'm very new to C and am just using nano to write the code and gcc to compile it. Here is my code:

Code:

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

void PrintPrimes(const int Max)
{
  int i = 0, j = 0;
  int* PossiblePrimes = (int*) malloc((Max - 2) * sizeof(int));
  printf("Test 2");
  if(!PossiblePrimes)
  {
    printf("Error: Out of Memory");
    return;
  }
  printf("Test 3");
  for(i = 0; i < Max - 2; i++)
  {
    *(PossiblePrimes + i) = i + 2;
  }
  printf("Test 4");
  i = 0;
  
  while(i < Max - 2)
  {
    if(PossiblePrimes[i] != 0)
    {
      for(j = 0; PossiblePrimes[i] * j < Max - 2; j++)
      {
        PossiblePrimes[(i * j) - 2] = 0;
      }
    }
    i++;
  }
  
  for(i = 0; i < Max - 2; i++)
  {
    if(PossiblePrimes[i] != 0)
      printf("&#37;d\n", PossiblePrimes[i]);
  }
  
  free(PossiblePrimes);
  
  return;
}

int main()
{
  int UserMax = 0;
  
  scanf("%d", &UserMax);
  printf("Do I get to here?");

  if(UserMax >= 2)
    PrintPrimes(UserMax);
    
  return 0;
}

Never mind if my algo isn't quite right yet, the problem is the printf in main never executes and instead I get a seg fault

Any idea or hints as to why that might be? I'm really stuck!

EDIT: It only seg faults if I input a number greater than 10!?

Thanks for your time, much appreciated.
James

**Dave_Sinkula** · 03-19-2008

It gets there, the seg fault occurs in the called function. Add the following after the printf:

Code:

  fflush(stdout);

And also think about adding newlines.

As to the real issue, look in here:

Code:

  while(i < Max - 2)
  {
    if(PossiblePrimes[i] != 0)
    {
      for(j = 0; PossiblePrimes[i] * j < Max - 2; j++)
      {
        PossiblePrimes[(i * j) - 2] = 0;
      }
    }
    i++;
  }

**jimmymax** · 03-19-2008

Ohh I see, so the problem in the called function happens before the output is flushed to the screen. Thanks for that.

**IceDane** · 03-19-2008

Originally Posted by jimmymax

Ohh I see, so the problem in the called function happens before the output is flushed to the screen. Thanks for that.

Exactly - it's basically the compiler rearranging things a bit to optimize.

**dwks** · 03-19-2008

Not exactly. The screen is only (usually) guaranteed to be flushed when a newline is printed. Unless you print a newline, or call fflush(stdout), you might not see anything at all.

Files are buffered such that they are usually only written to when BUFSIZ characters have been written to the file. So unless you fflush() or fclose() the file, you might not see the latest data in a file either.

In short -- if you're going to print something to the screen, either print a newline or call fflush() to make sure it gets displayed. (Or make sure more text is printed immediately afterwards, which contains a newline or whatever . . . .)

**QuantumPete** · 03-20-2008

If I remember correctly, doing

Code:

fprintf (stderr, "Blah");

Will also be flushed directly to the screen without a newline, because stderr is not buffered like stdio. Hence I'd reccomend using fprintf and stderr to print out errors.

QuantumPete

**dwks** · 03-21-2008

Looks like not always. http://mailman.linuxchix.org/piperma...st/000692.html

I'm pretty sure you can make streams unbuffered, line buffered, or . . . "block" buffered at will.

I do remember something about cerr being unbuffered and clog being line buffered in C++, even though both wrote to the standard error stream. I'm not sure what's guaranteed for C's stderr.

It does look like stderr is usually unbuffered by default.

The setbuf(blank) and setvbuf(blank) functions let you change the type and size of the buffers. By default, output to a terminal is line buffered, output to stderr is unbuffered, and all other I/O is fully buffered. See the setbuf(3c) man page for details.

http://techpubs.sgi.com/library/tpl/...html/ch05.html

I don't know how standard/unstandard this is.

[edit] I would definitely recommend using stderr to print errors, however. perror() uses it, and the user is more likely to see any messages printed to stderr, in case they redirected stdout. [/edit]

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