Storing a filename entered from the keyboard.

Completely Clueless · 09-17-2009, 11:46 AM

Hi guys,

I must have an attack of brainfade. This problem's got me beat. This code below when compiled just generates a segmentation fault. I've commented it so you can see what I'm trying to do. Where am I going wrong? Thanks:

Code:

 
#include <stdio.h>

int main(void)
{
    FILE *fp;
    int c;
    int b=0;
    char s[25];          //declare array 's' to hold the filename

printf("Enter local filename: ");
fgets(s,20,stdin);                  //read string from keyboard into   
                                    //array 's'


    fp = fopen(s , "r"); // no errorcheck    //open the file referrenced by 's' in read-mode.

    while((c = fgetc(fp)) != EOF)
          {
          b++;         //count number of characters in the file
          }

    fclose(fp);

printf("The number of characters in %s is:%d " ,s,b); //print out  
                                                      //total characters

    return 0;

}

zhjim · 09-17-2009, 12:06 PM

I first thought that you need a \0 to terminate the string but
http://www.cplusplus.com/reference/c.../cstdio/fgets/

states that it is appended. But what sruged me in fgets is that the newline char is also put into the string s.
But this all is just guessing and a quick shot. Maybe debugging with

http://www.ibm.com/developerworks/library/l-gdb/

will help

cheers Zhjim

druuna · 09-17-2009, 12:49 PM

Don't have a solution (yet), but I can point you to the actual error:

The file cannot be found (even though it is there).

I change the code a bit to get that answer:

Code:

#include <stdio.h>

int main(void)
{
  FILE *fp;
  int c;
  int b=0;
  char s[25];  

  printf("Enter local filename: ");
  fgets(s,20,stdin);                  

  fp = fopen(s,"r"); 

  if(fp==NULL) {
    printf("Error: can't open file.\n");
    return 1;
  }
  else {
    while((c = fgetc(fp)) != EOF)
    {
      b++;         
    }
    fclose(fp);
    printf("The number of characters in %s is:%d " ,s,b); 
    return 0;
  }
}

Output of a test run:

Quote:

$./blaat
Enter local filename: one.txt
Error: can't open file.
$ ls one.txt
one.txt

A clean, although unexpected, run. How is the file closed when an error occurs? (don't have the c knowledge to answer that myself)

An strace shows the following (relevant part, without the above addition):

Quote:

write(1, "Enter local filename: ", 22Enter local filename: ) = 22
read(0, one.txt
"one.txt\n", 1024) = 8
brk(0) = 0x804a000
brk(0x806b000) = 0x806b000
open("one.txt
", O_RDONLY) = -1 ENOENT (No such file or directory)
--- SIGSEGV (Segmentation fault) @ 0 (0) ---
+++ killed by SIGSEGV +++
Process 24775 detached

Wim Sturkenboom · 09-17-2009, 12:57 PM

Your first problem is that you don't check the result of fopen. If the file does not exist you continue with fp equal to NULL.

And fgets keeps the newline character in, so s contains a '\n' and therefore fopen WILL return NULL.

Basic debugging with some printfs and a perror would have revealed it.