How to create an int file

Question

I am trying to create a shared file, which has to be int.

int f;

However, when I reach

if ((fstat(f, &stbuf) != 0) || (!S_ISREG(stbuf.st_mode)))

It gives me an error. The file must contain a string like:

abcd

My guess is the following, but it does not work:

int main() {
  int f;
  void *memoria = NULL;
  int tam;
  struct stat stbuf;
  char string[15] = {0};

  f = open("fichero.txt", O_RDWR, S_IRUSR);

  // ERROR IS HERE!!
  if ((fstat(f, &stbuf) != 0) || (!S_ISREG(stbuf.st_mode))) {
    printf("Error");
  }

  tam = stbuf.st_size;
  printf("%d\n", tam);

  // Proyect the file
  memoria = mmap(0, tam, PROT_WRITE, MAP_SHARED, f, 0);
  // Copy the string into the file
  memcpy(memoria, "abcd", 5);

  munmap(memoria, tam);

  return 0;
}

Should I change the parameters in open?? What am I doing wrong? Thanks!

You don't check if the `open` succeeded and you don't output the specific error. That makes debugging impossible. — David Schwartz, Dec 08 '16 at 10:53
Segmentation fault. I think I am trying to access a file that I haven't created, but I do not want to do that, I want to actually generate it. — Eduardo Ramos, Dec 08 '16 at 10:54
The thing is that I am not trying to open it, I want to create it. I just do not know how to. — Eduardo Ramos, Dec 08 '16 at 10:55
Accept my edit don't copy and paste... I take time to indent your code! http://stackoverflow.com/review/suggested-edits/14534511 — Stargateur, Dec 08 '16 at 10:58
Check if `open` returns -1 which indicates that the file could not be opened and if yes, then don't proceed any further but display an error message. If you want to create a file, then maybe you need [`creat`](http://man7.org/linux/man-pages/man2/creat.2.html). BTW there is no such thing as an _int file_. — Jabberwocky, Dec 08 '16 at 11:07

score 0 · Accepted Answer · answered Dec 08 '16 at 11:05

0

You need to use the O_CREAT mode to create the file if it doesn't exist.

f = open("fichero.txt", O_RDWR | O_CREAT, S_IRUSR);

You should check for errors from open():

if (f == -1) {
    perror("open");
    exit(1);
}

answered Dec 08 '16 at 11:05

Barmar

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It tells me there is an error creating the file: Permission denied. – Eduardo Ramos Dec 08 '16 at 11:13
You're trying to create the file in a directory you don't have write permission to. – Barmar Dec 08 '16 at 11:14
Try using the full pathname so it creates the file in a directory you can write. – Barmar Dec 08 '16 at 11:14
It is the downloads directory, is that normal? Can I solve it with sudo? – Eduardo Ramos Dec 08 '16 at 11:14
Hey, so I am trying to copy my string like this memcpy(memoria,string,sizeof(string)); and it gives me a bus error. Do you have any idea why? (Last question I swear lol) – Eduardo Ramos Dec 08 '16 at 11:22
1

You can't grow a file with `mmap()`. If you just created the file, it will be empty, so there's no memory to map. You should use `ftruncate()` to set the length of the file before you map it. – Barmar Dec 08 '16 at 11:24
Thank you so much! – Eduardo Ramos Dec 08 '16 at 11:25

How to create an int file

1 Answers1