Problem in copying a file in C

akin81 · 03-28-2004, 07:10 PM

How to copy a regular file in C using open(), read() and write()?
It seems i have program creating the new file using open().
How can i make the permission to be the same with the old file?

I am a newbie in Linux programming

zekko · 03-28-2004, 07:52 PM

Let's see the code you already got.

I'm not sure how to copy the file with same permissions, I always have to chmod +x if its a executable, I would like to know this too.

rkef · 03-28-2004, 08:27 PM

$ man 2 stat

Code:

struct stat {
        [...]
        mode_t        st_mode;     /* protection */
        [...]
};

akin81 · 03-28-2004, 08:58 PM

so far i have the following code in my function
working with just copying the content first
but i dont know why the new file is still empty after i run this
is there any problem in "write(dest_file,buffer,size);"?

Code:

void copy_file(char *source, char *dest)
{
  int source_file, dest_file;
  char buffer[128];
  struct stat buf;
  
  if ( lstat(source,&buf)==0 ) {
    if ( !(buf.st_mode & S_IFREG) )
      printf("%s: is not a regular file\n",source);  
  }
  else
    printf("%s: file does not exist\n",source);
    
  source_file = open(source, O_RDONLY);
  if ( source_file!=0 ) {
    printf("%s: error in opening file\n", source);
    return;
  }
 
  dest_file = open(dest, O_CREAT, S_IRWXU);
  if ( dest_file!=0 ) {
    printf("%s: error in creating file\n", dest);
    return;
  }
    
  int size;
  while( (size=read(source_file, buffer, 128)) > 0 ) {
      write(dest_file,buffer,size);
  }
  
  close(source_file);
  close(dest_file);
    
  return;
}

rkef · 03-28-2004, 09:35 PM

That's odd. open should return unique file descriptors; your tests of (fd != 0) should always succeed, unless your implementation defines stdin, stdout, stderr to something other than 0, 1, and 2.

You should be testing that they aren't -1 or, in general, < 0.

zekko · 03-28-2004, 09:42 PM

I havent test this, but maybe it'll work ...

Code:

void copy_file(char *source, char *dest) {
     int source_file, dest_file, read_size;
     char buffer[128];
     struct stat buf;

     if ( lstat(source,&buf)==0 ) {
        if ( !(buf.st_mode & S_IFREG) )
        printf("%s: is not a regular file\n",source);  
    }
    else
        printf("%s: file does not exist\n",source);
        return;
    }
    if ((source_file = open(source, O_RDONLY)) < 0) {
        fprintf(stderr, "%s: Can't open file\n", source);
        return;
    }
    if ((dest_file = open(dest, O_CREAT, S_IRWXU)) < 0) {
        fprintf(stderr, "%s: error creating file\n", dest);
        return;
    }
    while (1) {
        read_size = read(source_file, buffer, sizeof(buffer));
        if (read_size == 0)
           break;
        if (read_size < 0) {
           fputs("Error while reading file\n", stderr);
           return;
        }
        if (write(dest_file, buffer, read_size) < 0) {
           fputs("Error while writing to file\n", stderr);
           return;
       }
    }
    close(source_file);
    close(dest_file);
}

akin81 · 03-29-2004, 12:19 AM

Thanks to both of you, for correcting my mistakes.

The problem now i got is "Error while writing to file".
I guess there is a problem in "dest_file = open(dest, O_CREAT, S_IRWXU)".
Is that ok to write into the file using O_CREAT?

I tried "close(dest_file);" and open it again by "dest_file = open(dest, O_WRONLY);".
Then i can write into it without writing error, but this way looks clumsy.

I read "man open" but i don't quite understand about O_CREAT.

If i create a file using "open(dest, O_CREAT, S_IRWXU)" but the file already exists, how can i overwrite it?
If i delete the existing file and create a new one with the same name, it looks clumsy again.

rkef · 03-29-2004, 01:42 AM

Quote:

If i create a file using "open(dest, O_CREAT, S_IRWXU)" but the file already exists, how can i overwrite it?

Quote:

O_TRUNC
If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0.

Quote:

"Error while writing to file"

That looks like it's from your code. Try altering your error message to print the system error message, with something like: printf("Error while writing to file: %s\n", strerror(errno));

Hko · 03-29-2004, 11:18 AM

Code:

#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>

#define BUFFERSIZE 128

int copy_file(char *source, char *dest)
{
	 int source_file, dest_file;
	 char buffer[BUFFERSIZE];
	 struct stat fstatus;
	 mode_t permissions;
	 ssize_t bytesread;

	 /* Opening source file */
	 source_file = open(source, O_RDONLY);
	 if (source_file < 0) {
		  perror("Opening source file");
		  return 0;
	 }

	 /* Get permissions of source file */
	 if (fstat(source_file, &fstatus) < 0) {
		  perror("Getting permissions of source file");
		  return 0;
	 }
	 permissions = fstatus.st_mode;    /* store permissions */

	 /* 
	  * Opening destination file for writing.
	  * We will set the same permissions as the source file later,
	  * because the umask will be in effect here.
	  */
	 dest_file = open(dest, O_WRONLY|O_CREAT|O_TRUNC,  S_IRUSR|S_IWUSR);
	 if (source_file < 0) {
		  perror("Opening destination file");
		  return 0;
	 }

	 /* Do the copying */
	 do {
		  bytesread = read(source_file, buffer, BUFFERSIZE);
		  if (bytesread < 0) {
			   perror("Copying");
			   return 0;
		  }
		  write(dest_file, buffer, bytesread);
	 } while (bytesread > 0);

	 /* Set permissions as stored from the source file */
	 if (fchmod(dest_file, permissions) < 0) {
		  perror("Setting permissions");
		  return 0;
	 }
	 close(source_file);
	 close(dest_file);
	 return 1;   /* Succesfull! */	 
}


int main(int argc, char *argv[])
{
	 if (argc != 3) {
		  fprintf(stderr, "Usage: %s <source> <destination>\n", argv[0]);
		  return 1;
	 }
	 if (!copy_file(argv[1], argv[2])) {
		  return 2;
	 }
	 printf("done\n");
	 return 0;
}

worldmagic · 04-01-2004, 06:15 AM

For you copying.. this function might be intresting..
Parts taken from its manpage..

--
#include <sys/sendfile.h>
--
ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
--
Because this copying is done within the kernel, sendfile() does not
need to spend time transferring data to and from user space.
--

akin81 · 04-02-2004, 12:31 AM

Thank you everyone. That really helps.

Quote:

Originally posted by worldmagic
For you copying.. this function might be intresting..
Parts taken from its manpage..

--
#include <sys/sendfile.h>
--
ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
--
Because this copying is done within the kernel, sendfile() does not
need to spend time transferring data to and from user space.
--

For my project it is not allowed to use function like this.
But thanks for telling, it may help in the future.

worldmagic · 04-06-2004, 08:06 AM

You can always say you found it 'grep'ing the kernel.. ;-)