Name

Library

Synopsis

Return values

Detailed description

The flags specified are formed by or ’ing the following values

O_RDONLY        open for reading only
O_WRONLY        open for writing only
O_RDWR          open for reading and writing
O_NONBLOCK      do not block on open
O_APPEND        append on each write
O_CREAT         create file if it does not exist
O_TRUNC         truncate size to 0
O_EXCL          error if create and file exists
O_SHLOCK        atomically obtain a shared lock
O_EXLOCK        atomically obtain an exclusive lock
O_DIRECT        eliminate or reduce cache effects
O_FSYNC         synchronous writes
O_NOFOLLOW      do not follow symlinks
Following options are currently not supported :
O_NONBLOCK, O_SHLOCK, O_EXLOCK, O_DIRECT, O_FSYNC, O_NOFOLLOW.

         

Opening a file with O_APPEND set causes each write on the file to be appended to the end.

If O_TRUNC is specified and the file exists, the file is truncated to zero length.

If O_EXCL is set with O_CREAT and the file already exists, open returns an error. This may be used to implement a simple exclusive access locking mechanism.

If O_EXCL is set and the last component of the pathname is a symbolic link, open will fail even if the symbolic link points to a non-existent name.

If the O_NONBLOCK flag is specified and the open system call would result in the process being blocked for some reason (e.g., waiting for carrier on a dialup line), open returns immediately. The descriptor remains in non-blocking mode for subsequent operations.This mode need not have any effect on files other than FIFOs.

If O_FSYNC is used in the mask, all writes will immediately be written to disk, the kernel will not cache written data and all writes on the descriptor will not return until the data to be written completes.

If O_NOFOLLOW is used in the mask and the target file passed to open is a symbolic link then the open will fail.

When opening a file, a lock with flock semantics can be obtained by setting O_SHLOCK for a shared lock, or O_EXLOCK for an exclusive lock. If creating a file with O_CREAT, the request for the lock will never fail (provided that the underlying file system supports locking).

O_DIRECT may be used to minimize or eliminate the cache effects of reading and writing. The system will attempt to avoid caching the data,read or written. If it cannot avoid caching the data, it will minimize the impact the data has on the cache. Use of this flag can drastically reduce performance if not used with care.

If successful, open returns a non-negative integer, termed a file descriptor. It returns -1 on failure. The file pointer used to mark the current position within the file is set to the beginning of the file.

When a new file is created it is given the group of the directory which contains it.

The new descriptor is set to remain open across execve system calls; see close and fcntl.

The system imposes a limit on the number of file descriptors open simultaneously by one process. The getdtablesize system call returns the current system limit.

Note

If the specified file is a symbolic link and the file it is pointing to is invalid, the symbolic link file will be removed automatically.

Limitation

Creation of a file with O_CREAT flag does not alter the time stamp of the parent directory. A new file has two time stamps namely: access time and modification time. The creation time stamp of a file is not supported as; the access time stamp is same as the modification time stamp.

Examples

/************************************************************************
* This example creates a file in the current working directory and
* opens it in read-write mode.
************************************************************************/
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main()
{
  int fd = 0;
   fd = open("Example.txt" , O_CREAT | O_EXCL , 0666);
   if(fd < 0 ) 
   {
      printf("Failed to create and open file in current working directory \n");
      return -1;
   }
   printf("File created and opened in current working directory \n"  );
   return 0;
}

         

Output

File created and opened in current working directory

         

Errors

See also

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