Contiki Coffee File System

Introduction

This tutorial is a introduction to cfs_open function of Coffee file system on Contiki OS 2.7.

You Will Learn

Through this tutorial you will learn about how the Coffee file system works.

Source Code

~/contiki-2.7/core/cfs/cfs-coffee.h
~/contiki-2.7/core/cfs/cfs-coffee.c

Coffee File System Open File

Data Structure Used In The System

struct file_desc {
  cfs_offset_t offset; 
  struct file *file; //  a file pointer which point to the corresponding file.
  uint8_t flags; //flag used to indicate the open mode of the file and whether the fd is free.
#if COFFEE_IO_SEMANTICS
  uint8_t io_flags;
#endif
};

struct file_desc coffee_fd_set[COFFEE_FD_SET_SIZE];

static int
get_available_fd(void)
{
  int i;

  for(i = 0; i < COFFEE_FD_SET_SIZE; i++) {
    if(coffee_fd_set[i].flags == COFFEE_FD_FREE) {
      return i;
    }
  }
  return -1;
}

Contiki is a Linux like system, everything is a file, and the opened file is represented by the file descriptor. The get_available_fd will return the smallest available fd by check the file_desc array in an ascending order, the first one with flag == 0(COFFEE_FD_FREE) is what we want. If all the flags are set, then return -1.

cfs_open

cf_open is used to open a file. If it successfully open the file then it returns the file descriptor(fd), otherwise it returns -1.
cfs_open source code:

int
cfs_open(const char *name, int flags)
{
  int fd;
  struct file_desc *fdp;

  fd = get_available_fd(); //find the smallest available fd, see section 4.1
  if(fd < 0) {
    PRINTF("Coffee: Failed to allocate a new file descriptor!\n");
    return -1;
  }

  fdp = &coffee_fd_set[fd];
  fdp->flags = 0; //set the fd to FREE

  fdp->file = find_file(name); //find the file corresponding to name(not exist, In flash but not cached, cached)

  /*** if there isn't any corresponding file, then try to create new file ***/
  if(fdp->file == NULL) {
    if((flags & (CFS_READ | CFS_WRITE)) == CFS_READ) {
      return -1;
    }
    fdp->file = reserve(name, page_count(COFFEE_DYN_SIZE), 1, 0);
    if(fdp->file == NULL) {
      return -1;
    }
    fdp->file->end = 0; // Since it's a new created file, the end will be set to 0
  }
   /*** find the file,seek for the end of the file***/ 
   else if(fdp->file->end == UNKNOWN_OFFSET) {
    fdp->file->end = file_end(fdp->file->page);
  }

  fdp->flags |= flags;
  fdp->offset = flags & CFS_APPEND ? fdp->file->end : 0; //if the flag is set to APPEND, then the offset is set to the end of the file, otherwise set to 0
  fdp->file->references++; //reference count will increment

  return fd;
}