Chapter 5: The SFTP subsystem

The SFTP subsystem

SFTP stands for "Secure File Transfer Protocol". It enables you to safely transfer files between the local and the remote computer. It reminds a lot of the old FTP protocol.

SFTP is a rich protocol. It lets you do over the network almost everything that you can do with local files:

• send files
• modify only a portion of a file
• receive files
• receive only a portion of a file
• get file owner and group
• get file permissions
• set file owner and group
• set file permissions
• remove files
• rename files
• create a directory
• remove a directory
• retrieve the list of files in a directory
• get the target of a symbolic link
• create symbolic links
• get information about mounted filesystems.

The current implemented version of the SFTP protocol is version 3. All functions aren't implemented yet, but the most important are.

Opening and closing a SFTP session

Unlike with remote shells and remote commands, when you use the SFTP subsystem, you don't handle directly the SSH channels. Instead, you open a "SFTP session".

The function sftp_new() creates a new SFTP session. The function sftp_init() initializes it. The function sftp_free() deletes it.

As you see, all the SFTP-related functions start with the "sftp_" prefix instead of the usual "ssh_" prefix.

The example below shows how to use these functions:

#include <libssh/sftp.h>
 
int sftp_helloworld(ssh_session session)
{
  sftp_session sftp;
  int rc;
 
  sftp = sftp_new(session);
  if (sftp == NULL)
  {
    fprintf(stderr, "Error allocating SFTP session: %s\n",
            ssh_get_error(session));
    return SSH_ERROR;
  }
 
  rc = sftp_init(sftp);
  if (rc != SSH_OK)
  {
    fprintf(stderr, "Error initializing SFTP session: code %d.\n",
            sftp_get_error(sftp));
    sftp_free(sftp);
    return rc;
  }
 
  ...
 
  sftp_free(sftp);
  return SSH_OK;
}

Analyzing SFTP errors

In case of a problem, the function sftp_get_error() returns a SFTP-specific error number, in addition to the regular SSH error number returned by ssh_get_error_number().

Possible errors are:

• SSH_FX_OK: no error
• SSH_FX_EOF: end-of-file encountered
• SSH_FX_NO_SUCH_FILE: file does not exist
• SSH_FX_PERMISSION_DENIED: permission denied
• SSH_FX_FAILURE: generic failure
• SSH_FX_BAD_MESSAGE: garbage received from server
• SSH_FX_NO_CONNECTION: no connection has been set up
• SSH_FX_CONNECTION_LOST: there was a connection, but we lost it
• SSH_FX_OP_UNSUPPORTED: operation not supported by libssh yet
• SSH_FX_INVALID_HANDLE: invalid file handle
• SSH_FX_NO_SUCH_PATH: no such file or directory path exists
• SSH_FX_FILE_ALREADY_EXISTS: an attempt to create an already existing file or directory has been made
• SSH_FX_WRITE_PROTECT: write-protected filesystem
• SSH_FX_NO_MEDIA: no media was in remote drive

Creating a directory

The function sftp_mkdir() takes the "SFTP session" we just created as its first argument. It also needs the name of the file to create, and the desired permissions. The permissions are the same as for the usual mkdir() function. To get a comprehensive list of the available permissions, use the "man 2 stat" command. The desired permissions are combined with the remote user's mask to determine the effective permissions.

The code below creates a directory named "helloworld" in the current directory that can be read and written only by its owner:

#include <libssh/sftp.h>
#include <sys/stat.h>
 
int sftp_helloworld(ssh_session session, sftp_session sftp)
{
  int rc;
 
  rc = sftp_mkdir(sftp, "helloworld", S_IRWXU);
  if (rc != SSH_OK)
  {
    if (sftp_get_error(sftp) != SSH_FX_FILE_ALREADY_EXISTS)
    {
      fprintf(stderr, "Can't create directory: %s\n",
              ssh_get_error(session));
        return rc;
    }
  }
 
  ...
 
  return SSH_OK;
}

Unlike its equivalent in the SCP subsystem, this function does NOT change the current directory to the newly created subdirectory.

Writing to a file on the remote computer

You handle the contents of a remote file just like you would do with a local file: you open the file in a given mode, move the file pointer in it, read or write data, and close the file.

The sftp_open() function is very similar to the regular open() function, excepted that it returns a file handle of type sftp_file. This file handle is then used by the other file manipulation functions and remains valid until you close the remote file with sftp_close().

The example below creates a new file named "helloworld.txt" in the newly created "helloworld" directory. If the file already exists, it will be truncated. It then writes the famous "Hello, World!" sentence to the file, followed by a new line character. Finally, the file is closed:

#include <libssh/sftp.h>
#include <sys/stat.h>
#include <fcntl.h>
 
int sftp_helloworld(ssh_session session, sftp_session sftp)
{
  int access_type = O_WRONLY | O_CREAT | O_TRUNC;
  sftp_file file;
  const char *helloworld = "Hello, World!\n";
  int length = strlen(helloworld);
  int rc, nwritten;
 
  ...
 
  file = sftp_open(sftp, "helloworld/helloworld.txt",
                   access_type, S_IRWXU);
  if (file == NULL)
  {
    fprintf(stderr, "Can't open file for writing: %s\n",
            ssh_get_error(session));
    return SSH_ERROR;
  }
 
  nwritten = sftp_write(file, helloworld, length);
  if (nwritten != length)
  {
    fprintf(stderr, "Can't write data to file: %s\n",
            ssh_get_error(session));
    sftp_close(file);
    return SSH_ERROR;
  }
 
  rc = sftp_close(file);
  if (rc != SSH_OK)
  {
    fprintf(stderr, "Can't close the written file: %s\n",
            ssh_get_error(session));
    return rc;
  }
 
  return SSH_OK;
}

Reading a file from the remote computer

A synchronous read from a remote file is done using sftp_read(). This section describes how to download a remote file using sftp_read(). The next section will discuss more about synchronous/asynchronous read/write operations using libssh sftp API.

Files are normally transferred in chunks. A good chunk size is 16 KB. The following example transfers the remote file "/etc/profile" in 16 KB chunks. For each chunk we request, sftp_read() blocks till the data has been received:

// Good chunk size
#define MAX_XFER_BUF_SIZE 16384
 
int sftp_read_sync(ssh_session session, sftp_session sftp)
{
  int access_type;
  sftp_file file;
  char buffer[MAX_XFER_BUF_SIZE];
  int nbytes, nwritten, rc;
  int fd;
 
  access_type = O_RDONLY;
  file = sftp_open(sftp, "/etc/profile",
                   access_type, 0);
  if (file == NULL) {
      fprintf(stderr, "Can't open file for reading: %s\n",
              ssh_get_error(session));
      return SSH_ERROR;
  }
 
  fd = open("/path/to/profile", O_CREAT);
  if (fd < 0) {
      fprintf(stderr, "Can't open file for writing: %s\n",
              strerror(errno));
      return SSH_ERROR;
  }
 
  for (;;) {
      nbytes = sftp_read(file, buffer, sizeof(buffer));
      if (nbytes == 0) {
          break; // EOF
      } else if (nbytes < 0) {
          fprintf(stderr, "Error while reading file: %s\n",
                  ssh_get_error(session));
          sftp_close(file);
          return SSH_ERROR;
      }
 
      nwritten = write(fd, buffer, nbytes);
      if (nwritten != nbytes) {
          fprintf(stderr, "Error writing: %s\n",
                  strerror(errno));
          sftp_close(file);
          return SSH_ERROR;
      }
  }
 
  rc = sftp_close(file);
  if (rc != SSH_OK) {
      fprintf(stderr, "Can't close the read file: %s\n",
              ssh_get_error(session));
      return rc;
  }
 
  return SSH_OK;
}

Performing an asynchronous read/write on a file on the remote computer

sftp_read() performs a "synchronous" read operation on a remote file. This means that sftp_read() will first request the server to read some data from the remote file and then would wait until the server response containing data to read (or an error) arrives at the client side.

sftp_write() performs a "synchronous" write operation on a remote file. This means that sftp_write() will first request the server to write some data to the remote file and then would wait until the server response containing information about the status of the write operation arrives at the client side.

If your client program wants to do something other than waiting for the response after requesting a read/write, the synchronous sftp_read() and sftp_write() can't be used. In such a case the "asynchronous" sftp aio API should be used.

Please go through Chapter 10: The SFTP asynchronous I/O for a detailed description of the sftp aio API.

The sftp aio API provides two categories of functions :

• sftp_aio_begin_*() : For requesting a read/write from the server.
• sftp_aio_wait_*() : For waiting for the response of a previously issued read/write request from the server.

Hence, the client program can call sftp_aio_begin_*() to request a read/write and then can perform any number of operations (other than waiting) before calling sftp_aio_wait_*() for waiting for the response of the previously issued request.

We call read/write operations performed in the manner described above as "asynchronous" read/write operations on a remote file.

Listing the contents of a directory

The functions sftp_opendir(), sftp_readdir(), sftp_dir_eof(), and sftp_closedir() enable to list the contents of a directory. They use a new handle_type, "sftp_dir", which gives access to the directory being read.

In addition, sftp_readdir() returns a "sftp_attributes" which is a pointer to a structure with information about a directory entry:

• name: the name of the file or directory
• size: its size in bytes
• etc.

sftp_readdir() might return NULL under two conditions:

• when the end of the directory has been met
• when an error occurred

To tell the difference, call sftp_dir_eof().

The attributes must be freed with sftp_attributes_free() when no longer needed.

The following example reads the contents of some remote directory:

int sftp_list_dir(ssh_session session, sftp_session sftp)
{
  sftp_dir dir;
  sftp_attributes attributes;
  int rc;
 
  dir = sftp_opendir(sftp, "/var/log");
  if (!dir)
  {
    fprintf(stderr, "Directory not opened: %s\n",
            ssh_get_error(session));
    return SSH_ERROR;
  }
 
  printf("Name                       Size Perms    Owner\tGroup\n");
 
  while ((attributes = sftp_readdir(sftp, dir)) != NULL)
  {
    printf("%-20s %10llu %.8o %s(%d)\t%s(%d)\n",
     attributes->name,
     (long long unsigned int) attributes->size,
     attributes->permissions,
     attributes->owner,
     attributes->uid,
     attributes->group,
     attributes->gid);
 
     sftp_attributes_free(attributes);
  }
 
  if (!sftp_dir_eof(dir))
  {
    fprintf(stderr, "Can't list directory: %s\n",
            ssh_get_error(session));
    sftp_closedir(dir);
    return SSH_ERROR;
  }
 
  rc = sftp_closedir(dir);
  if (rc != SSH_OK)
  {
    fprintf(stderr, "Can't close directory: %s\n",
            ssh_get_error(session));
    return rc;
  }
}