Tcl

Write Text at a 32-bit Offset in a Remote SFTP File

Demonstrates the Chilkat SFtp.WriteFileText32 method, which encodes text and writes it at a specific 32-bit byte offset in an open handle. The arguments are the handle, the offset, the charset, and the text.

Background: Explicit-offset writes enable random access — patching a fixed-length record in place, for instance, without rewriting the whole file. The offset is a byte position, not a character position, so with a variable-width encoding like UTF-8 you must account for the encoded size of preceding characters to land where you intend. After the write, the sequential position continues just past the bytes written.

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Tcl


load ./chilkat.dll

set success 0

#  Demonstrates the SFtp.WriteFileText32 method, which encodes text and writes it at a specific
#  32-bit byte offset in an open handle.  The 1st argument is the handle, the 2nd is the offset,
#  the 3rd is the charset, and the 4th is the text.

set sftp [new_CkSFtp]

#  Connect, authenticate, and initialize the SFTP subsystem.
set port 22
set success [CkSFtp_Connect $sftp "sftp.example.com" $port]
if {$success == 0} then {
    puts [CkSFtp_lastErrorText $sftp]
    delete_CkSFtp $sftp
    exit
}

#  Normally you would not hard-code the password in source.  You should instead obtain it
#  from an interactive prompt, environment variable, or a secrets vault.
set password "mySshPassword"

set success [CkSFtp_AuthenticatePw $sftp "mySshLogin" $password]
if {$success == 0} then {
    puts [CkSFtp_lastErrorText $sftp]
    delete_CkSFtp $sftp
    exit
}

set success [CkSFtp_InitializeSftp $sftp]
if {$success == 0} then {
    puts [CkSFtp_lastErrorText $sftp]
    delete_CkSFtp $sftp
    exit
}

#  Open the remote file for read/write so an existing file can be modified in place.
set handle [CkSFtp_openFile $sftp "subdir/record.dat" "readWrite" "openOrCreate"]
if {[CkSFtp_get_LastMethodSuccess $sftp] == 0} then {
    puts [CkSFtp_lastErrorText $sftp]
    delete_CkSFtp $sftp
    exit
}

#  Overwrite 5 bytes starting at offset 10.  The offset is a byte position, not a character
#  position, so it must account for the encoded size of any multibyte characters.
set offset 10
set success [CkSFtp_WriteFileText32 $sftp $handle $offset "utf-8" "HELLO"]
if {$success == 0} then {
    puts [CkSFtp_lastErrorText $sftp]
    delete_CkSFtp $sftp
    exit
}

set success [CkSFtp_CloseHandle $sftp $handle]
if {$success == 0} then {
    puts [CkSFtp_lastErrorText $sftp]
    delete_CkSFtp $sftp
    exit
}

puts "Wrote at the specified offset."

CkSFtp_Disconnect $sftp

delete_CkSFtp $sftp