Tcl eof Command

last modified April 3, 2025

The Tcl eof command checks if an end-of-file condition has occurred on a channel. It's essential for file and stream processing in Tcl. The command returns 1 if EOF is reached, 0 otherwise.

Basic Definition

The eof command tests whether a channel has reached its end. It's commonly used with file operations and socket communications. The command takes a channel identifier as its argument.

Syntax: eof channelId. The channelId must be an open channel like a file handle or socket. The command returns a boolean value indicating EOF status.

Basic File EOF Check

This example demonstrates checking EOF while reading a file line by line.

basic_eof.tcl

set file [open "data.txt" r]
while {![eof $file]} {
    gets $file line
    puts $line
}
close $file

This opens a file and reads it line by line until EOF. The eof command checks the condition before each read. Note that gets might return an empty string at EOF.

EOF with Read Operations

This example shows how EOF works with direct read operations.

eof_read.tcl

set file [open "data.bin" r]
set data [read $file]
if {[eof $file]} {
    puts "Reached end of file"
}
close $file

Here we read the entire file content at once. After reading, we check if we've reached EOF. With read, EOF is typically reached after the operation.

EOF in Network Communication

The eof command is also useful for detecting closed network connections.

eof_socket.tcl

set server [socket -server accept 12345]
proc accept {chan addr port} {
    if {[eof $chan]} {
        puts "Connection closed by client"
        close $chan
        return
    }
    puts [gets $chan]
}
vwait forever

This creates a simple server that checks for EOF on incoming connections. When a client disconnects, EOF is detected and the channel is closed. This prevents hanging on dead connections.

EOF with Standard Input

The eof command can check for EOF on standard input.

eof_stdin.tcl

puts "Enter text (Ctrl+D to end):"
while {![eof stdin]} {
    gets stdin line
    if {![eof stdin]} {
        puts "You entered: $line"
    }
}

This script reads from standard input until EOF (Ctrl+D in Unix, Ctrl+Z in Windows). We check EOF after gets to avoid processing empty lines at EOF. This pattern is common for interactive input.

EOF in Binary File Processing

When processing binary files, EOF checks are crucial for reading fixed-size blocks.

eof_binary.tcl

set file [open "image.jpg" rb]
while {![eof $file]} {
    set chunk [read $file 1024]
    puts "Read [string length $chunk] bytes"
    # Process chunk here
}
close $file

This reads a binary file in 1024-byte chunks until EOF. The eof check ensures we stop when the file ends. The last chunk might be smaller than 1024 bytes.

EOF with Multiple Channels

This example demonstrates checking EOF on multiple channels simultaneously.

eof_multiple.tcl

set file1 [open "file1.txt" r]
set file2 [open "file2.txt" r]

while {1} {
    if {[eof $file1] && [eof $file2]} {
        break
    }
    if {![eof $file1]} { gets $file1 line1 }
    if {![eof $file2]} { gets $file2 line2 }
    # Process lines
}
close $file1
close $file2

This script reads from two files until both reach EOF. We check each file's EOF status separately. This pattern is useful for merging or comparing files.

Best Practices

Check after read: Verify EOF after read operations for accuracy.
Handle empty reads: EOF might coincide with empty data.
Close channels: Always close channels after detecting EOF.
Error handling: Combine EOF checks with error handling.
Buffering: Be aware of buffering effects on EOF detection.

This tutorial covered the Tcl eof command with practical examples showing its usage in file processing, network communication, and input handling.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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