fclose — Close file identifiers in MATLAB and RunMat.

fclose closes files previously opened with fopen. Pass a file identifier, a vector of identifiers, or 'all' (or no arguments) to close handles. Status and optional message outputs, including invalid-identifier behavior, follow MATLAB semantics.

Syntax

status = fclose()
status = fclose(fid)
status = fclose("all")
[status, msg] = fclose(...)

Inputs

Returns

Returned values from fclose depend on how many outputs the caller requests.

Errors

How fclose works

• status = fclose(fid) closes the file represented by the numeric identifier fid. The status is 0 on success and -1 if the identifier is invalid.
• status = fclose([fid1 fid2 ...]) closes a vector of identifiers. If any identifier is invalid, status is -1 and message explains the failure.
• status = fclose('all') (or fclose() with no arguments) closes every open file except the standard streams (0, 1, 2).
• [status, message] = fclose(...) returns the diagnostic message. On success, message is an empty character vector.
• Identifiers 0, 1, and 2 refer to standard input, output, and error. fclose treats them as already-open handles and simply returns success.
• RunMat keeps file metadata in a registry shared with fopen, ensuring MATLAB-compatible behaviour across subsequent I/O builtins.

Does RunMat run fclose on the GPU?

fclose does not perform GPU computation. If the argument resides on a GPU array (for example, 'all' stored in a gpuArray), RunMat gathers the value to host memory before dispatching the host-only close logic.

Examples

Close a file after writing data

[fid, msg] = fopen('results.txt', 'w');
if fid == -1
    error('Failed to open file: %s', msg);
end
fprintf(fid, 'Simulation complete\n');
status = fclose(fid)

Expected output:

status = 0

Close all open files at once

% Close every file except stdin/stdout/stderr
status = fclose('all')

Expected output:

status = 0

Handle invalid file identifiers gracefully

[status, message] = fclose(9999);
if status == -1
    fprintf('Close failed: %s\n', message);
end

Expected output:

status = -1;
message = 'Invalid file identifier. Use fopen to generate a valid file ID.'

Close multiple file identifiers together

fids = fopen('all');
status = fclose(fids)

Expected output:

status = 0

Detect failures with the second output

[fid, msg] = fopen('data.bin', 'r');
assert(fid ~= -1, msg);
fclose(fid);
[status, message] = fclose(fid);  % closes again, returns -1 and an error string

Expected output:

status = -1;
message = 'Invalid file identifier. Use fopen to generate a valid file ID.'

Close files using the no-argument form

% Equivalent to fclose('all')
status = fclose()

Expected output:

status = 0

Using fclose with coding agents

Open a RunMat example with live inputs, then ask the agent to explain how fclose changes the result.

FAQ

Related Io functions

Open-source implementation

Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how fclose is executed, line by line, in Rust.

• View the source for fclose in Rust on GitHub
• Learn how the RunMat runtime works
• Found a bug? Open an issue with a minimal reproduction.

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