filewrite — Write text or raw bytes to files in MATLAB and RunMat.

filewrite(filename, data) writes text or raw byte content to a file and returns the number of bytes written. By default it overwrites existing content; use 'WriteMode','append' to append. Encoding options and input coercions follow MATLAB semantics.

Syntax

count = filewrite(filename, data)
count = filewrite(filename, data, encoding)
count = filewrite(filename, data, "Encoding", encoding)
count = filewrite(filename, data, ..., "WriteMode", mode)

Inputs

Returns

Errors

How filewrite works

• Accepts file names supplied as character vectors, string scalars, or scalar string arrays.
• Accepts data supplied as character vectors, string scalars, string arrays, or numeric arrays of bytes (uint8/double values in the range 0–255). Logical arrays map to bytes 0 and 1.
• By default the file is truncated (overwrite mode). Pass 'WriteMode','append' to add to an existing file.
• Encoding is optional. Provide either a positional encoding argument or the 'Encoding', value keyword pair. Supported encodings mirror fileread: auto (default), utf-8, ascii, latin1, and raw.
• When writing text data with filewrite and Encoding set to ascii or latin1, an error is raised if any characters fall outside the permitted code page.
• When writing raw numeric bytes, encoding is ignored except to validate ASCII requests.
• Returns the number of bytes written as a double scalar. Ignoring the output behaves like MATLAB (data is still written).

Does RunMat run filewrite on the GPU?

filewrite performs synchronous host file I/O. If either the path or data reside on the GPU (for example through lazy residency), RunMat gathers those values before performing the write. No GPU kernels are launched and providers do not need to implement specialised hooks for this builtin.

Examples

Write Text To A New File

bytes = filewrite("notes.txt", "Hello, RunMat!")

Expected output:

bytes = 14

Append Text Without Overwriting

filewrite("log.txt", "First line\n");
bytes = filewrite("log.txt", "Second line\n", 'WriteMode', 'append')

Expected output:

bytes = 13

Specify A Particular Encoding

bytes = filewrite("latin1.txt", ['E' 's' 'p' 'a' char(241) 'a'], 'Encoding', 'latin1')

Expected output:

bytes = 6

Write Raw Bytes From A Numeric Array

payload = [1 2 3 255];
bytes = filewrite("data.bin", payload, 'Encoding', 'raw')

Expected output:

bytes = 4

Export A String Array As Newline-Separated Text

lines = ["alpha", "beta", "gamma"];
bytes = filewrite("items.txt", lines)

Expected output:

bytes = 16

Handle Invalid ASCII Characters

try
    filewrite("ascii.txt", ['c' 'a' 'f' char(233)], 'Encoding', 'ascii');
catch err
    disp(err.message);
end

Expected output:

filewrite: character 'é' (U+00E9) cannot be encoded as ASCII

Using filewrite with coding agents

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

FAQ

Related Io functions

Open-source implementation

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

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

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