fopen — Open files and return file identifiers in MATLAB and RunMat.

fopen opens a file and returns a numeric file identifier (fid) used by other I/O builtins. Permission strings, machine-format specifiers, encoding handling, metadata query forms, and 'all' listing behavior follow MATLAB semantics.

Syntax

fid = fopen(filename)
fid = fopen(filename, permission)
fid = fopen(filename, permission, machinefmt)
fid = fopen(filename, permission, machinefmt, encoding)
[fid, msg] = fopen(filename, ...)
[fid, msg, machinefmt, encoding] = fopen(filename, ...)

fid = fopen(filename)
fid = fopen(filename, permission)
fid = fopen(filename, permission, machinefmt)
fid = fopen(filename, permission, machinefmt, encoding)
[fid, msg] = fopen(filename, ...)
[fid, msg, machinefmt, encoding] = fopen(filename, ...)
filename = fopen(fid)
[filename, permission, machinefmt, encoding] = fopen(fid)
fids = fopen("all")
fids = fopen("all", machinefmt)
[fids, names, machinefmts, encodings] = fopen("all")
[fids, names, machinefmts, encodings] = fopen("all", machinefmt)

Inputs

Returns

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

Errors

How fopen works

• fid = fopen(filename) opens an existing file for reading. The call fails (returning fid = -1) when the file cannot be opened.
• fid = fopen(filename, permission) opens a file using the requested permission string ('r', 'w', 'a', 'r+', 'w+', 'a+', plus optional 'b'/'t'). Permissions map to the same semantics as MATLAB: 'w' truncates, 'a' appends, and '+' enables reading and writing.
• fid = fopen(filename, permission, machinefmt, encoding) records the requested machine format ('native', 'ieee-le', 'ieee-be') and text encoding (defaults to UTF-8 in text mode, binary otherwise) for later inspection.
• [fid, message] = fopen(...) returns an empty character vector on success and the operating-system error message on failure.
• [filename, permission, machinefmt, encodingOut] = fopen(fid) queries an existing file identifier.
• [fid_list, names, machinefmts, encodings] = fopen('all') lists every open file (including standard input/output/error) and returns cell arrays containing metadata for each entry.
• [fid_list, names] = fopen('all', machinefmt) filters the listing to files whose recorded machine format (for example 'ieee-be' or 'native') matches the requested value.
• RunMat gathers GPU-resident filename arguments before opening files; the resulting handles always live on the host. Providers do not implement GPU kernels for file I/O.

Does RunMat run fopen on the GPU?

fopen always executes on the CPU. When a filename argument resides on a GPU array, RunMat gathers it to host memory before opening the file. File identifiers and their associated metadata are managed by a host-side registry that other builtins (such as fclose, fread, and fwrite) consult.

Examples

Open a File for Reading

[fid, message] = fopen('data/input.txt', 'r');
if fid == -1
    error('Failed to open file: %s', message);
end

Write Text to a New File

[fid, message] = fopen('logs/session.log', 'w');
if fid == -1
    error('Failed to create log file: %s', message);
end
fprintf(fid, 'Session started\n');
fclose(fid)

Append Binary Data

[fid, message] = fopen('signals.bin', 'ab+');
if fid == -1
    error('Failed to open binary log: %s', message);
end
fwrite(fid, rand(1, 1024), 'double');
fclose(fid)

Query File Metadata

fid = fopen('config.json');
[filename, permission, machinefmt, encoding] = fopen(fid);
disp(filename);
disp(permission);
fclose(fid)

List All Open Files

[fid_list, names] = fopen('all');
disp(fid_list);
disp(names)

Handle Missing Files Gracefully

[fid, message] = fopen('does_not_exist.txt', 'r');
if fid == -1
    fprintf('Failed to open file: %s\n', message);
end

Specify Machine Format and Encoding

[fid, message, machinefmt, encoding] = fopen('report.txt', 'w', 'ieee-le', 'latin1');
if fid == -1
    error('Failed to open report: %s', message);
end
fprintf(fid, 'olá mundo');
fclose(fid)

Using fopen with coding agents

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

FAQ

fid = fopen('data.txt', 'r');
cleanup = onCleanup(@() fclose(fid));
% ... use fid freely ...

Related Io functions

Open-source implementation

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

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

About RunMat

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