frewind — Rewind the file position indicator to the beginning of an open file.
frewind(fid) moves the file position indicator back to the beginning of the file associated with fid. It is equivalent to calling fseek(fid, 0, 'bof') and is commonly used to re-read a file after an initial pass without closing and reopening it.
How frewind works in RunMat
frewind(fid)rewinds the file position indicator to byte offset 0 for the file identified byfid.fidmust be a non-negative integer returned byfopen. Passing an invalid identifier raises an error.- Standard input/output identifiers (
0,1,2) are silently accepted and treated as a no-op; RunMat does not support seeking on those streams. frewindproduces no output value. Assigning the result offrewindto a variable yields an empty output list.- After
frewind, subsequent calls tofread,fgets, orfscanfstart from the beginning of the file.
How frewind runs on the GPU
frewind is a host-only operation. If fid resides on the GPU (for example inside a gpuArray), RunMat gathers the scalar to host memory before consulting the file registry. File handles always remain on the CPU, so there are no provider hooks or fusion paths for this builtin.
GPU memory and residency
No. File identifiers are always owned by the host-side registry maintained by fopen. Wrapping a scalar identifier in gpuArray does not keep the file handle on the GPU—the runtime gathers the scalar before performing the seek. The rewind operation itself executes entirely on the host.
Examples
Write then rewind and read back binary data
fid = fopen('data.bin', 'w+b');
fwrite(fid, [1 2 3], 'double');
frewind(fid);
values = fread(fid, 'double');
fclose(fid);
delete('data.bin')Read a text file twice without reopening
fid = fopen('notes.txt', 'r');
first_line = fgets(fid);
frewind(fid);
same_line = fgets(fid);
fclose(fid)Rewind a binary file opened for reading bytes
fid = fopen('payload.bin', 'w+b');
fwrite(fid, uint8(1:6), 'uint8');
frewind(fid);
bytes = fread(fid, 4, 'uint8');
fclose(fid);
delete('payload.bin')FAQ
What kind of input does frewind accept?
Pass a scalar numeric file identifier returned by fopen. The identifier must be a non-negative integer. Character vectors and strings are not accepted.
Does frewind return a value?
No. frewind is a void function and produces no output. Capturing its result assigns an empty output list.
How does frewind compare to fseek?
frewind(fid) is exactly equivalent to fseek(fid, 0, 'bof'). Use frewind when you only ever need to return to the beginning; use fseek when you need to jump to arbitrary positions.
Can I call frewind on a write-only file?
Yes, as long as the underlying file was opened with a mode that supports seeking (e.g. 'w+' or 'w+b'). Rewinding a write-only stream positions the cursor at the start so subsequent writes overwrite from the beginning.
Can I call frewind on a closed file?
No. Passing a closed or never-opened identifier raises "Invalid file identifier. Use fopen to generate a valid file ID."
Related functions to explore
These functions work well alongside frewind. Each page has runnable examples you can try in the browser.
fopen, fclose, fread, fwrite, feof, fgets, fileread, filewrite, fprintf
Open-source implementation
Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how frewind works, line by line, in Rust.
- View frewind.rs on GitHub
- Learn how the runtime works
- Found a bug? Open an issue with a minimal reproduction.
About RunMat
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