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Use os.ReadFile when the complete file fits comfortably in memory, os.WriteFile to write a byte slice, and File.Read with a reusable buffer when you want to process data in chunks. With chunked reads, only buf[:n] contains bytes returned by that call. Use io.ReadFull when a record must have an exact size.
How Go slices work as file buffers
A Go slice describes a region of an underlying array. Assigning a slice to another slice does not automatically copy its bytes; both can refer to the same array. The slice passed to a read also specifies the maximum number of bytes the operation can place in the buffer: the Read method has the signature Read(buf []byte) (n int, err error). See the Go Authors’ Effective Go discussion of slices.
For a read, the returned count n is essential. It tells you how many bytes from the beginning of the buffer were filled. The rest of the slice may contain old or otherwise irrelevant data, so process buf[:n], not all of buf.
Read a complete file into a byte slice
For a small or otherwise bounded file whose complete contents can fit in memory, os.ReadFile is the straightforward option:
data, err := os.ReadFile("input.bin")
if err != nil {
return err
}
// data contains the complete file.On success, err is nil; a successful whole-file read does not return io.EOF. If the operation fails, the error is a *PathError. The returned []byte can be inspected or modified. See the Go os.ReadFile documentation.
This convenience comes with a memory trade-off: the complete contents are returned as a slice. For a very large file or an unbounded upload, reading everything at once may use more memory than the application can afford. Use a bounded buffer and process chunks instead.
Write a byte slice to a file
Use os.WriteFile to create or replace a file from a []byte:
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if err := os.WriteFile("output.bin", data, 0o644); err != nil {
return err
}If the file does not exist, os.WriteFile creates it with the requested permission bits, subject to the process umask. If it already exists, the function truncates it before writing and does not change its permissions. The Go documentation also warns that a failure partway through the multiple system calls may leave a partially written file. See os.WriteFile.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallIf readers must not see a partially replaced file, a common pattern is to write a temporary file, confirm writing and closing it succeed, and then rename it into place. Whether that replacement is atomic depends on the filesystem and platform; os.WriteFile itself does not promise an all-or-nothing update.
Read sequentially with a reusable slice
File.Read reads up to the buffer’s length and returns the number of bytes read. At end of file it returns 0, io.EOF. A loop should process any bytes returned before handling the error, because a read can return both a positive count and an error.
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f, err := os.Open("input.bin")
if err != nil {
return err
}
defer f.Close()
buf := make([]byte, 32*1024)
for {
n, err := f.Read(buf)
if n > 0 {
consume(buf[:n])
}
if err == io.EOF {
break
}
if err != nil {
return err
}
}The 32 KiB buffer in this example is a chosen buffer size, not a guarantee about how much any individual read returns. A read may return fewer bytes than requested. The Go Authors document the contract as: “Read reads up to len(b) bytes from the File and stores them in b.” See os.File.Read.
Because the buffer is reused, a consumer that needs to retain a chunk after the next read must copy it; retaining a slice of buf otherwise means retaining a view of the same backing array that subsequent reads overwrite.
Read an exact number of bytes
A single Read is not a request for an exact-length result. When a file format requires a complete fixed-size header or record, use io.ReadFull:
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header := make([]byte, 16)
if _, err := io.ReadFull(f, header); err != nil {
return err // io.ErrUnexpectedEOF indicates a partial header
}io.ReadFull keeps reading until the buffer is full or an error prevents it. If the input ends after some but not all bytes have arrived, it returns io.ErrUnexpectedEOF; its count equals the buffer length exactly when the error is nil. See io.ReadFull.
Read at an offset
Use ReadAt when the access pattern calls for a specific offset rather than the current sequential position:
chunk := make([]byte, 4096)
n, err := f.ReadAt(chunk, 1<<20)
if err != nil && err != io.EOF {
return err
}
process(chunk[:n])ReadAt begins at the supplied offset and reports an error if it reads fewer bytes than the requested buffer length. If the data format requires the entire chunk, check that the count is the expected length and treat a short read as incomplete, or use io.ReadFull with a suitable section reader. See os.File.ReadAt.
Best Value
Read all bytes from a reader that is not a file
io.ReadAll reads from an io.Reader until EOF and returns the bytes as a slice. It is useful when the input is a reader rather than a filename, but it still accumulates all data in memory; it is not a memory-bounded streaming alternative for unbounded input. The standard library’s filesystem implementation illustrates a growing slice by repeatedly reading into the unused capacity. See io.ReadAll.
For code that accepts an io.Reader, bytes.NewReader lets a test use an existing slice as a read-only, seekable input without creating a temporary file:
r := bytes.NewReader(input)
copy, err := io.ReadAll(r)
if err != nil {
return err
}The resulting *bytes.Reader implements io.Reader, io.ReaderAt, and related interfaces. See bytes.NewReader.
Choose the read or write pattern that fits
| Pattern | Best fit | Key behavior |
|---|---|---|
os.ReadFile |
Read a bounded file completely | Returns the full file in a byte slice; whole contents occupy memory. |
File.Read with a reusable slice |
Process sequential input in chunks | Returns a count that may be smaller than the buffer; consume only buf[:n]. |
io.ReadFull |
Require an exact-size header or record | Fills the requested slice or reports an error; a truncated input yields io.ErrUnexpectedEOF. |
File.ReadAt |
Read from a particular file offset | Starts at the specified offset and reports a short-read error when fewer than the requested bytes are read. |
io.ReadAll |
Read an entire stream exposed as an io.Reader |
Reads until EOF and accumulates all bytes in memory. |
os.WriteFile |
Create or truncate a file from a byte slice | Existing contents are truncated; a failure can leave a partial write. |
Use the current standard-library names
For new code, use os.ReadFile and os.WriteFile for whole-file operations, and io.ReadAll for reading a complete stream. The older ioutil.ReadFile and ioutil.WriteFile names are deprecated aliases; see the Go documentation for ioutil.ReadFile and ioutil.WriteFile.
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