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ENOSTR
This page examines UCRT compatibility status: object is not a stream. The CRT compatibility layer rejected an object for an operation requiring stream semantics.
The returning interface matters
Microsoft C runtime compatibility: Ucrt compatibility status: object is not a stream is exposed by the Microsoft CRT compatibility layer. _doserrno and the immediately preceding CRT call can preserve the Win32 or runtime condition summarized by errno. The wording attached to ENOSTR is a clue to the original interface, not proof that a similarly named modern subsystem returned it.
Microsoft documents that some errno constants exist for UNIX compatibility and that errno is not necessarily the operating-system error code. Preserve both layers during diagnosis.
Incident record
| Evidence | Question it answers |
|---|---|
| CRT function, descriptor provenance, mode, errno/_doserrno, and object type. | Ties this error to one live API object instead of a translated message. |
| Thread-local errno captured before another CRT call changes it. | Shows whether this error arose in the caller ABI, wrapper, or provider. |
| Paired Win32 handle/socket state and the actual CRT descriptor conversion. | Preserves the first boundary before errno can be overwritten. |
| Known-good call using the same UCRT build and calling convention. | Provides a same-platform known-good comparison. |
Capture errno immediately after the failing call that produced this error before formatting, tracing, allocation, or cleanup code runs. Retain the function return value as well because many C interfaces signal failure there and use errno only as secondary detail.
Controlled comparisons
- Open the intended stream-capable object using the matching CRT API. A changed errno after this test is a new boundary, not another spelling of this error.
- Log errno with _get_errno and _doserrno at the immediate failure site. Repeat the original operation after the control so the result remains relevant to this error.
- Verify the numeric value against the target UCRT header rather than a Unix table. Keep the binary, header ABI, and provider fixed.
Two observations are especially discriminating here: CRT function, descriptor provenance, mode, errno/_doserrno, and object type. Use this control for comparison: open the intended stream-capable object using the matching CRT API. Those two observations separate a stale identifier or unsupported request from a provider-wide failure without broad changes to the host.
Nearby interpretations
| Tempting assumption | Safer interpretation |
|---|---|
| The English phrase is assumed to be portable | this error belongs to Microsoft C runtime compatibility; verify the target headers and returning API before applying another platform's errno table |
| A retry succeeded and erased the original evidence | The retry can replace this error with a different state; preserve the first call, numeric value, and object generation beforehand |
If a wrapper converts this error to an exception or Boolean, instrument below that wrapper. The decisive record is the syscall, ioctl, CRT function, or compatibility entry point that first exposed the value.
Resolution criteria
Targeted correction. Validate descriptor/mode before the call and avoid mixing Win32 handles with CRT descriptors.
Regression proof. Correct descriptors work and mismatched handles are rejected locally. Verify one supported boundary case and one deliberate negative case so handling of this error remains explicit.
Technical references
References for Microsoft C runtime compatibility ENOSTR on the deployed platform version.
- Microsoft: errno constants — defines the status namespace used.
- Microsoft: _get_errno — documents the API or lifecycle boundary behind this error.
- Microsoft: _doserrno and errno — provides ABI, implementation, or protocol context.
- The Open Group: errno.h — supports the portability and verification limits.
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