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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.
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. |
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.
Nearby interpretations
| Tempting assumption | Safer interpretation |
|---|---|
| The English phrase is assumed to be portable | ENOSTR 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 | A retry can replace the original errno with a different state; preserve the first call, numeric value, and object generation beforehand |
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.
Technical references
- 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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