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Mar 28, 2025
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Update C4799 warning page #5272

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30 changes: 24 additions & 6 deletions docs/error-messages/compiler-warnings/compiler-warning-level-1-c4799.md
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---
description: "Learn more about: Compiler Warning (level 1) C4799"
title: "Compiler Warning (level 1) C4799"
ms.date: "11/04/2016"
description: "Learn more about: Compiler Warning (level 1) C4799"
ms.date: "03/28/2025"
f1_keywords: ["C4799"]
helpviewer_keywords: ["C4799"]
ms.assetid: 8ecbd06f-c778-4371-a2fb-c690b6743ec8
---
# Compiler Warning (level 1) C4799

> No EMMS at end of function '*function*'
> function '*function*' has no EMMS instruction

The function has at least one MMX instruction, but does not have an `EMMS` instruction. When a multimedia instruction is used, an `EMMS` instruction or `_mm_empty` intrinsic should also be used to clear the multimedia tag word at the end of the MMX code.
The function has at least one MMX instruction, but does not have an `EMMS` instruction. When a multimedia instruction is used, an `EMMS` instruction or [`_mm_empty`](https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#text=_mm_empty) intrinsic should also be used to clear the multimedia tag word at the end of the MMX code.

You may get C4799 when using ivec.h, indicating that the code does not use properly execute the EMMS instruction before returning. This is a false warning for these headers. You may turn these off by defining _SILENCE_IVEC_C4799 in ivec.h. However, be aware that this will also keep the compiler from giving correct warnings of this type.
You may get C4799 when using `ivec.h`, indicating that the code does not properly execute the `EMMS` instruction before returning. This is a false warning for these headers. You may turn these off by defining `_SILENCE_IVEC_C4799` in `ivec.h`. However, be aware that this will also keep the compiler from giving correct warnings of this type.

For related information, see [Intel's MMX Instruction Set](../../assembler/inline/intel-s-mmx-instruction-set.md).

## Example

The following example generates C4799:

```cpp
// C4799.cpp
// compile with: /W1
// processor: x86
#include <mmintrin.h>

int main()
{
__m64 m = _mm_setzero_si64();

// Uncomment the following line to resolve the warning:
// _mm_empty();
} // C4799
```
2 changes: 1 addition & 1 deletion docs/error-messages/compiler-warnings/compiler-warnings-by-compiler-version.md
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Expand Up @@ -953,7 +953,7 @@ These warnings, and all warnings in later versions, are suppressed by using the
| [C4792](compiler-warning-level-3-c4792.md) | `function 'function' declared using sysimport and referenced from native code; import library required to link` |
| [C4794](compiler-warning-level-1-c4794.md) | `segment of thread local storage variable 'name' changed from 'segment' to 'segment'` |
| C4798 | `native code generated for p-code function 'name' with exception handler or unwind semantics` |
| [C4799](compiler-warning-level-1-c4799.md) | `function 'name' has no EMMS instruction` |
| [C4799](compiler-warning-level-1-c4799.md) | `function 'function' has no EMMS instruction` |
| [C4803](compiler-warning-level-1-c4803.md) | `'declaration': the raise method has a different storage class from that of the event, 'declaration'` |
| [C4810](compiler-warning-level-1-c4810.md) | `value of pragma pack(show) == number` |
| [C4811](compiler-warning-level-1-c4811.md) | `value of pragma conform(forScope, show) == value` |
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2 changes: 1 addition & 1 deletion docs/error-messages/compiler-warnings/compiler-warnings-c4600-through-c4799.md
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Expand Up @@ -170,7 +170,7 @@ The articles in this section describe Microsoft C/C++ compiler warning messages
|[Compiler warning (level 1 and level 3) C4793](compiler-warning-level-1-and-3-c4793.md)|'*function*': function compiled as native:<br> *reason*|
|[Compiler warning (level 1) C4794](compiler-warning-level-1-c4794.md)|segment of thread local storage variable '%s' changed from '%s' to '%s'|
|Compiler warning C4798|native code generated for p-code function 'name' with exception handler or unwind semantics|
|[Compiler warning (level 1) C4799](compiler-warning-level-1-c4799.md)|function 'function' has no `EMMS` instruction|
|[Compiler warning (level 1) C4799](compiler-warning-level-1-c4799.md)|function '*function*' has no EMMS instruction|

## See also

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