---
title: increment_anonymous_segment_sz
description: Increments or decrements the size of an anonymous memory segment by
  a delta value
source_url:
  html: https://thru.org/docs/spec/vm/syscalls/increment_anonymous_segment_sz/
  md: https://thru.org/docs/spec/vm/syscalls/increment_anonymous_segment_sz.md
---

# increment_anonymous_segment_sz

## Overview

The `increment_anonymous_segment_sz` syscall modifies the size of an anonymous memory segment by adding or subtracting a delta value. This is useful for dynamic memory allocation and deallocation.

## Syscall Code

**Code:** `0x01` (`TN_SYSCALL_CODE_INCREMENT_ANONYMOUS_SEGMENT_SZ`)

## C SDK Function

```c
ulong tsys_increment_anonymous_segment_sz(void* segment_addr, ulong delta, void** addr);
```

## Arguments

**`segment_addr` · *void** · **required***\*

Virtual address that identifies the anonymous segment to modify. The segment type and index are extracted from this address.

**`delta` · *ulong* · **required****

Delta value to add to the current segment size. Can be positive (expand) or negative (shrink). If zero, returns the current segment boundary address without modification.

**`addr` · *void*** · **required**\*\*

Pointer to store the resulting segment boundary address.

## Return Value

Returns a syscall result code:

**`Success` · *ulong***

- `TN_VM_SYSCALL_SUCCESS` (0) - Operation completed successfully

**`Error Codes` · *ulong***

- `TN_VM_ERR_SYSCALL_INVALID_SEGMENT_ID` (-21) - Invalid segment type or index
- `TN_VM_ERR_SYSCALL_INVALID_SEGMENT_SIZE` (-28) - Size overflow/underflow detected
- `TN_VM_ERR_SYSCALL_INSUFFICIENT_PAGES` (-26) - Not enough free pages for expansion
- `TN_VM_ERR_SYSCALL_UNFREEABLE_PAGE` (-29) - Cannot free pages when shrinking

## Resource Consumption

### Compute Units

- **Base cost**: `TN_VM_SYSCALL_BASE_COST` (512 units)

- **Delta cost**: Additional units based on delta value

  - **Positive delta**: Additional units equal to the delta value
  - **Negative delta**: No additional cost beyond base
  - **Zero delta**: No additional cost (query operation)

- **Total formula**: `base_cost + max(0, delta)`

### Memory Pages

- **Page allocation**: New pages allocated from transaction pool when expanding
- **Page deallocation**: Freed pages returned to transaction pool when shrinking
- **Page constraints**: Pages can only be freed if allocated in current or later call frames

## Side Effects

- **Memory allocation/deallocation**: Pages allocated when expanding or freed when shrinking
- **Output parameter**: Sets the addr parameter to the segment boundary address
- **Segment state**: Updates segment size and page mappings

## Usage Notes

- When delta is 0, returns current segment size without modification
- For positive delta: allocates new pages and returns old boundary
- For negative delta: frees pages and returns new boundary
- Overflow/underflow protection prevents invalid size calculations
- Only works with anonymous data segments (heap and stack types)

## Example

```c
#include <thru-sdk/c/tn_sdk_syscall.h>

// Increase heap segment by 4096 bytes
void* segment_addr = TSDK_ADDR(TSDK_SEG_TYPE_HEAP, 0, 0);
void* boundary_addr;
ulong result = tsys_increment_anonymous_segment_sz(segment_addr, 4096, &boundary_addr);

if (result == TN_VM_SYSCALL_SUCCESS) {
    // boundary_addr now contains the address of the old segment boundary
    // Segment has been expanded by 4096 bytes
}

// Check current size without modification
result = tsys_increment_anonymous_segment_sz(segment_addr, 0, &boundary_addr);
// boundary_addr contains current segment boundary address
```