---
title: log
description: Outputs debug/diagnostic information to the console
source_url:
  html: https://thru.org/docs/spec/vm/syscalls/log/
  md: https://thru.org/docs/spec/vm/syscalls/log.md
---

# log

## Overview

The `log` syscall outputs data to the console for debugging and diagnostic purposes. It prints the specified data directly to stdout.

## Syscall Code

**Code:** `0x0C` (`TN_SYSCALL_CODE_LOG`)

## C SDK Function

```c
ulong tsys_log(void const* data, ulong data_len);
```

## Arguments

**`data` · *void const** · **required***\*

Pointer to the data to be logged.

**`data_len` · *ulong* · **required****

Length of the data to log in bytes. Must not exceed the maximum log data size.

## Return Value

Returns a syscall result code:

**`Success` · *ulong***

- `TN_VM_SYSCALL_SUCCESS` (0) - Data logged successfully

**`Error Codes` · *ulong***

- `TN_VM_ERR_SYSCALL_LOG_DATA_TOO_LARGE` (-30) - Data exceeds maximum log size
- `TN_VM_ERR_SYSCALL_INVALID_ADDRESS` (-22) - Invalid virtual address for data

## Resource Consumption

### Compute Units

- **Base cost**: None (log syscalls don’t charge compute units)
- **Total cost**: 0 units
- **Rationale**: Logging is considered a debugging aid and is not charged

### Memory Pages

- **Page usage**: No page allocation
- **I/O overhead**: Minimal system overhead for stdout operations
- **Buffer management**: Uses system stdout buffer

## Side Effects

- **Console output**: Writes data directly to stdout
- **Buffer flushing**: Flushes stdout buffer after writing

## Size Limitations

- Maximum log data size: `TN_VM_LOG_DATA_MAX_SIZE` (1024 bytes)
- Attempts to log more data will result in an error
- No truncation is performed - the entire operation fails

## Output Behavior

- Data is output byte-for-byte to stdout
- No formatting, newlines, or prefixes are added
- Output is immediately flushed to ensure visibility
- Binary data is output as-is (may include non-printable characters)

## Usage Notes

- Intended for debugging and diagnostic purposes only
- Should not be used for production data exchange
- Performance impact is minimal but output is synchronous
- Consider the security implications of logging sensitive data

> **Caution:**
>
> The log syscall outputs raw data to stdout. Be careful not to log sensitive information such as private keys, authentication tokens, or user data in production environments.

## Example

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

// Log a simple debug message
char debug_msg[] = "Debug: Account balance updated\n";
ulong result = tsys_log(debug_msg, strlen(debug_msg));

// Log binary data (e.g., account hash)
uchar account_hash[32];
get_account_hash(account_hash);
result = tsys_log(account_hash, 32);

// Log formatted data
char buffer[256];
int len = snprintf(buffer, sizeof(buffer), "Account %lu: balance = %llu\n",
                   account_id, balance);
if (len > 0 && len < sizeof(buffer)) {
    result = tsys_log(buffer, len);
}
```