---
title: "How big buffer should I supply to the DHCP_init() function?"
url: "https://maker.wiznet.io/forum/10920"
markdown_url: "https://maker.wiznet.io/forum/10920/md"
type: "Forum topic"
category: "Ethernet Chips"
author: "Eerik"
created: "2015-03-19T12:37:23+09:00"
last_activity: "2017-05-04T08:35:04+09:00"
language: "en"
views: 1351
replies: 1
source: "WIZnet Makers forum (https://maker.wiznet.io/forum/)"
---

# How big buffer should I supply to the DHCP_init() function?

## Question

Asked by Eerik on 2015-03-19 in Ethernet Chips.

How big buffer should I supply to the DHCP_init() function?

I can see in the driver the following:

```
void DHCP_init(uint8_t s, uint8_t * buf)
{
   uint8_t zeroip[4] = {0,0,0,0};
   getSHAR(DHCP_CHADDR);
   if((DHCP_CHADDR[0] | DHCP_CHADDR[1]  | DHCP_CHADDR[2] | DHCP_CHADDR[3] | DHCP_CHADDR[4] | DHCP_CHADDR[5]) == 0x00)
   {
      // assing temporary mac address, you should be set SHAR before call this function. 
      DHCP_CHADDR[0] = 0x00;
      DHCP_CHADDR[1] = 0x08;
      DHCP_CHADDR[2] = 0xdc;      
      DHCP_CHADDR[3] = 0x00;
      DHCP_CHADDR[4] = 0x00;
      DHCP_CHADDR[5] = 0x00; 
      setSHAR(DHCP_CHADDR);     
   }

	DHCP_SOCKET = s; // SOCK_DHCP
	pDHCPMSG = (RIP_MSG*)buf;
	DHCP_XID = 0x12345678;

	// WIZchip Netinfo Clear
	setSIPR(zeroip);
	setSIPR(zeroip);
	setGAR(zeroip);

	reset_DHCP_timeout();
	dhcp_state = STATE_DHCP_INIT;
}
```

and

```
/*
 * @brief DHCP message format
 */ 
typedef struct {
	uint8_t  op;            ///< @ref DHCP_BOOTREQUEST or @ref DHCP_BOOTREPLY
	uint8_t  htype;         ///< @ref DHCP_HTYPE10MB or @ref DHCP_HTYPE100MB
	uint8_t  hlen;          ///< @ref DHCP_HLENETHERNET
	uint8_t  hops;          ///< @ref DHCP_HOPS
	uint32_t xid;           ///< @ref DHCP_XID  This increase one every DHCP transaction.
	uint16_t secs;          ///< @ref DHCP_SECS
	uint16_t flags;         ///< @ref DHCP_FLAGSBROADCAST or @ref DHCP_FLAGSUNICAST
	uint8_t  ciaddr[4];     ///< @ref Request IP to DHCP sever
	uint8_t  yiaddr[4];     ///< @ref Offered IP from DHCP server
	uint8_t  siaddr[4];     ///< No use 
	uint8_t  giaddr[4];     ///< No use
	uint8_t  chaddr[16];    ///< DHCP client 6bytes MAC address. Others is filled to zero
	uint8_t  sname[64];     ///< No use
	uint8_t  file[128];     ///< No use
	uint8_t  OPT[OPT_SIZE]; ///< Option
} RIP_MSG;
```

and

```
#define OPT_SIZE                 312               /// Max OPT size of @ref RIP_MSG
#define RIP_MSG_SIZE             (236+OPT_SIZE)    /// Max size of @ref RIP_MSG
```

So 548 byte would be ok?

## Replies

### Reply 1 by Coccoliso, 2015-03-19

1K is sufficient but keep in mind that after you’ve gotten the address you can easily reuse the socket then put the buffer size that you need to use in your application.

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Source: https://maker.wiznet.io/forum/10920