When should I use pragma pack?

PinoyAko · 05-23-2011, 05:27 AM

When should I use pragma pack?

I found this code on some source files while other sources do not have the pragma pack()

Code:

#pragma pack (1)
typedef struct _net_ip_header_t {
	uint8 hdr_len:4;		// header length
	uint8 version:4;		// version
	uint8 tos;				// type of service
	uint16 tot_len;			// total length
	uint16 id;				// identification
	uint16 flags_offset;	// fragment offset field
	uint8 ttl;				// time to live
	uint8 proto;			// protocol
	uint16 cksum;			// checksum
	ip_addr src_ip;			// source ip address
	ip_addr dst_ip;			// destination ip address
} net_ip_header_t;
#pragma pack ()

dwhitney67 · 05-23-2011, 05:31 AM

The use of pragma pack is to ensure that the data structure is byte-aligned versus word-aligned. For example, the following struct takes up 8 bytes (perhaps not on all systems), but on typical Linux machines:

Code:

struct Foo
{
   int  val;   /* takes up 4 bytes */
   char ch;    /* takes up 1 byte  */
};

/* Yet struct Foo takes up 8 bytes */

With the pragma pack directive, the struct above will only take up 5-bytes.