LinuxQuestions.org - When should I use pragma pack?

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When should I use pragma pack?

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 ()

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.