[Zz] use of typedef struct, typedef union, and Union

(1) the well-known use of typedef improves code Portability:

// On the platform that supports long double, the most precise type is real <br/> typedef long double real; <br/> // if the highest precision of the other platform is float <br/> typedef float real; 

 

(2) Use of typedef struct:

Method 1

Struct MSG {<br/> byte index; <br/> int Len; <br/> char data [10]; <br/> }; <br/> struct msg a; <br/>. index = 0x1a; <br/> //... omitted 

Method 2

Typedef struct message {<br/> byte index; <br/> int Len; <br/> char data [10]; <br/>} MSG; // New Type <br/> MSG; <br/> MSG. index = 0x1a; 

Method 3

Struct MSG {<br/> byte index; <br/> int Len; <br/> char data [10]; <br/>}a; <br/>. index = 0x1a; 

 

(3) Use of union

Union intbyte <br/>{< br/> unsigned char byte [4]; <br/> int real; <br/> }; 

 

 

Reference from:

Http://lionwq.spaces.eepw.com.cn/articles/article/item/17197 #

 

Audio/Video Encoding/Decoding Algorithms often involve data compression, sound decoding, image scaling, and other issues.
Here we use an example to recommend a wonderful union usage (this method is provided by equator and used in our company's image processing algorithms ). In this example, the union structure n64u is used to implement the 8-byte n64 type and the single-byte C0 ~ To achieve data compression and decomposition.

 

# Include <stdio. h> <br/> # define in <br/> # define out <br/> # define inout <br/> typedef unsigned long n64; <br/> typedef unsigned int n32; <br/> typedef unsigned short n16; <br/> typedef unsigned char n8; </P> <p> typedef struct _ s8t {<br/> unsigned char C0, C1, C2, C3, C4, C5, C6, C7; <br/>} s8t; </P> <p>/* use n64 and C0 ~ C7 occupies the same memory zone to implement mutual conversion */<br/> typedef Union {<br/> n64 n64; <br/> struct {<br/> n32 l0, L1; <br/>}u32; <br/> struct {<br/> long l0, L1; <br/>}s32; <br/> struct {<br/> unsigned short S0, S1, S2, S3; <br/>}separate; <br/> struct {<br/> short S0, S1, S2, S3; <br/>}s16; <br/> struct {<br/> unsigned char C0, c1, C2, C3, C4, C5, C6, C7; <br/>}u8; <br/> struct {<br/> char C0, C1, C2, C3, c4, C5, C6, C7; <br/>} S8; <br/>} n64u; </P> <p> # define max_data_compressed_num 8 </P> <p> int compress64_8 (in const n64 * SRC, in n16 N, out n8 * DST ); <br/> int uncompress8_64 (in const n8 * SRC, in n16 N, out n64 * DST); </P> <p> int compress64_8 (in const n64 * SRC, in n16 N, out n8 * DST) <br/>{< br/> n64u n64u_data; <br/> Register n64 * n64ptr = (n64 *) SRC; <br/> Register n8 * n8ptr = DST; <br/> n16 I = 0, num = N; </P> <p> If (null = n64ptr | null = n8ptr | n <1) <br/>{< br/> printf ("invalid Param, SRC 0x % x, DST 0x % x, N % d/N ", n64ptr, n8ptr, n); <br/> return 0; <br/>}</P> <p> for (I = 0; I <num; I ++) <br/>{< br/> n64u_data.n64 = * n64ptr ++; </P> <p>/* use the simple mean method to compress eight points into one point */<br/> * n8ptr ++ = (n64u_data.u8.c0 + n64u_data.u8.c1 + n64u_data.u8.c2 +/<br/> n64u_data.u8.c3 + n64u_data.u8.c4 + n64u_data.u8.c5 +/<br/> n64u_data.u8.c6 + n64u_data.u8.c7) /(sizeof (n64)/sizeof (n8); <br/>}< br/> return 1; <br/>}</P> <p> int uncompress8_64 (in const n8 * SRC, in n16 N, out n64 * DST) <br/>{< br/> n64u n64u_data; <br/> Register n64 * n64ptr = DST; <br/> Register n8 * n8ptr = (n8 *) SRC; <br/> Register n8 n8data; <br/> n16 I = 0, num = N; </P> <p> If (null = n64ptr | null = n8ptr | n <1) <br/>{< br/> printf ("invalid Param, SRC 0x % x, DST 0x % x, N % d/N ", n64ptr, n8ptr, n); <br/> return 0; <br/>}</P> <p> for (I = 0; I <num; I ++) <br/> {<br/> n8data = * n8ptr ++; </P> <p>/* decompress one point into eight points and adopt simple restoration, so there is distortion */<br/> n64u_data.u8.c0 = n8data; <br/> latency = n8data; <br/> n64u_data.u8.c2 = n8data; <br/> n64u_data.u8.c3 = n8data; <br/> latency = n8data; <br/> n64u_data.u8.c5 = n8data; <br/> n64u_data.u8.c6 = n8data; <br/> n64u_data.u8.c7 = n8data; </P> <p> * n64ptr ++ = n64u_data.n64; <br/>}< br/> int main (INT argc, char * argv []) <br/>{< br/> n64 n64data [max_data_compressed_num]; <br/> n8 n8data [max_data_compressed_num]; <br/> s8t s8t_data [max_data_compressed_num] <br/>=< br/> {/* data to be compressed, one data occupies one byte */<br/> {1, 2, 3, 4, 5, 6, 7, 8},/* 8 bytes are compressed into a single byte */<br/> {2, 2, ,}, <br/> {,}, <br/> }, <br/> {,}, <br/> {,}, <br/>, }, <br/> {,} <br/>}; </P> <p> n16 I, n = max_data_compressed_num; </P> <p> printf ("data:/N"); <br/> for (I = 0; I <n; I ++) <br/> {<br/> n64data [I] = * (n64 *) & s8t_data [I]; </P> <p> printf ("% 3u % 3u % 3u % 3u % 3u % 3u % 3u % 3u/N", <br/> s8t_data [I]. c0, s8t_data [I]. c1, s8t_data [I]. c2, <br/> s8t_data [I]. c3, s8t_data [I]. c4, s8t_data [I]. c5, <br/> s8t_data [I]. c6, s8t_data [I]. c7); <br/>}< br/> printf ("/N"); </P> <p> compress64_8 (n64data, N, n8data ); </P> <p> printf ("compressed to:/N"); <br/> for (I = 0; I <n; I ++) <br/>{< br/> printf ("% 3u", n8data [I]); <br/>}< br/> printf ("/n"); </P> <p> uncompress8_64 (n8data, N, n64data ); </P> <p> printf ("uncompressed to:/N"); <br/> for (I = 0; I <n; I ++) <br/>{< br/> * (n64 *) & s8t_data [I] = n64data [I]; </P> <p> printf ("% 3u % 3u % 3u % 3u % 3u % 3u % 3u % 3u/N", <br/> s8t_data [I]. c0, s8t_data [I]. c1, s8t_data [I]. c2, <br/> s8t_data [I]. c3, s8t_data [I]. c4, s8t_data [I]. c5, <br/> s8t_data [I]. c6, s8t_data [I]. c7); <br/>}< br/> printf ("/N "); <br/>}</P> <p>/* <br/> running result: <br/> data: <br/> 1 2 3 4 5 6 7 8 <br/> 2 2 2 3 4 5 6 7 7 <br/> 3 2 3 4 5 6 7 6 <br/> 4 2 3 4 5 6 7 5 <br/> 5 2 3 4 5 6 7 4 <br/> 6 2 3 4 5 6 7 3 <br/> 7 2 3 4 5 6 7 2 <br/> 8 7 6 5 4 3 2 1 <br/> compressed: <br/> 4 4 4 4 4 4 4 4 <br/> uncompressed: <br/> 4 4 4 4 4 4 4 4 4 <br/> 4 4 4 4 4 4 4 4 <br/> 4 4 4 4 4 4 4 4 <br/> 4 4 4 4 4 4 4 4 <br/> 4 4 4 4 4 4 4 <br/> 4 4 4 4 4 4 4 4 <br/> 4 4 4 4 4 4 4 4 <br/> 4 4 4 4 4 4 4 <br/> */<br/>