由于read、readv、write和writev函數一次讀或寫有時并不能滿足所要求的數據，因此需要多次調用直到要求的字節數或者出錯。針對這4個系統調用，編寫了對應的xxxn版本，實現如下
1
/**********************************************************************************************************
2
In the following four functions,the optional parameter tran indicate the number of bytes read or written,or -1 if an error occurred.
3 On success,return true indicate all data had been read or written successfully,otherwise in other cases,return false indicate error or partial success.
4
**********************************************************************************************************/
5
6
bool readn(int fd,void* buf,size_t cnt,ssize_t* tran/**//*=NULL*/)
7
{
8 size_t left = cnt;
9 ssize_t ret;
10 char* ptr = (char*)buf;
11
12
while(left > 0){
13 ret = read(fd,ptr,left);
14 if(ret > 0){
15 left -= ret;
16 ptr += ret;
17
}else if(0==ret || left != cnt)
18 break;
19 else{
20 if(tran) *tran = -1;
21 return false;
22 }
23 }
24 if(tran) *tran = cnt-left;
25 return 0==left;
26}
27
28bool writen(int fd,const void* buf,size_t cnt,ssize_t* tran/**//*=NULL*/)
29{
30 size_t left = cnt;
31 ssize_t ret;
32 char* ptr = (char*)buf;
33
34 while(left > 0){
35 ret = write(fd,ptr,left);
36 if(ret > 0){
37 left -= ret;
38 ptr += ret;
39 }else if(0==ret || left != cnt)
40 break;
41 else
42 if(tran) *tran = -1;
43 return false;
44 }
45 if(tran) *tran = cnt-left;
46 return 0==left;
47}
48
49static int get_iov_tran_index(const struct iovec* iov,int iovcnt,size_t trans,size_t& tran)
50{
51 size_t cnt = 0; int i;
52
53 for(i=0;i < iovcnt;++i){
54 cnt += iov[i].iov_len;
55 if(trans < cnt){
56 tran = iov[i].iov_len - (cnt - tran);
57 break;
58 }
59 }
60 return i;
61}
62
63bool readvn(int fd,const struct iovec* iov,int iovcnt,ssize_t* tran/**//*=NULL*/)
64{
65 if(iovcnt > IOV_MAX){
66 if(tran) *tran = -1;
67 errno = EINVAL;
68 return false;
69 }
70 size_t all_cnt = 0,all_tran = 0,one_tran;
71 ssize_t ret;
72
73 struct iovec _iov[IOV_MAX];
74 int i;
75 for(i=0;i < iovcnt;++i){
76 _iov[i] = iov[i];
77 all_cnt += iov[i].iov_len;
78 }
79
80 i = 0;
81 do{
82 ret = readv(fd,&_iov[i],iovcnt-i);
83 if(ret > 0){
84 all_tran += ret;
85 if(all_tran==all_cnt)
86 break;
87
88 i = get_iov_tran_index(iov,iovcnt,all_tran,one_tran);
89 assert(i < iovcnt);
90 _iov[i].iov_base = iov[i].iov_base + one_tran;
91 _iov[i].iov_len = iov[i].iov_len - one_tran;
92
93 }else if(0==ret)
94 break;
95 else{
96 if(tran) *tran = -1;
97 return false;
98 }
99 }while(all_tran < all_cnt);
100
101 if(tran) *tran = all_tran;
102 return all_tran==all_cnt;
103}
104
105bool writevn(int fd,const struct iovec* iov,int iovcnt,ssize_t* tran/**//*=NULL*/)
106{
107 if(iovcnt > IOV_MAX){
108 if(tran) *tran = -1;
109 errno = EINVAL;
110 return false;
111 }
112 size_t all_cnt = 0,all_tran = 0,one_tran;
113 ssize_t ret;
114
115 struct iovec _iov[IOV_MAX];
116 int i;
117 for(i=0;i < iovcnt;++i){
118 _iov[i] = iov[i];
119 all_cnt += iov[i].iov_len;
120 }
121
122 i = 0;
123 do{
124 ret = writev(fd,&_iov[i],iovcnt-i);
125 if(ret > 0){
126 all_tran += ret;
127 if(all_tran==all_cnt)
128 break;
129
130 i = get_iov_tran_index(iov,iovcnt,all_tran,one_tran);
131 assert(i < iovcnt);
132 _iov[i].iov_base = iov[i].iov_base + one_tran;
133 _iov[i].iov_len = iov[i].iov_len - one_tran;
134
135 }else if(0==ret)
136 break;
137 else{
138 if(tran) *tran = -1;
139 return false;
140 }
141 }while(all_tran < all_cnt);
142
143 if(tran) *tran = all_tran;
144 return all_tran==all_cnt;
145} 從以上代碼可看出，readvn和writevn的實現并不是循環對每一個緩沖區簡單地調用readn或writen，而是多次調用原生的readv或writev，因為對于讀寫多個緩沖區，使用readv或writev的效率通常要比多次調用read或write高，所以這樣做就會盡可能減少系統調用的次數，提高效率。
posted on 2013-08-02 19:44
春秋十二月 閱讀(1785)
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