man(2) readv writev

#include <sys/uio.h>

ssize_t readv(int fd, const struct iovec *iov, int iovcnt);

unix高级环境编程中的定义：

【ssize_t readv(int filedes,const struct iovec iov[ ],int iovcnt) ;
ssize_t writev(int filedes,const struct iovec iov[ ],int iovcnt) ; 】

----------------------------------------------------------------------------

1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <sys/stat.h>
4 #include <fcntl.h>
5 #include <sys/uio.h>
6 #include <unistd.h>
7
8 int main()
9 {
10 struct iovec iov[2];
11 char buf[12],str[12];
12 int fd = open("1.txt",O_RDONLY);
13 if(fd == -1)
14 return -1;
15 iov[0].iov_base = buf;
16 iov[0].iov_len = 12;
17 iov[1].iov_base = str;
18 iov[1].iov_len = 12;
19 int n = readv(fd, iov, 2);

buf[11] = 0;//添加这个

20 printf("n:%d\n", n);
21 //printf("buf:%s\n",buf);
22 //printf("str:%s\n",str);
23 puts(buf);
24 puts(str);
25 close(fd);
26 return 0;
27 }

----------------------------------------------------------------------------

运行结果很奇怪：第一个缓冲区溢出了

tarena@ubuntu:~/mon$ ./a.out
n:23
buf:aaaaaaaaaaaabbbbbbbbbb

str:bbbbbbbbbb

解决办法：

int main()
9 {
10 struct iovec iov[3];
11 char buf[13],str[13],str2[13];
12 int fd = open("1.txt",O_RDONLY);
13 if(fd == -1)
14 return -1;
15 iov[0].iov_base = buf;
16 iov[0].iov_len = 12;
17 iov[1].iov_base = str;
18 iov[1].iov_len = 12;
19 iov[2].iov_base = str2;
20 iov[2].iov_len = 12;
21 int n = readv(fd, iov, 3);
22 buf[12] = 0;
23 str[12] = 0;
24 printf("n:%d\n", n);
25 printf("buf:%s\n",buf);
26 printf("str:%s\n",str);
27 printf("str2:%s\n",str2);
28 close(fd);
29 return 0;
30 }

当你要读入12个的时候申请13个存储空间读入的长度iov_len = 12，并且手动在末尾添加字符串结束标志。

----------------------------------------------------------------------------

ssize_t writev(int fd, const struct iovec *iov, int iovcnt);

----------------------------------------------------------------------------

1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <sys/stat.h>
4 #include <fcntl.h>
5 #include <sys/uio.h>
6 #include <unistd.h>
7 #include <string.h>
8 int main()
9 {
10 struct iovec iov[2];
11 char buf[12] = "hello,lyy",str[12] = "nihao,lyy";
12 int fd = open("2.txt",O_WRONLY | O_CREAT);
13 if(fd == -1)
14 return -1;
15 iov[0].iov_base = buf;
16 iov[0].iov_len = strlen(buf);
17 iov[1].iov_base = str;
18 iov[1].iov_len = strlen(str);
19 int n = writev(fd, iov, 2);
20 printf("n:%d\n", n);
21 close(fd);
22 return 0;
23 }

----------------------------------------------------------------------------

运行结果正确，但是为什么需要超级用户才能查看呢？

tarena@ubuntu:~/mon$ sudo cat 2.txt
hello,lyynihao,lyytarena@ubuntu:~/mon$

----------------------------------------------------------------------------

ssize_t preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);

1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <sys/stat.h>
4 #include <fcntl.h>
5 #include <sys/uio.h>
6 #include <unistd.h>
7 int main()
8 {
9 int fd = open("4.txt",O_RDWR|O_CREAT);
10 if(fd == -1) return -1;
11 struct iovec iov[3];
12 char str1[12],str2[12],str3[12];
13 iov[0].iov_base = str1;
14 iov[0].iov_len = 3;
15 iov[1].iov_base = str2;
16 iov[1].iov_len = 4;
17 iov[2].iov_base = str3;
18 iov[2].iov_len = 5;
19 preadv(fd, iov, 3, 4);
20 str1[iov[0].iov_len] = 0;
21 str2[iov[1].iov_len] = 0;
22 str3[iov[2].iov_len] = 0;
23 printf("str1:%s\nstr2:%s\nstr3:%s\n",str1,str2,str3);
24 close(fd);
25 return 0;
26 }

ssize_t pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);

1 #include <stdio.h>
2 #include <sys/types.h>
3 #include <sys/stat.h>
4 #include <fcntl.h>
5 #include <sys/uio.h>
6 #include <unistd.h>
7 #include <string.h>
8 int main()
9 {
10 int fd = open("4.txt",O_RDWR|O_CREAT);
11 if(fd == -1) return -1;
12 struct iovec iov[3];
13 char str1[12] = "hello,lyy",str2[12] = "nihao,lyy",str3[36] = "nice to meet you";
14 iov[0].iov_base = str1;
15 iov[0].iov_len = strlen(str1);
16 iov[1].iov_base = str2;
17 iov[1].iov_len = strlen(str2);
18 iov[2].iov_base = str3;
19 iov[2].iov_len = strlen(str3);
20 pwritev(fd, iov, 3, 3);
21 close(fd);
22 return 0;
23 }

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这两个举例不太好举

ssize_t preadv2(int fd, const struct iovec *iov, int iovcnt, off_t offset, int flags);

flags:RWF_SYNC  RWF_DSYNC

ssize_t pwritev2(int fd, const struct iovec *iov, int iovcnt, off_t offset, int flags);

flags:RWF_NOWAI

---------------------------------------------

RWF_HIPRI只用于下列条件

其中this feature is usable only on a file descriptor opened using the O_DIRECT flag.

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#include <unistd.h>

ssize_t pread(int fd, void *buf, size_t count, off_t offset);

从第offset个开始读

1 #include <stdio.h>
2 #include <string.h>
3 #include <unistd.h>
4 #include <sys/types.h>
5 #include <sys/stat.h>
6 #include <fcntl.h>
7
8 int main()
9 {
10 char buf[12];
11 int fd = open("3.txt",O_RDONLY);
12 int n = pread(fd, buf, 12,3);
13 buf[n] = 0;
14 printf("n = %d buf:%s\n",n,buf);
15 close(fd);
16 return 0;
17 }

返回值比实际读到的多一个，手动buf[n]置为0

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ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);

从第offset个开始写，从0开始；（略）