readv/writev系统调用
名称：readv/writev
功能：散布读/聚集写
头文件：#include <sys/uio.h>
函数原形： ssize_t readv(int filedes,const struct iovec *iov,int iovcnt);
ssize_t writev(int filedes,const struct iovec *iov,int iovcnt);
参数：filedes 文件描述符
iov 指向iovec结构数组的一个指针。
iovcnt 数组元素的个数
返回值：若成功则返回已读、写的字节数，若出错则返回-1
readv和writev函数用于在一次函数调用中读、写多个非连续缓冲区。有时也将这两个函数成为散布读和聚集写。
这两个函数的第二个参数是指向iovec结构数组的一个指针：
struct iovec{
void *iov_base;
size_t iov_len;
};
writev以顺序iov[0]至iov[iovcnt-1]从缓冲区中聚集输出数据。writev返回输出的字节总数，通常，它应等于所有缓冲区长度之和。
readv则将读入的数据按上述同样顺序散布读到缓冲区中。readv总是先填满一个缓冲区，然后再填写下一个。readv返回读到的总字节数。如果遇到文件结尾，已无数据可读，则返回0。
下面就是读多个缓冲区的例子：
/*12_4.c*/
#include <sys/uio.h>
#include <stdio.h>
#include <fcntl.h>
int main(int argc,char *argv[])
{
ssize_t size;
char buf1[9];
char buf2[9];
struct iovec iov[2];
fd1=open(argv[1],O_RDONLY);
fd2=open(argv[2],O_RDONLY);
fd3=open(argv[3],O_WRONLY);
size=read(fd1,buf1,sizeof(buf1));
printf(“%s size is:%d\n”,argv[1],size);
size=read(fd2,buf2,sizeof(buf2));
printf(“%s size is:%d\n”,argv[2],size);
iov[0].iov_base=buf1;
iov[0].iov_len=sizeof(buf1);
iov[1].iov_base=buf2;
iov[1].iov_len=sizeof(buf2);
size=writev(fd3,iov,2));
printf(“%s size is:%d\n”,argv[3],size);
close(fd1);
close(fd2);
close(fd3);
}
先用vi或cat建立三个文件（test1,test2,test3），test写入123456789，test写入abcdefghi. test3为空。
然后运行命令：
#./12_4 test1 test2 test3
在屏幕上会输出：
test1 size is:9
test2 size is:9
test3 size is:18
打开test3，文件内容为123456789abcdefghi.
程序先把test1和test2的内容分别读到缓冲区buf1和buf2中。然后用write把buf1和buf2的内容写至test3.
NAME
sys/uio.h - definitions for vector I/O operations
SYNOPSIS
[XSI] #include <sys/uio.h>
DESCRIPTION
The <sys/uio.h> header shall define the iovec structure that includes at least the following members:
void *iov_base Base address of a memory region for input or output. size_t iov_len The size of the memory pointed to by iov_base.The <sys/uio.h> header uses the iovec structure for scatter/gather I/O.
The ssize_t and size_t types shall be defined as described in <sys/types.h>.
The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.
ssize_t readv(int, const struct iovec *, int); ssize_t writev(int, const struct iovec *, int);
APPLICATION USAGE
The implementation can put a limit on the number of scatter/gather elements which can be processed in one call. The symbol {IOV_MAX} defined in <limits.h> should always be used to learn about the limits instead of assuming a fixed value.
RATIONALE
Traditionally, the maximum number of scatter/gather elements the system can process in one call were described by the symbolic value {UIO_MAXIOV}. In IEEE Std 1003.1-2001 this value is replaced by the constant {IOV_MAX} which can be found in <limits.h>.
FUTURE DIRECTIONS
None.
SEE ALSO
<limits.h>, <sys/types.h>, the System Interfaces volume of IEEE Std 1003.1-2001, read(), write()
CHANGE HISTORY
First released in Issue 4, Version 2.
Issue 6
Text referring to scatter/gather I/O is added to the DESCRIPTION.