readv ＆ writev

readv and writev The feature is that it allows a single system call to read in or write out One or more buffers . These operations are called Spread reading and Centralized writing .

Because the input data from the read operation is scattered into multiple application buffers , The output data from multiple application buffers is provided to a single write operation .

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

The second argument to these two functions points to iovec A pointer to an array of structures ,iov It's an array of structures , Each of its elements indicates a buffer in memory ：

struct iovec{
    void *iov_base; //starting address of buffer
    size_t iov_len; //size of buffer
}

 

 

writev In order iov[0],iov[1] to iov[iovcnt-1] Aggregate output data from buffer .

readv Then the read data is scattered into the buffer in the same order as above ,readv Always fill a buffer first , Then fill in the next .

 

With these two functions , When you want to write a linked list together , Just let iov Each element of the array contains the address and length of each item in the linked list , And then iov And the number of its elements are passed as parameters to writev(), These data can be written at once .

 

Function implementation ：
 

experience Dispersive readout and Centralized write The meaning of ：

#include <stdio.h>
#include <sys/uio.h>
#include <fcntl.h>
 
#define IOVENT		3
#define PER_IOVENT	5

int main(void)
{
	char buf1[PER_IOVENT],buf2[PER_IOVENT],buf3[PER_IOVENT];
	
	struct iovec iov[IOVENT];// Distribute 3 Structures are stored together  abcdefghijklnmo
	iov[0].iov_base = buf1;
	iov[0].iov_len = PER_IOVENT;
	iov[1].iov_base = buf2;
	iov[1].iov_len = PER_IOVENT;
	iov[2].iov_base = buf3;
	iov[2].iov_len = PER_IOVENT;

	int fd = open("a.txt",O_RDWR);
	if(fd < 0){
		perror("open");
		return -1;
	}
	int rsize = readv(fd, iov, IOVENT);//a.txt The content of   Dispersive readout   Into three structures 
	printf("buf1 = %s\n",buf1);
	printf("buf2 = %s\n",buf2);
	printf("buf3 = %s\n",buf3);

	close(fd);

	fd = open("b.txt", O_RDWR|O_CREAT, S_IRUSR|S_IWUSR);
	if(fd < 0){
		perror("open");
		return -1;
	}

	int wsize = writev(fd,iov,IOVENT);// The contents of the three structures   Centralized write  b.txt in 

	close(fd);
	
	return 0;
}

#if 0

 Function execution ：

[email protected]:~/study/network$ ./a.out
buf1 = abcdefghijklnmo
buf2 = fghijklnmo
buf3 = klnmo
[email protected]:~/study/network$ cat a.txt
abcdefghijklnmo
[email protected]:~/study/network$ cat b.txt
[email protected]:~/study/network$ 
[email protected]:~/study/network$ 

#endif