linux c 文件 read（读） 和 write (写) 代码分析

read code:

[root@luozhonghua 03]# cat ex03-read-01.c

/*文件ex03-open-03.c,
O_CREAT和O_EXCL的使用*/
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>


int main(void)
{
int fd = -1,i;
ssize_t size = -1;
/*存放数据的缓冲区*/
char buf[10];
char filename[] = "test.txt";
/*打开文件，如果文件不存在，则报错*/
fd = open(filename,O_RDONLY);
if(-1 == fd){
/*文件已经存在*/
printf("Open file %s failure,fd:%d\n",filename,fd);
} else {
/*文件不存在，创建并打开*/
printf("Open file %s success,fd:%d\n",filename,fd);
}


/*循环读取数据，直到文件末尾或者出错*/
while(size){
/*每次读取10个字节数据*/
size = read(fd, buf,10);
if( -1 == size) {
/*读取数据出错*/
close(fd);/*关闭文件*/
printf("read file error occurs\n");
/*返回*/
return -1;
}else{
/*读取数据成功*/
if(size >0 ){
/*获得size个字节数据*/
printf("read %d bytes:",size);
/*打印引号*/
printf("\"");
/*将读取的数据打印出来*/
for(i = 0;i<size;i++){
printf("%c",*(buf+i));
}
/*打印引号并换行*/
printf("\"\n");
}else{
printf("reach the end of file\n");
}
}
}


return 0;

}

[root@luozhonghua 03]# ./ex03-read-01
Open file test.txt success,fd:3
read 10 bytes:"aaaaaaaaaa"
read 10 bytes:"aaaaaaaaaa"
read 10 bytes:"aaaaaaaaaa"
read 5 bytes:"aaaa
"
reach the end of file

-----write

[root@luozhonghua 03]# cat ex03-write-01.c
/*文件ex03-write-01.c,
O_CREAT和O_EXCL的使用*/
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>


int main(void)
{
int fd = -1,i;
ssize_t size = -1;
int input = 0;
/*存放数据的缓冲区*/
char buf[]="quick brown fox jumps over the lazy dog";
char filename[] = "test.txt";
/*打开文件，如果文件不存在，则报错*/
fd = open(filename,O_RDWR|O_TRUNC);
if(-1 == fd){
/*文件已经存在*/
printf("Open file %s failure,fd:%d\n",filename,fd);
} else {
/*文件不存在，创建并打开*/
printf("Open file %s success,fd:%d\n",filename,fd);
}


/*将数据写入到文件test.txt中*/
size = write(fd, buf,strlen(buf));
printf("write %d bytes to file %s\n",size,filename);
/*关闭文件*/
close(fd);


return 0;
}
[root@luozhonghua 03]# cat text.txt
cat: text.txt: No such file or directory
[root@luozhonghua 03]# cat test.txt
quick brown fox jumps over the lazy dog