c open close read write lseek

几个系统函数
open

 int open(const char *pathname, int flags, ...);   //mode
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O_RDONLY 只读
O_WRONLY 只写
O_RDWR 读写
O_CREAT 若不存在创建
O_ APPEND 末尾添加

如果是有O_CREAT，最后参数是权限参数，否则忽略
S_IRWXU 0700 用户权限读写执行

close

int close(int fd);
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例

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
 
int main()
{
    int fd = open("test", O_RDONLY | O_CREAT， S_IRWXU);
 
    printf("fd = %d\n", fd);
    if (fd == -1) 
        printf("open failed\n");
    close(fd);
    return 0;
}
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read

size_t read (int fd, void* buf, size_t cnt);
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write

size_t write (int fd, void* buf, size_t cnt); 
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lseek

off_t lseek(int fd, off_t offset, int whence);
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whence
SEEK_CUR 当前偏移 最后指向当前偏移+offset
SEEK_SET 把offset设为当前偏移
SEEK_END 指向结尾 可以用来获取文件大小

例

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
 
int main()
{
    int fd = open("test", O_RDONLY | O_CREAT，S_IRWXU);
 
    printf("fd = %d\n", fd);
    if (fd == -1) 
        printf("open failed\n");

	char mem[256];
	
	memset(mem,'a',256);
	write(fd,mem,256);
	lseek(fd,128,SEEK_SET);
	memset(mem,0,256);
	int res = read(fd,mem,256);
	printf("res = %d\n", res);
	 int filesize = lseek(fd,0,SEEK_END);
	 printf("filesize = %d\n", filesize);

    close(fd);
    return 0;
}
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输出

fd = 3
res = 128
filesize = 256
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