linux文件操作的一些函数

dup, dup2, dup3 - duplicate a file descriptor

these system calls create a copy of the file descriptor oldfd.

dup() uses the lowest-numbered unused descriptor for the new descriptor.

dup2() makes newfd be the copy of oldfd, closing newfd first if necessary, but note the following:

*

If  oldfd is not a valid file descriptor, then the call fails, and  newfd is not closed.

*

If  oldfd is a valid file descriptor, and  newfd has the same value as  oldfd, then  dup2() does nothing, and returns  newfd.

#include<unistd.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<stdio.h>
int main(int argc ,char *argv[])
{
        int fd=open("./1.c",O_RDONLY);
        printf("original fd is %d\n",fd);
        int new_fd=dup(fd);
        printf("new fd is %d\n",new_fd);

}

— Macro: int O_ACCMODE

This macro stands for a mask that can be bitwise-ANDed with the file status flag value to produce a value representing the file access mode. The mode will be O_RDONLY, O_WRONLY, or O_RDWR. (On GNU/Hurd systems it could also be zero, and it never includes the O_EXEC bit.)

unlink - delete a name and possibly the file it refers to

unlink() deletes a name from the file system. If that name was the last link to a file and no processes have the file open the file is deleted and the space it was using is made available for reuse.

#include<fcntl.h>
#include<stdio.h>
int main(void)
{
        if(open("cc",O_RDWR)<0)
            printf("open error\n");
        if(unlink("cc")<0)
            printf("unlink error\n");
        printf("file unlinked\n");
        sleep(15);
        printf("done\n");
        exit(0);

}