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);

}