Linux内核编程--常见的文件操作

#include <unistd.h>
int dup(int fd); /*复制一个现有文件的描述符*/
int dup2(int fd, int fd2); /*把一个文件的旧描述符复制到一个新描述符上*/

int fd = open(filename, O_WRONLY, 0666);
dup2(fd, STDOUT_FILENO); 
dup2(fd, STDERR_FILENO);
dup2(fd, STDERR_FILENO);

#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <string.h>

int main(){
    int fd_old = open("test.txt", O_RDWR | O_CREAT, 0666);
    int fd_new = dup(fd_old);
    int fd_new_2 = dup2(fd_old, 6);

    if((fd_new == -1) || (fd_new_2 == -1)){   
        perror("dup or dup2 failed");
        return -1;
    }

    printf("fd_old : %d, fd_new : %d \n, fd_new_2 : %d \n", fd_old, fd_new, fd_new_2);
    close(fd_old);

    char *str = "hello world!";
    int ret = write(fd_new, str, strlen(str));
    if(ret == -1){
        perror("write failed");
        return -2;
    }

    close(fd_new);
    return 0;
}

fd_old : 3, fd_new : 4, fd_new_2 : 6

#include <fcntl.h>
int fcntl(int fd, int cmd, ... );
--fd表示需要操作的文件描述符，cmd表示对文件描述符进行如何操作
--常用的cmd:
(1) 复制一个已有的描述符（cmd = F_DUPFD/F_DUPFD_CLOEXEC)
(2) 获取/设置文件描述符标志（cmd = F_GETFD/SETFD)
(3) 获取/设置文件状态标志（cmd = F_GETFL/F_SETFL)
(4) 获取/设置异步I/O所有权（cmd = F_GETOWN/F_SETOWN)
(5) 获取/设置记录锁（cmd = F_GETLK/F_SETLK/F_SETLKW)

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>

int main()
{
  int flags;
  int append_flag;
  int nonblock_flag;
  int access_mode;
  int fd; /* File Descriptor */
  char *text1 = "abcdefghij";
  char *text2 = "0123456789";
  char read_buffer[25];

  memset(read_buffer, '\0', 25);

  /* create a new file */
  fd = creat("test.txt",S_IRWXU);
  write(fd, text1, 10);
  close(fd);

  /* open the file with read/write access */
  fd = open("test.txt", O_RDWR);
  read(fd, read_buffer, 24);
  printf("first read is \'%s\'\n",read_buffer);

  /* reset file pointer to the beginning of the file */
  lseek(fd, 0, SEEK_SET);
  /* set append flag to prevent overwriting existing text */
  fcntl(fd, F_SETFL, O_APPEND);
  write(fd, text2, 10);
  lseek(fd, 0, SEEK_SET);
  read(fd, read_buffer, 24);
  printf("second read is \'%s\'\n",read_buffer);

  close(fd);
  unlink("test.txt");

  return 0;
}

first read is 'abcdefghij'
second read is 'abcdefghij0123456789'

#include <sys/stat.h>
int stat(const char *restrict pathname, struct stat *restrict buf);
int lstat(const char *restrict pathname, struct stat *restrict buf);
int fstat(int fd, struct stat *buf);

struct stat {
     dev_t     st_dev;         /* ID of device containing file */
     dev_t     st_rdev;        /* Device ID (if special file) */
     ino_t     st_ino;         /* Inode number */
     mode_t    st_mode;        /* File type and mode */
     nlink_t   st_nlink;       /* Number of hard links */
     uid_t     st_uid;         /* User ID of owner */
     gid_t     st_gid;         /* Group ID of owner */               
     off_t     st_size;        /* Total size, in bytes */
     blksize_t st_blksize;     /* Block size for filesystem I/O */
     blkcnt_t  st_blocks;      /* Number of 512B blocks allocated */
     struct timespec st_atim;  /* Time of last access */
     struct timespec st_mtim;  /* Time of last modification */
     struct timespec st_ctim;  /* Time of last status change */
};

#include <sys/stat.h>
#include <unistd.h>
main()
{
    struct stat buf;
    stat("/etc/yum", &buf);
    printf("/etc/yum file_size = %d \n", buf.st_size);
    printf("/etc/yum file_mode = %s \n", buf.st_mode);
    printf("/etc/yum Device_ID = %d \n", buf.st_dev);
}

/etc/yum file_size = 4096
/etc/yum file_mode = 16877
/etc/yum Device_ID = 64769

stat [OPTION]... FILE...

  -L, --dereference     
  -f, --file-system     display file system status instead of file status
  -c  --format=FORMAT   use the specified FORMAT instead of the default;
                          output a newline after each use of FORMAT
      --printf=FORMAT   like --format, but interpret backslash escapes,
                          and do not output a mandatory trailing newline;
                          if you want a newline, include \n in FORMAT
  -t, --terse    print the information in terse form

[root@VM-4-13-centos cpp_learning]# ls -l hello.txt
-rw-r--r-- 1 root root 12 Apr 13 00:07 hello.txt

#include <sys/stat.h>
mode_t umask(mode_t, cmask);

S_IRUSR  当前用户可读
S_IWUSR  当前用户可写
S_IXUSR  当前用户可执行
S_IRGRP  用户组可读
S_IWGRP  用户组可写
S_IXGRP  用户组可执行
S_IROTH  其他用户可读
S_IWOTH  其他用户可写
S_IXOTH  其他用户可执行

#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#include <stdio.h>

main() {
  int fd;
  mode_t oldmask;
  printf("Your old umask is %i\n",oldmask=umask(S_IRWXG));
  if ((fd = creat("test.txt", S_IRWXU|S_IRWXG)) < 0)
    perror("creat() error");
  else {
    system("ls -l test.txt");
    close(fd);
    unlink("test.txt");
  }
  umask(oldmask);
}

Your old umask is 36
-rwx------ 1 root root 0 Apr 13 10:24 test.txt

umask [-p] [-S] [mode]