File creation and opening in LInux

Library function:
       #include <fcntl.h>

Function prototype

       int open(const char *pathname, int flags);
       int open(const char *pathname, int flags, mode_t mode)；

pathname : The path name of the file to be opened or created, which can be an absolute path or a relative path.

flags : When opening a file, you can pass in multiple parameter options, and use one or more of the following to perform an "or" operation to form flags;

O_RDONLY : read-only open;

O_WRONLY : write-only open;

O_RDWR : open for reading and writing; (only one of the above three variables can be specified)

O_CREAT : If the file does not exist, create it. Need to use the mode (file permission flag) option to indicate the access permission of the new file;

O_APPEND : additional write, each write will be appended to the end of the file;

O_TRUNC : Delete all the contents of the original file and then open the file;

O_EXCL : If the file to be created already exists, an error will occur when using O_CREAT and -1 will be returned. If the file does not exist, it will be created and the return value will be greater than 0;

mode  :      r : read permission ( 0400 );  w : write permission ( 0200 ); x : executable permission ( 0100 ); read and write ( 0600 ); read, write and execute ( 0700 );

Return value : If successful, it returns the descriptor of the open file. The file descriptor is a non-negative integer (usually 3). If it fails, it returns -1.
        The file identifiers 0, 1, and 2 represent standard input, standard output, and standard error output, respectively, and are replaced by the constants STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO, respectively.

#include <fcntl.h>
#include <stdio.h>
int main()
{
    int fd;
    fd = open("./file1",O_RDWR);
    
    if(fd == -1){
        printf(" 打开 file1 文件失败 \n");
        fd = open("./file1",O_RDWR|O_CREAT,0600);

        if(fd > 0){
           printf(" 创建 file1 文件成功 \n");        
        }
    }

    return 0;
}