Question

#include<apue.h>
#include<unistd.h>
#include<fcntl.h>
#include<string.h>

int main(int argc , char *argv[])
{
    int fd = open("test.txt",O_WRONLY | O_CREAT | O_TRUNC);
    int pid,status;
    const char *str_for_parent = "str_for_parent\n";
    const char *str_for_child = "str_for_child\n";
    if ((pid = fork()) < 0)
        err_sys("fork error");
    else if (pid == 0)
    {
        write(fd,str_for_child,strlen(str_for_child)+1);
        _exit(0);
    }
    wait(&status);
    write(fd,str_for_parent,strlen(str_for_parent)+1);

    return 0;
}

The test.txt is created by open().But it’s permission(---------x) is different with those files(-rw-rw-r--) created by touch or any other softwares in my system.my umask is 0002

Answer 1

open actually takes an (optional) third parameter:

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

The mode of the new file is set to the AND of mode and the inverse of your umask (ie, mode & ~umask). Since you’re not passing mode, it’s getting initialized to some garbage, and you’re getting the wrong mode as a result.

Generally if you’re passing O_CREAT, you should pass a mode of 0666, unless you have some specific reason to use some other mode (eg, if you want to make sure the file is not readable by other users regardless of what the umask is set to).