umask的理解

在创建文件时有一个缺省的权限，这个缺省的权限与umask所设立的权限相“AND”，得到该文件最终的权限。所以，文件不可能得到比缺省权限更大的权限，除非文件创建后再改。

手册说：（关于fopen)
Any created files will have mode
S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP |S_IROTH | S_IWOTH (0666),
as modified by the process's umask value

如umask(022), 则用0666 & ~022， 结果是0644.

验证程序如下：

#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>

void create1file(int i);

void main(void) {
    mode_t mode;
    int i = 0;

    printf("umask was not set:\n");
    create1file(i++);

    mode = umask(0777);
    create1file(i++);
    printf("umask was set to 0777,last umask is: %04o\n",mode);

    mode = umask(0222);
    create1file(i++);
    printf("umask was set to 0777,last umask is: %04o\n",mode);

    mode = umask(0000);
    create1file(i++);
    printf("umask was set to 0777,last umask is: %04o\n",mode);
}

void create1file(int i)
{
    FILE* fp;
    char fn[10];

    sprintf(fn,"file%d",i);
    fp = fopen(fn,"w");

    fclose(fp);
}

输出结果：

$ ./a
umask was not set:
umask was set to 0777,last umask is: 0022
umask was set to 0777,last umask is: 0777
umask was set to 0777,last umask is: 0222

$ ls -al file*
-rw-r--r-- 1 a16000 mkgroup 0 Aug 29 15:41 file0
---------- 1 a16000 mkgroup 0 Aug 29 15:41 file1
-r--r--r-- 1 a16000 mkgroup 0 Aug 29 15:41 file2
-rw-rw-rw- 1 a16000 mkgroup 0 Aug 29 15:41 file3