C语言 使用 C 在 Linux 中创建文件
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Create a file in Linux using C
提问by michael
I am trying to create a write only file in C on Linux (Ubuntu). This is my code:
我正在尝试在 Linux (Ubuntu) 上用 C 创建一个只写文件。这是我的代码：
int fd2 = open ("/tmp/test.svg", O_RDWR|O_CREAT);
if (fd2 != -1) {
//....
}
But why do the files I created have 'xr' mode? How can I create it so that I can open it myself at command prompt?
但是为什么我创建的文件有“xr”模式？如何创建它以便我可以在命令提示符下自己打开它？
------xr-- 1 michael michael 55788 2010-03-06 21:57 test.txt*
------xr-- 1 michael michael 9703 2010-03-06 22:41 test.svg*
回答by Jonathan Leffler
You need the three-argument form of open()when you specify O_CREAT. When you omit the third argument, open()uses whatever value happens to be on the stack where the third argument was expected; this is seldom a coherent set of permissions (in your example, it appears that decimal 12 = octal 014 was on the stack).
open()指定 O_CREAT 时需要三参数形式。当您省略第三个参数时，open()使用堆栈上恰好出现第三个参数的任何值；这很少是一组连贯的权限（在您的示例中，十进制 12 = 八进制 014 似乎在堆栈中）。
The third argument is the permissions on the file - which will be modified by the umask()value.
第三个参数是文件的权限 - 它将被umask()值修改。
int fd2 = open("/tmp/test.svg", O_RDWR | O_CREAT, S_IRUSR | S_IRGRP | S_IROTH);
Note that you can create a file without write permissions (to anyone else, or any other process) while still being able to write to it from the current process. There is seldom a need to use execute bits on files created from a program - unless you are writing a compiler (and '.svg' files are not normally executables!).
请注意，您可以在没有写入权限的情况下创建文件（对其他人或任何其他进程），同时仍然可以从当前进程写入文件。很少需要在从程序创建的文件上使用执行位 - 除非您正在编写编译器（并且“.svg”文件通常不是可执行文件！）。
The S_xxxx flags come from <sys/stat.h>and <fcntl.h>— you can use either header to get the information (but open()itself is declared in <fcntl.h>).
S_xxxx 标志来自<sys/stat.h>和<fcntl.h>- 您可以使用任一标头来获取信息（但open()它本身在 中声明<fcntl.h>）。
Note that the fixed file name and the absence of protective options such as O_EXCLmake even the revised open()call somewhat unsafe.
请注意，固定的文件名和缺少保护选项（例如，O_EXCL即使修改后的open()调用也有些不安全）。
回答by kompally santosh
Give access permissions as the third parameter:
授予访问权限作为第三个参数：
int fd2 = open("/tmp/test.svg", O_RDWR|O_CREAT, 0777); // Originally 777 (see comments)
if (fd2 != -1) {
// use file descriptor
close(fd2);
}
By doing this, all read, write and execute permissions will be given to user, group and others. Modify the 3rd parameter according to your use.
通过这样做，所有读取、写入和执行权限都将授予用户、组和其他人。根据您的使用修改第三个参数。
