宏：整数常量 O_ACCMODE

Macro: int O_ACCMODE

This macro stands for a mask that can be bitwise-ANDed with the file status flag value to produce a value representing the file access mode. The mode will be O_RDONLY, O_WRONLY, or O_RDWR. (In the GNU system it could also be zero, and it never includes the O_EXEC bit.)

翻译：

这个宏作为一个掩码以与文件状态标识值做AND位运算，产生一个表示文件访问模式的值。这模式将是O_RDONLY, O_WRONLY, 或 O_RDWR（在GNU系统中，也可能是零，并且从不包括 O_EXEC 位）

O_ACCMODE<0003>：读写文件操作时，用于取出flag的低2位

O_RDONLY<00>：只读打开

O_WRONLY<01>：只写打开

O_RDWR<02>：读写打开

#include "apue.h"
#include <fcntl.h>

int
main(int argc, char *argv[])
{
int  val;

if (argc != 2)
err_quit("usage: a.out <descriptor#>");

if ((val = fcntl(atoi(argv[1]), F_GETFL, 0)) < 0)
err_sys("fcntl error for fd %d", atoi(argv[1]));

switch (val & O_ACCMODE) {
case O_RDONLY:
printf("read only");
break;

case O_WRONLY:
printf("write only");
break;

case O_RDWR:
printf("read write");
break;

default:
err_dump("unknown access mode");
}

if (val & O_APPEND)
printf(", append");
if (val & O_NONBLOCK)
printf(", nonblocking");
#if defined(O_SYNC)
if (val & O_SYNC)
printf(", synchronous writes");
#endif
#if !defined(_POSIX_C_SOURCE) && defined(O_FSYNC)
if (val & O_FSYNC)
printf(", synchronous writes");
#endif
putchar('/n');
exit(0);
}

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