splice()函数,'SPLICE_F_MOVE' 'SPLICE_F_NONBLOCK' 'SPLICE_F_MORE' undeclared

1.编译含有splice()函数的程序时出现，'SPLICE_F_MOVE'  undeclared，'SPLICE_F_NONBLOCK' ‘SPLICE_F_MORE' 也是一样undeclared！

2.使用man splice查看，发现要定义宏_GNU_SOURCE

 #define _GNU_SOURCE         /* See feature_test_macros(7) */
 #include <fcntl.h>

 ssize_t splice(int fd_in, loff_t *off_in, int fd_out, loff_t *off_out, size_t len, unsigned int flags);

我们跟着后面宏的注释See feature_test_macros(7)

 $:man feature_test_macros

man手册上面的描述是:特性测试宏允许程序编译时控制系统头文件的宏定义！

 NAME
        feature_test_macros - feature test macros

 SYNOPSIS
        #include <features.h>

 DESCRIPTION
        Feature test macros allow the programmer to control the definitions that are exposed by system header files when a program is compiled.

3.我们找找看，splice()函数的这个几个flags参数宏定义在哪里。splice()函数定义在<fcntl.h>头文件里面，查看头文件

 $:vim /usr/include/fcntl.h

然而里面并没有这几个宏，但里面有包含了<features.h>，<bits/fcntl.h>文件，我们跟进去这2个文件。我的系统是64位的kail。

bits文件夹在/usr/include/x86_64-linux-gnu/bits/

 $:vim /usr/include/features.h
 $:vim /usr/include/x86_64-linux-gnu/bits/fcntl.h

这2个里面都也没有，<bits/fcntl.h>文件里面又包含了<bits/fcntl-linux.h>。我们继续跟进。

 $:vim /usr/include/x86_64-linux-gnu/bits/fcntl-linux.h

这次我们找到了：

 /* Flags for SPLICE and VMSPLICE.  */
 # define SPLICE_F_MOVE         /* Move pages instead of copying.  */
 # define SPLICE_F_NONBLOCK     /* Don't block on the pipe splicing
                         (but we may still block on the fd
                         we splice from/to).  */
 # define SPLICE_F_MORE         /* Expect more data.  */
 # define SPLICE_F_GIFT         /* Pages passed in are a gift.  */

4.现在我们根据这几个宏定义的上下文来查看跟_GNU_SOURCE宏的联系。

这4个宏包含在#ifdef __USE_GNU里面，我回头在看看features.h

 $:vim /usr/include/features.h

直接搜索__USE_GNU，发现里面有这个定义，跟_GNU_SOURCE关联。

#ifdef  _GNU_SOURCE
# define __USE_GNU
#endif

5.如果不注重里面的包含细节，直接用grep搜索，简单粗暴！！！

$:grep -rn 'SPLICE_F' /usr/include/