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

1.編譯含有splice()函數的程式時出現，'SPLICE_F_MOVE'  undeclared，'SPLICE_F_NONBLOCK' ‘SPLICE_F_MORE' 也是一樣undeclared！

2.使用man splice查看，發現要定義巨集_GNU_SOURCE

1 #define _GNU_SOURCE         /* See feature_test_macros(7) */
2 #include <fcntl.h>
3 
4 ssize_t splice(int fd_in, loff_t *off_in, int fd_out, loff_t *off_out, size_t len, unsigned int flags);

我們跟著後面巨集的註釋See feature_test_macros(7)

1 $:man feature_test_macros

man手冊上面的描述是:特性測試巨集允許程式編譯時控制系統頭文件的巨集定義！

1 NAME
2        feature_test_macros - feature test macros
3 
4 SYNOPSIS
5        #include <features.h>
6 
7 DESCRIPTION
8        Feature test macros allow the programmer to control the definitions that are exposed by system header files when a program is compiled.

3.我們找找看，splice()函數的這個幾個flags參數巨集定義在哪裡。splice()函數定義在<fcntl.h>頭文件裡面，查看頭文件

1 $:vim /usr/include/fcntl.h

然而裡面並沒有這幾個巨集，但裡面有包含了<features.h>，<bits/fcntl.h>文件，我們跟進去這2個文件。我的系統是64位的kail。

bits文件夾在/usr/include/x86_64-linux-gnu/bits/

1 $:vim /usr/include/features.h
2 $:vim /usr/include/x86_64-linux-gnu/bits/fcntl.h

這2個裡面都也沒有，<bits/fcntl.h>文件裡面又包含了<bits/fcntl-linux.h>。我們繼續跟進。

1 $:vim /usr/include/x86_64-linux-gnu/bits/fcntl-linux.h

這次我們找到了：

1 /* Flags for SPLICE and VMSPLICE.  */
2 # define SPLICE_F_MOVE      1   /* Move pages instead of copying.  */
3 # define SPLICE_F_NONBLOCK  2   /* Don't block on the pipe splicing
4                         (but we may still block on the fd
5                         we splice from/to).  */
6 # define SPLICE_F_MORE      4   /* Expect more data.  */
7 # define SPLICE_F_GIFT      8   /* Pages passed in are a gift.  */

4.現在我們根據這幾個巨集定義的上下文來查看跟_GNU_SOURCE巨集的聯繫。

這4個巨集包含在#ifdef __USE_GNU裡面，我回頭在看看features.h

1 $:vim /usr/include/features.h

直接搜索__USE_GNU，發現裡面有這個定義，跟_GNU_SOURCE關聯。

#ifdef  _GNU_SOURCE
# define __USE_GNU  1
#endif

5.如果不註重裡面的包含細節，直接用grep搜索，簡單粗暴！！！

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