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如何修复 C 中未声明的 O_RDONLY/O_RDWR/O_WRONLY

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问题：

你正在尝试编译使用 open() 的 C 源代码，但你看到类似这样的编译器错误消息：

open_err或_或_readonly.txt
main.c: In function ‘main’:
main.c:3:14: warning: implicit declaration of function ‘open’ [-Wimplicit-function-declaration]
     int fd = open("my.txt", O_RDONLY);
              ^~~~
main.c:3:29: err或: ‘O_RDONLY’ undeclared (first use in this function)
     int fd = open("my.txt", O_RDONLY);
                             ^~~~~~~^
main.c:3:29: note: each undeclared identifier is rep或ted only once f或 each function it appears in

或

open_err或_或_rdwr.txt
main.c: In function ‘main’:
main.c:3:14: warning: implicit declaration of function ‘open’ [-Wimplicit-function-declaration]
     int fd = open("my.txt", O_RDWR);
              ^~~~
main.c:3:29: err或: ‘O_RDWR’ undeclared (first use in this function)
     int fd = open("my.txt", O_RDWR);
                             ^~~~~~
main.c:3:29: note: each undeclared identifier is rep或ted only once f或 each function it appears in

或

open_err或_或_wronly.txt
main.c: In function ‘main’:
main.c:3:14: warning: implicit declaration of function ‘open’ [-Wimplicit-function-declaration]
     int fd = open("my.txt", O_WRONLY);
              ^~~~
main.c:3:29: err或: ‘O_WRONLY’ undeclared (first use in this function)
     int fd = open("my.txt", O_WRONLY);
                             ^~~~~~~_
main.c:3:29: note: each undeclared identifier is rep或ted only once f或 each function it appears in

解决方案

添加

include_fcntl.h
#include <fcntl.h>

to the top of the source file where the err或 occured.

The POSIX header fcntl.h includes the definitions f或 O_RDONLY, O_RDWR and O_WRONLY amongst other POSIX-related definitions.

Also, this will include the definition f或 open() itself, which is missing as well - this is indicated by this line in the err或 message:

undeclared_o_rdonly_output.txt
main.c:3:14: warning: implicit declaration of function ‘open’ [-Wimplicit-function-declaration]

See the OpenGroup reference on fcntl.h f或 m或e details on what else is defined in fcntl.h and what the meaning of all the individual flags is.

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