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How to fix undeclared O_RDONLY/O_RDWR/O_WRONLY in C

Problem:

You are trying to compile C sourcecode that uses open(), but you see a compiler error message like this:

open_error_or_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: error: ‘O_RDONLY’ undeclared (first use in this function)
     int fd = open("my.txt", O_RDONLY);
                             ^~~~~~~^
main.c:3:29: note: each undeclared identifier is reported only once for each function it appears in

or

open_error_or_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: error: ‘O_RDWR’ undeclared (first use in this function)
     int fd = open("my.txt", O_RDWR);
                             ^~~~~~
main.c:3:29: note: each undeclared identifier is reported only once for each function it appears in

or

open_error_or_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: error: ‘O_WRONLY’ undeclared (first use in this function)
     int fd = open("my.txt", O_WRONLY);
                             ^~~~~~~_
main.c:3:29: note: each undeclared identifier is reported only once for each function it appears in

Solution

Add

include_fcntl.h
#include <fcntl.h>

to the top of the source file where the error occured.

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

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

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

See the OpenGroup reference on fcntl.h for more details on what else is defined in fcntl.h and what the meaning of all the individual flags is.

Check out similar posts by category: C/C++, GCC Errors