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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:

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

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

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>

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:

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.