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--- pragma-once [February 18, 2026 at 03:23] – yanevskiv
+++ pragma-once [May 14, 2026 at 11:38] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
+# #pragma once
+In C and C++ programming languages `#pragma once` is a non-standard preprocessor directive that is equivalent to [[header-guard|header guards]].
+You just put `#pragma once` at the top, rather than wrapping the entire header within header guards.
+Header guards:
+```cpp
+// image.h
+#ifndef __IMAGE_H__
+#define __IMAGE_H__
+struct Image {
+    ...
+};
+#endif /* __IMAGE_H__ */
+```
+Pragma once:
+```cpp
+// image.h
+#pragma once
+struct Image {
+    ...
+};
+```
+## Should I use #pragma once?
+It's up to you! It's widely supported by almost all C and C++ compilers -- even though it's not technically defined by the standard in either programming language.
+An advantage is that there is less chance you will make a typo like this:
+```c
+#ifndef __HEADER_H_
+#define __HEADER_H__
+/* ... */
+#endif
+```
+There is no good reason not to use `#pragma once` although my preferences still rather lean towards header guards simply because I feel most C and C++ programmers will find them more familiar.
+## What is #pragma?
+`#pragma` is a preprocessor directive that instructs the compiler to employ some non-standard behavior -- beyond the language itself. `#pragma` directive itself is part of the C standard, but what exactly `#pragma` *does* is up to the compilers.
+### GCC example
+As an example -- if you use the [[gcc|GCC]] compiler -- you can use `#pragma GCC poison` in order to ban (or "poison") certain identifiers. It's [well-known](https://stackoverflow.com/questions/1694036/why-is-the-gets-function-so-dangerous-that-it-should-not-be-used) among C programmers that `gets()` is a dangerous function that should not be used. So you use `#pragma GCC poison gets` to ban it altogether!
+Well, maybe gets() is a bad example because GCC already bans it and tells you to use fgets(). ;)
+You can read more about GCC `#pragma` directive [here](https://gcc.gnu.org/onlinedocs/cpp/Pragmas.html).
+```c
+/*
+Compile: gcc main.c
+Error:
+    main.c: In function ‘main’:
+    main.c:24:5: error: attempt to use poisoned ‘gets’
+|     gets(str);
+          |     ^
+    main.c:19:20: note: poisoned here
+| #pragma GCC poison gets
+          |                    ^~~~
+    main.c:24:5: error: implicit declaration of function ‘gets’; did you mean ‘fgets’? [-Wimplicit-function-declaration]
+|     gets(str);
+          |     ^~~~
+          |     fgets
+*/
+#include <stdio.h>
+#pragma GCC poison gets
+int main()
+{
+    char str[BUFSIZ];
+    gets(str);
+    puts(str);
+    return 0;
+}
+```
+### OpenMP example
+A multithreading library [[openmp|OpenMP]] uses `#pragma` extensively directive extensively!
+All OpenMP directives start with `#pragma omp`.
+The following example creates two competing threads -- one that prints a red letter 'a', the other that prints a green letter 'b'. You cancel the program with a SIGINT signal by pressing Ctrl + C.
+You have to remember to compile the source code with `-fopenmp` option.
+```c
+/*
+ Compile: gcc -fopenmp main.c -o omp_two_threads
+ Run:     ./omp_two_threads
+ Result:  aaaaaaabbbbbbbaaaaaaabbbbb
+ Stop:    Ctrl + C
+*/
+#include <stdio.h>
+#include <unistd.h>
+#include <omp.h>
+#define RED   "\e[31m"
+#define GREEN "\e[32m"
+#define NONE  "\e[0m"
+#define A_STR RED "a" NONE
+#define B_STR GREEN "b" NONE
+int main()
+{
+    #pragma omp parallel num_threads(2)
+    {
+        int tid = omp_get_thread_num();
+        for ( ; ; ) {
+            #pragma omp crticial
+            {
+                printf("%s", tid == 0 ? A_STR : B_STR);
+            }
+        }
+    }
+    return 0;
+}
+```
+## Links
+ - https://gcc.gnu.org/onlinedocs/cpp/Pragmas.html
+ - https://en.wikipedia.org/wiki/Pragma_once