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C language
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Comments serve as a sort of in-code documentation. When inserted into a program, they are effectively ignored by the compiler; they are solely intended to be used as notes by the humans that read source code.

## Contents

### Syntax

 /* comment */ (1) // comment (2) (since C99)
1) Often known as "C-style" or "multi-line" comments.
2) Often known as "C++-style" or "single-line" comments.

All comments are removed from the program at translation phase 3 by replacing each comment with a single whitespace character.

### C-style

C-style comments are usually used to comment large blocks of text or small fragments of code; however, they can be used to comment single lines. To insert text as a C-style comment, simply surround the text with /* and */. C-style comments tell the compiler to ignore all content between /* and */. Although it is not part of the C standard, /** and **/ are often used to indicate documentation blocks; this is legal because the second asterisk is simply treated as part of the comment.

Except within a character constant, a string literal, or a comment, the characters /* introduce a comment. The contents of such a comment are examined only to identify multibyte characters and to find the characters */ that terminate the comment. C-style comments cannot be nested.

### C++-style

C++-style comments are usually used to comment single lines of text or code; however, they can be placed together to form multi-line comments. To insert text as a C++-style comment, simply precede the text with // and follow the text with the new line character. C++-style comments tell the compiler to ignore all content between // and a new line.

Except within a character constant, a string literal, or a comment, the characters // introduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the new-line character that terminates the comment. C++-style comments can be nested:

//  y = f(x);   // invoke algorithm

A C-style comment may appear within a C++-style comment:

//  y = f(x);   /* invoke algorithm */

A C++-style comment may appear within a C-style comment; this is a mechanism for excluding a small block of source code:

/*
y = f(x);   // invoke algorithms
z = g(x);
*/
(since C99)

### Notes

Because comments are removed before the preprocessor stage, a macro cannot be used to form a comment and an unterminated C-style comment doesn't spill over from an #include'd file.

/* An attempt to use a macro to form a comment. */
/* But, a space replaces characters "//".       */
#ifndef DEBUG
#define PRINTF //
#else
#define PRINTF printf
#endif
...
PRINTF("Error in file %s at line %i\n", __FILE__, __LINE__);

Besides commenting out, other mechanisms used for source code exclusion are:

#if 0
puts("this will not be compiled");
/* no conflict with C-style comments */
// no conflict with C++-style comments
#endif

and

if(0) {
puts("this will be compiled but not be executed");
/* no conflict with C-style comments */
// no conflict with C++-style comments
}

The introduction of // comments in C99 was a breaking change in some rare circumstances:

a = b //*divisor:*/ c
+ d; /* C89 compiles a = b / c + d;
C99 compiles a = b + d; */

### Example

#include <stdio.h>
/*
multiple lines.
*/

/* Or, just one line. */

// C++-style comments can comment one line.

// Or, they can
// be strung together.

int main(void)
{
// The below code won't be run
// puts("Hello");

// The below code will be run
puts("World");

// A note regarding backslash + newline.
// Despite belonging to translation phase 2 (vs phase 3 for comments),
// '\' still determines which portion of the source code is considered
// This comment will be promoted to the next line \
puts("Won't be run"); // may issue a warning "multi-line comment"
puts("Hello, again");
}

Output:

World
Hello, again

### References

• C17 standard (ISO/IEC 9899:2018):