C Programming Comments
C supports two different styles of comments: Block Comments and Single Line Comments.
Block Comments
Block Comments begin with a /* and end with a */ and may span multiple lines. For example:
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* Program: Hello.c
* Author: R. Ostapiuk
************************************************************************/
#include <stdio.h>
/* Application begins here */
int main(void)
{
printf("Hello, world!\n"); /* Display "Hello, world!" in terminal */
}
Lines 1-4 are comprised of one long comment that spans four lines. The comment begins with the /* at the beginning of line 1 and ends with the */ at the end of line 4. Every other character between those two delimiters is ignored by the compiler.
Line 7 shows another example where a block comment is used for just a single line. While line 10 shows a similar construct to add a comment at the end of a line of code.
Single Line Comments
Single line comments begin with a // and run to the end of the line. These are sometimes called "C++ style comments" because they were developed for C++ and were not originally part of the ANSI C standard. They may not span multiple lines. For example:
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// Program: Hello.c
// Author: R. Ostapiuk
//====================================================
#include <stdio.h>
// Application begins here
int main(void)
{
printf("Hello, world!\n"); // Display "Hello, world!" in terminal
}
Here we have replaced each block comment from the first example with an equivalent form using a line comment.
Lines 1-4 are now four separate comments with each line starting with //.
Lines 7 and 10 were also converted into single-line comments that are more appropriate for their situations.
Nesting Comments
Block comments may not be nested within other block comments. However, single-line comments may be nested within a block comment.
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code_here; // This is a valid way to nest comments
*/
/*
code_here; /* This is not a valid way to nest comments */
code_here; /* What happened here? */
*/
Lines 1-3 show that it is perfectly valid to nest a single-line comment within a block comment. In fact, this is a widespread practice when testing programs. From time to time, you may want to temporarily remove a block of code from your program. Using block comment delimiters as shown here will have that effect as long as the only other comments between the delimiters are single-line comments.
Lines 5-8 show what can go wrong when block comments are nested within one another. A comment begins with the /* at the beginning of line 5. Because of line 5, the opening delimiter /* on line 6 is completely ignored and the closing delimiter */ at the end of line 6 is matched with the opening delimiter on line 5. This leaves the code on line 7 uncommented, followed by a complete comment at the end of line 7.
Therefore, at the beginning of line 8, we have a closing delimiter */ that is unmatched by an opening delimiter which will cause a syntax error.
Best Practices
Ideally, you should use single-line comments throughout most of your code, reserving block comments for times when you want to temporarily remove a block of code or for when you really need multi-line comments. Below is an example of how many programmers use the two comment styles.
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* Program: Hello.c
* Author: R. Ostapiuk
************************************************************************/
#include <stdio.h>
/************************************************************************
* Function: main()
* (some details about what main does here...)
************************************************************************/
int main(void)
{
int i; // Loop Count Variable
char *p; // Pointer to output string
/*
printf("Hello, world!\n"); // Display "Hello, world!" in terminal
*/
...
}
