> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # C Line Comment A line comment in C is a lexical construct that instructs the compiler to ignore all text starting from the comment delimiter up to the end of the physical or logical line. During Translation Phase 3 of the compilation process, the source file is decomposed into preprocessing tokens and sequences of white-space characters. At this stage, the entire comment is identified as white-space and replaced by a single space character before syntactic analysis begins. ## Syntax A line comment is introduced by the character sequence of two consecutive forward slashes (`//`) and is terminated by the next newline character (`\n`). ```c theme={"dark"} // [text to be ignored by the compiler] int x = 5; // The comment can also follow a valid statement ``` ## Technical Mechanics * **Lexical Analysis:** The `//` sequence is a character sequence that introduces a comment. Neither the `//` delimiter nor the comment itself constitutes a preprocessing token; they are categorized strictly as white-space. * **Exclusions:** The C standard dictates that the `//` sequence carries no special meaning and does not initiate a comment if it appears inside a character constant, a string literal, or an existing block comment (`/* ... */`). * **Nesting:** Line comments do not nest. Once the `//` sequence successfully initiates a comment, all subsequent characters—including other comment delimiters like `/*` or `//`—are treated as part of the comment until the newline character. ```c theme={"dark"} char* path = "http://example.com"; /* The // inside the string is not a comment */ /* Block comment containing // which is ignored */ // Line comment containing /* which is ignored ``` ## Line Splicing (Continuation) Because the C preprocessor resolves line splicing (Translation Phase 2) before it processes comments (Translation Phase 3), a line comment can be extended to the next physical line using a backslash (`\`) immediately preceding the newline character. ```c theme={"dark"} // This is a single logical line comment that spans \ across multiple physical lines in the source code \ because of the backslash line-continuation character. int y = 10; ``` ## Standardization Line comments were not part of the original ANSI C standard (C89/C90), which only supported block comments (`/* ... */`). They were officially adopted into the language specification with the **C99 standard** (ISO/IEC 9899:1999), borrowing the syntax from C++. In strict C89 mode (e.g., compiling with `gcc -std=c89` or `gcc -ansi`), the `//` sequence is not recognized as a comment delimiter. This leads to distinct lexical parsing behaviors compared to C99: * An expression like `a // b` is parsed as the identifier `a`, followed by two consecutive division operators (`/` and `/`), and the identifier `b`. This results in a hard syntax error. * An expression like `a //*b*/ c` is parsed as the identifier `a`, followed by a single division operator (`/`), a block comment (`/*b*/`), and the identifier `c`. This evaluates as valid division (`a / c`). In contrast, under C99 rules, the initial `//` initiates a line comment, causing the compiler to ignore `*b*/ c` entirely and leaving the expression incomplete. The compiler will only accept `//` as a comment in pre-C99 environments if compiler extensions are active (e.g., using GNU C89 mode via `gcc -std=gnu89`). In that specific extended mode, adding the `-pedantic` flag will successfully compile the code but emit a warning regarding the use of C++ style comments.

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