Fundamentals of Embedded Programming | What are the Keywords in C Language and Which Words are Easily Confused with Keywords?

01Introduction:

In the previous article, we discussed three keywords in C language. Liu thought about it and wondered what other keywords exist in C language. After gathering some information, the summary is as follows, based on different standards.

02C89/C90 Standard (32 Keywords)

Data Types Control Flow Storage Classes Others <span>int</span> <span>if</span> <span>auto</span> <span>sizeof</span> <span>char</span> <span>else</span> <span>extern</span> <span>typedef</span> <span>float</span> <span>switch</span> <span>register</span> <span>const</span> <span>double</span> <span>case</span> <span>static</span> <span>volatile</span> <span>void</span> <span>default</span> <span>enum</span> <span>short</span> <span>for</span> <span>struct</span> <span>long</span> <span>do</span> <span>union</span> <span>signed</span> <span>while</span> <span>return</span> <span>unsigned</span> <span>break</span> <span>goto</span> <span>continue</span>

035 New Keywords Added in C99 Standard

Keyword	Usage
<span>_Bool</span>	Boolean type (requires inclusion of <span><stdbool.h></span>)
<span>_Complex</span>	Complex type (requires inclusion of <span><complex.h></span>)
<span>_Imaginary</span>	Imaginary type (requires inclusion of <span><complex.h></span>)
<span>inline</span>	Inline function declaration
<span>restrict</span>	Pointer qualifier (optimizes memory access)

047 New Keywords Added in C11 Standard

Keyword	Usage
<span>_Alignas</span>	Memory alignment specification
<span>_Alignof</span>	Get memory alignment value
<span>_Atomic</span>	Support for atomic operations
<span>_Generic</span>	Generic selection expression
<span>_Noreturn</span>	Marks a function that does not return
<span>_Static_assert</span>	Static assertion at compile time
<span>_Thread_local</span>	Thread-local storage

03Conclusion

//C89/C90
auto        break       case        char        const       continue
default     do          double      else        enum        extern
float       for         goto        if          int         long
register    return      short       signed      sizeof      static
struct      switch      typedef     union       unsigned    void
volatile    while

// C99 New Keywords
_Bool       _Complex    _Imaginary  inline      restrict

// C11 New Keywords
_Alignas    _Alignof    _Atomic     _Generic    _Noreturn
_Static_assert          _Thread_local

Notes:

1. Distinguishing Standards:

• C89/C90 is the classic standard, supported by all compilers.
• C99/C11 new keywords require compiler support (e.g., GCC uses <span><span>-std=c99</span></span> or <span><span>-std=c11</span></span> to enable).

Disabled Identifiers: Keywords cannot be used as variable names or function names (e.g., <span><span>int float;</span></span> is illegal).

Common Confusions:

• <span><span>main</span></span> is not a keyword, but a predefined function name.
• <span><span>true</span></span>/<span><span>false</span></span> are macros in <span><span><stdbool.h></span></span>, not keywords.
• __IO is not a keyword.
• __nop() is not a keyword.