Statements
Statements are fragments of the C program that are executed in sequence. The body of any function is a compound statement, which, in turn is a sequence of statements and declarations:
int main(void) { // start of a compound statement int n = 1; // declaration (not a statement) n = n+1; // expression statement printf("n = %d\n", n); // expression statement return 0; // return statement } // end of compound statement, end of function body
There are five types of statements:
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An attribute specifier sequence (attr-spec-seq) can be applied to an unlabeled statement, in which case (except for an expression statement) the attributes are applied to the respective statement. |
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Labels
Any statement can be labeled, by providing a name followed by a colon before the statement itself.
attr-spec-seq(optional)(since C23) identifier :
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attr-spec-seq(optional)(since C23) case constant-expression :
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attr-spec-seq(optional)(since C23) default :
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Any statement (but not a declaration) may be preceded by any number of labels, each of which declares identifier to be a label name, which must be unique within the enclosing function (in other words, label names have function scope).
Label declaration has no effect on its own, does not alter the flow of control, or modify the behavior of the statement that follows in any way.
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A label shall be followed by a statement. |
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A label can appear without its following statement. If a label appears alone in a block, it behaves as if it is followed by a null statement. The optional attr-spec-seq is applied to the label. |
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Compound statements
A compound statement, or block, is a brace-enclosed sequence of statements and declarations.
{ statement | declaration...(optional) }
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attr-spec-seq(optional) { unlabeled-statement | label | declaration...(optional) }
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The compound statement allows a set of declarations and statements to be grouped into one unit that can be used anywhere a single statement is expected (for example, in an if statement or an iteration statement):
if (expr) // start of if-statement { // start of block int n = 1; // declaration printf("%d\n", n); // expression statement } // end of block, end of if-statement
Each compound statement introduces its own block scope.
The initializers of the variables with automatic storage duration declared inside a block and the VLA declarators are executed when flow of control passes over these declarations in order, as if they were statements:
int main(void) { // start of block { // start of block puts("hello"); // expression statement int n = printf("abc\n"); // declaration, prints "abc", stores 4 in n int a[n*printf("1\n")]; // declaration, prints "1", allocates 8*sizeof(int) printf("%zu\n", sizeof(a)); // expression statement } // end of block, scope of n and a ends int n = 7; // n can be reused }
Expression statements
An expression followed by a semicolon is a statement.
expression(optional) ;
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attr-spec-seq expression ;
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(2) | (since C23) | |||||||
Most statements in a typical C program are expression statements, such as assignments or function calls.
An expression statement without an expression is called a null statement. It is often used to provide an empty body to a for or while loop. It can also be used to carry a label in the end of a compound statement or before a declaration:
puts("hello"); // expression statement char *s; while (*s++ != '\0') ; // null statement
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The optional attr-spec-seq is applied to the expression. An attr-spec-seq followed by |
(since C23) |
Selection statements
The selection statements choose between one of several statements depending on the value of an expression.
attr-spec-seq(optional)(since C23) if ( expression ) statement
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attr-spec-seq(optional)(since C23) if ( expression ) statement else statement
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attr-spec-seq(optional)(since C23) switch ( expression ) statement
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Iteration statements
The iteration statements repeatedly execute a statement.
attr-spec-seq(optional)(since C23) while ( expression ) statement
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attr-spec-seq(optional)(since C23) do statement while ( expression ) ;
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attr-spec-seq(optional)(since C23) for ( init-clause ; expression(optional) ; expression(optional) ) statement
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Jump statements
The jump statements unconditionally transfer flow control.
attr-spec-seq(optional)(since C23) break ;
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attr-spec-seq(optional)(since C23) continue ;
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attr-spec-seq(optional)(since C23) return expression(optional) ;
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attr-spec-seq(optional)(since C23) goto identifier ;
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References
- C17 standard (ISO/IEC 9899:2018):
- 6.8 Statements and blocks (p: 106-112)
- C11 standard (ISO/IEC 9899:2011):
- 6.8 Statements and blocks (p: 146-154)
- C99 standard (ISO/IEC 9899:1999):
- 6.8 Statements and blocks (p: 131-139)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.6 STATEMENTS
See also
| C++ documentation for Statements
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