call function in struct initialization

Consider the following snippet:

struct my_struct {

     int a;

     int b;

     int c;

};





void my_func(unsigned long x)

{

    struct my_struct m = {

         { 0, 1, 2 },

         { 11, 22, htonl(x) },

         { 0x1, 0xa, 0xbb }

    };

    ...

}

Is it legal/portable to call a function inside of a structure initialization block?

asked Nov 24 '18 at 1:16

Mark

1,68242658

Do you mean in general or just for automatic variables as in your snippet? What does your standard compliant compiler say? What do you expect? Why?

– too honest for this site
Nov 24 '18 at 1:39

This compiles cleanly [under gcc] as you've presented it. However, if you made m a global (file scope variable), it will not. The compiler flags htonl: error: initializer element is not constant. I believe most C compilers will behave in a similar manner. Side note: C++ does allow this at global scope [in your example, it warns about ntonl being unsigned vs the signed int c, but this would be easy enough to fix]

– Craig Estey
Nov 24 '18 at 1:52

add a comment |

Consider the following snippet:

struct my_struct {

     int a;

     int b;

     int c;

};





void my_func(unsigned long x)

{

    struct my_struct m = {

         { 0, 1, 2 },

         { 11, 22, htonl(x) },

         { 0x1, 0xa, 0xbb }

    };

    ...

}

Is it legal/portable to call a function inside of a structure initialization block?

asked Nov 24 '18 at 1:16

Mark

1,68242658

Do you mean in general or just for automatic variables as in your snippet? What does your standard compliant compiler say? What do you expect? Why?

– too honest for this site
Nov 24 '18 at 1:39

This compiles cleanly [under gcc] as you've presented it. However, if you made m a global (file scope variable), it will not. The compiler flags htonl: error: initializer element is not constant. I believe most C compilers will behave in a similar manner. Side note: C++ does allow this at global scope [in your example, it warns about ntonl being unsigned vs the signed int c, but this would be easy enough to fix]

– Craig Estey
Nov 24 '18 at 1:52

add a comment |

Consider the following snippet:

struct my_struct {

     int a;

     int b;

     int c;

};





void my_func(unsigned long x)

{

    struct my_struct m = {

         { 0, 1, 2 },

         { 11, 22, htonl(x) },

         { 0x1, 0xa, 0xbb }

    };

    ...

}

Is it legal/portable to call a function inside of a structure initialization block?

asked Nov 24 '18 at 1:16

Mark

1,68242658

Consider the following snippet:

struct my_struct {

     int a;

     int b;

     int c;

};





void my_func(unsigned long x)

{

    struct my_struct m = {

         { 0, 1, 2 },

         { 11, 22, htonl(x) },

         { 0x1, 0xa, 0xbb }

    };

    ...

}

Is it legal/portable to call a function inside of a structure initialization block?

c function struct

asked Nov 24 '18 at 1:16

Mark

1,68242658

asked Nov 24 '18 at 1:16

Mark

1,68242658

asked Nov 24 '18 at 1:16

Mark

1,68242658

asked Nov 24 '18 at 1:16

Mark

1,68242658

asked Nov 24 '18 at 1:16

Mark

1,68242658

Do you mean in general or just for automatic variables as in your snippet? What does your standard compliant compiler say? What do you expect? Why?

– too honest for this site
Nov 24 '18 at 1:39

This compiles cleanly [under gcc] as you've presented it. However, if you made m a global (file scope variable), it will not. The compiler flags htonl: error: initializer element is not constant. I believe most C compilers will behave in a similar manner. Side note: C++ does allow this at global scope [in your example, it warns about ntonl being unsigned vs the signed int c, but this would be easy enough to fix]

– Craig Estey
Nov 24 '18 at 1:52

add a comment |

Do you mean in general or just for automatic variables as in your snippet? What does your standard compliant compiler say? What do you expect? Why?

– too honest for this site
Nov 24 '18 at 1:39

This compiles cleanly [under gcc] as you've presented it. However, if you made m a global (file scope variable), it will not. The compiler flags htonl: error: initializer element is not constant. I believe most C compilers will behave in a similar manner. Side note: C++ does allow this at global scope [in your example, it warns about ntonl being unsigned vs the signed int c, but this would be easy enough to fix]

– Craig Estey
Nov 24 '18 at 1:52

Do you mean in general or just for automatic variables as in your snippet? What does your standard compliant compiler say? What do you expect? Why?

– too honest for this site
Nov 24 '18 at 1:39

This compiles cleanly [under gcc] as you've presented it. However, if you made m a global (file scope variable), it will not. The compiler flags htonl: error: initializer element is not constant. I believe most C compilers will behave in a similar manner. Side note: C++ does allow this at global scope [in your example, it warns about ntonl being unsigned vs the signed int c, but this would be easy enough to fix]

– Craig Estey
Nov 24 '18 at 1:52

add a comment |

2 Answers
2

active

oldest

votes

Is it legal/portable to call a function inside of a structure initialization block?

Initialization ($6.7.8/1)

[...]

|     initializer-list:

|             designationopt initializer

|             initializer-list , designationopt initializer

~> initializer-lists consist of initializers

Initialization ($6.7.8/4)

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals

~> Initializers consist of expressions. (constant-expr. for objects with static storage duration)

Expressions (§6.5/1):

An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof.

~> A function call is an expression.

edited Nov 24 '18 at 2:24

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

1

We be sticking to the code...

– David C. Rankin
Nov 24 '18 at 2:20

@DavidC.Rankin Sorry, what do you mean?

– Swordfish
Nov 24 '18 at 2:22

Sorry, that's the funny line for "Pirates of the Caribbean" that fit with the good cites to the "code".

– David C. Rankin
Nov 24 '18 at 2:36

@DavidC.Rankin oh, legal-code. Now I get it. Yes, I'm a lawyer at heart ;)

– Swordfish
Nov 24 '18 at 2:41

add a comment |

Yes, it is legal, as long as you are initializing an object with automatic storage duration (as in your example). For objects with static storage duration that would not be legal, since such objects allow only constant expressions in their initializers.

Also keep in mind though that in C evaluations of initializer expressions are indeterminately sequenced with respect to one another. Which means that if you have multiple function calls among your initializers and these functions' results depend on some shared state, these initalizers might behave unpredictably

int foo()

{

  static int a;

  return ++a;

}



int main()

{

  struct { int x, y; } s = { foo(), foo() };

  /* Can be `{ 1, 2 }` or `{ 2, 1 }`... */

}

With regard to portability, one can note that C89/90 did not permit this (formally in C89/90 all {}-enclosed initializers has to be constant expressions, even for automatic objects), but most popular C89/90 compilers supported this regardless.

edited Nov 24 '18 at 4:35

answered Nov 24 '18 at 2:04

AnT

258k32411655

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

Is it legal/portable to call a function inside of a structure initialization block?

Initialization ($6.7.8/1)

[...]

|     initializer-list:

|             designationopt initializer

|             initializer-list , designationopt initializer

~> initializer-lists consist of initializers

Initialization ($6.7.8/4)

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals

~> Initializers consist of expressions. (constant-expr. for objects with static storage duration)

Expressions (§6.5/1):

An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof.

~> A function call is an expression.

edited Nov 24 '18 at 2:24

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

1

We be sticking to the code...

– David C. Rankin
Nov 24 '18 at 2:20

@DavidC.Rankin Sorry, what do you mean?

– Swordfish
Nov 24 '18 at 2:22

Sorry, that's the funny line for "Pirates of the Caribbean" that fit with the good cites to the "code".

– David C. Rankin
Nov 24 '18 at 2:36

@DavidC.Rankin oh, legal-code. Now I get it. Yes, I'm a lawyer at heart ;)

– Swordfish
Nov 24 '18 at 2:41

add a comment |

Is it legal/portable to call a function inside of a structure initialization block?

Initialization ($6.7.8/1)

[...]

|     initializer-list:

|             designationopt initializer

|             initializer-list , designationopt initializer

~> initializer-lists consist of initializers

Initialization ($6.7.8/4)

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals

~> Initializers consist of expressions. (constant-expr. for objects with static storage duration)

Expressions (§6.5/1):

An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof.

~> A function call is an expression.

edited Nov 24 '18 at 2:24

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

1

We be sticking to the code...

– David C. Rankin
Nov 24 '18 at 2:20

@DavidC.Rankin Sorry, what do you mean?

– Swordfish
Nov 24 '18 at 2:22

Sorry, that's the funny line for "Pirates of the Caribbean" that fit with the good cites to the "code".

– David C. Rankin
Nov 24 '18 at 2:36

@DavidC.Rankin oh, legal-code. Now I get it. Yes, I'm a lawyer at heart ;)

– Swordfish
Nov 24 '18 at 2:41

add a comment |

Is it legal/portable to call a function inside of a structure initialization block?

Initialization ($6.7.8/1)

[...]

|     initializer-list:

|             designationopt initializer

|             initializer-list , designationopt initializer

~> initializer-lists consist of initializers

Initialization ($6.7.8/4)

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals

~> Initializers consist of expressions. (constant-expr. for objects with static storage duration)

Expressions (§6.5/1):

An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof.

~> A function call is an expression.

edited Nov 24 '18 at 2:24

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

Is it legal/portable to call a function inside of a structure initialization block?

Initialization ($6.7.8/1)

[...]

|     initializer-list:

|             designationopt initializer

|             initializer-list , designationopt initializer

~> initializer-lists consist of initializers

Initialization ($6.7.8/4)

All the expressions in an initializer for an object that has static storage duration shall be constant expressions or string literals

~> Initializers consist of expressions. (constant-expr. for objects with static storage duration)

Expressions (§6.5/1):

An expression is a sequence of operators and operands that specifies computation of a value, or that designates an object or a function, or that generates side effects, or that performs a combination thereof.

~> A function call is an expression.

edited Nov 24 '18 at 2:24

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

edited Nov 24 '18 at 2:24

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

answered Nov 24 '18 at 2:02

Swordfish

9,05211335

1

We be sticking to the code...

– David C. Rankin
Nov 24 '18 at 2:20

@DavidC.Rankin Sorry, what do you mean?

– Swordfish
Nov 24 '18 at 2:22

Sorry, that's the funny line for "Pirates of the Caribbean" that fit with the good cites to the "code".

– David C. Rankin
Nov 24 '18 at 2:36

@DavidC.Rankin oh, legal-code. Now I get it. Yes, I'm a lawyer at heart ;)

– Swordfish
Nov 24 '18 at 2:41

add a comment |

1

We be sticking to the code...

– David C. Rankin
Nov 24 '18 at 2:20

@DavidC.Rankin Sorry, what do you mean?

– Swordfish
Nov 24 '18 at 2:22

Sorry, that's the funny line for "Pirates of the Caribbean" that fit with the good cites to the "code".

– David C. Rankin
Nov 24 '18 at 2:36

@DavidC.Rankin oh, legal-code. Now I get it. Yes, I'm a lawyer at heart ;)

– Swordfish
Nov 24 '18 at 2:41

We be sticking to the code...

– David C. Rankin
Nov 24 '18 at 2:20

@DavidC.Rankin Sorry, what do you mean?

– Swordfish
Nov 24 '18 at 2:22

Sorry, that's the funny line for "Pirates of the Caribbean" that fit with the good cites to the "code".

– David C. Rankin
Nov 24 '18 at 2:36

@DavidC.Rankin oh, legal-code. Now I get it. Yes, I'm a lawyer at heart ;)

– Swordfish
Nov 24 '18 at 2:41

add a comment |

int foo()

{

  static int a;

  return ++a;

}



int main()

{

  struct { int x, y; } s = { foo(), foo() };

  /* Can be `{ 1, 2 }` or `{ 2, 1 }`... */

}

edited Nov 24 '18 at 4:35

answered Nov 24 '18 at 2:04

AnT

258k32411655

add a comment |

int foo()

{

  static int a;

  return ++a;

}



int main()

{

  struct { int x, y; } s = { foo(), foo() };

  /* Can be `{ 1, 2 }` or `{ 2, 1 }`... */

}

edited Nov 24 '18 at 4:35

answered Nov 24 '18 at 2:04

AnT

258k32411655

add a comment |

int foo()

{

  static int a;

  return ++a;

}



int main()

{

  struct { int x, y; } s = { foo(), foo() };

  /* Can be `{ 1, 2 }` or `{ 2, 1 }`... */

}

edited Nov 24 '18 at 4:35

answered Nov 24 '18 at 2:04

AnT

258k32411655

int foo()

{

  static int a;

  return ++a;

}



int main()

{

  struct { int x, y; } s = { foo(), foo() };

  /* Can be `{ 1, 2 }` or `{ 2, 1 }`... */

}

edited Nov 24 '18 at 4:35

answered Nov 24 '18 at 2:04

AnT

258k32411655

edited Nov 24 '18 at 4:35

answered Nov 24 '18 at 2:04

AnT

258k32411655

answered Nov 24 '18 at 2:04

AnT

258k32411655

answered Nov 24 '18 at 2:04

AnT

258k32411655

add a comment |

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