if a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

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Computer Systems: A Programmer's Perspective says about two sections in a ELF relocatable object file:

• .data Initialized global and static C variables. Local C variables are maintained at run time on the stack and do not appear in
  either the .data or .bss sections.




• .bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero. This section
  occupies no actual space in the object file; it is merely a
  placeholder. Object file formats distinguish between initialized and
  uninitialized variables for space efficiency: unini- tialized variables
  do not have to occupy any actual disk space in the object file. At run
  time, these variables are allocated in memory with an initial value of
  zero.

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

if a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

The requirements for .data and for .bss are not mutually exclusive.
The .data requirement of "Initialized global and static C variables" doesn't say that such variables must be initialized to nonzero. So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

Thanks.

elf

share|improve this question

edited 4 hours ago

asked 4 hours ago

Tim

24.2k69233424

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Why don't you write a small C program, and test what happens?
  – dirkt
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  It doesn't matter if it's "implicitly" or "explicitly" initialized -- If it starts out as 0, it's in the bss.
  – mosvy
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  @mosvy .data is only for Initialized global and static C variables initialized to nonzero?
  – Tim
  4 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  that's the way it is by default. but you can force a variable into the .data section with eg. int var __attribute((section (".data")));.
  – mosvy
  1 hour ago
  

  
  
  
  

add a comment | 

up vote
1
down vote

favorite

Computer Systems: A Programmer's Perspective says about two sections in a ELF relocatable object file:

• .data Initialized global and static C variables. Local C variables are maintained at run time on the stack and do not appear in
  either the .data or .bss sections.




• .bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero. This section
  occupies no actual space in the object file; it is merely a
  placeholder. Object file formats distinguish between initialized and
  uninitialized variables for space efficiency: unini- tialized variables
  do not have to occupy any actual disk space in the object file. At run
  time, these variables are allocated in memory with an initial value of
  zero.

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

if a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

The requirements for .data and for .bss are not mutually exclusive.
The .data requirement of "Initialized global and static C variables" doesn't say that such variables must be initialized to nonzero. So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

Thanks.

elf

share|improve this question

edited 4 hours ago

asked 4 hours ago

Tim

24.2k69233424

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Why don't you write a small C program, and test what happens?
  – dirkt
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  It doesn't matter if it's "implicitly" or "explicitly" initialized -- If it starts out as 0, it's in the bss.
  – mosvy
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  @mosvy .data is only for Initialized global and static C variables initialized to nonzero?
  – Tim
  4 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  that's the way it is by default. but you can force a variable into the .data section with eg. int var __attribute((section (".data")));.
  – mosvy
  1 hour ago
  

  
  
  
  

add a comment | 

up vote
1
down vote

favorite

up vote
1
down vote

favorite

Computer Systems: A Programmer's Perspective says about two sections in a ELF relocatable object file:

• .data Initialized global and static C variables. Local C variables are maintained at run time on the stack and do not appear in
  either the .data or .bss sections.




• .bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero. This section
  occupies no actual space in the object file; it is merely a
  placeholder. Object file formats distinguish between initialized and
  uninitialized variables for space efficiency: unini- tialized variables
  do not have to occupy any actual disk space in the object file. At run
  time, these variables are allocated in memory with an initial value of
  zero.

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

if a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

The requirements for .data and for .bss are not mutually exclusive.
The .data requirement of "Initialized global and static C variables" doesn't say that such variables must be initialized to nonzero. So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

Thanks.

elf

share|improve this question

edited 4 hours ago

asked 4 hours ago

Tim

24.2k69233424

Computer Systems: A Programmer's Perspective says about two sections in a ELF relocatable object file:

• .data Initialized global and static C variables. Local C variables are maintained at run time on the stack and do not appear in
  either the .data or .bss sections.




• .bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero. This section
  occupies no actual space in the object file; it is merely a
  placeholder. Object file formats distinguish between initialized and
  uninitialized variables for space efficiency: unini- tialized variables
  do not have to occupy any actual disk space in the object file. At run
  time, these variables are allocated in memory with an initial value of
  zero.

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

if a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

The requirements for .data and for .bss are not mutually exclusive.
The .data requirement of "Initialized global and static C variables" doesn't say that such variables must be initialized to nonzero. So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

Thanks.

elf

elf

share|improve this question

edited 4 hours ago

asked 4 hours ago

Tim

24.2k69233424

share|improve this question

edited 4 hours ago

asked 4 hours ago

Tim

24.2k69233424

share|improve this question

share|improve this question

edited 4 hours ago

edited 4 hours ago

edited 4 hours ago

asked 4 hours ago

Tim

24.2k69233424

asked 4 hours ago

Tim

24.2k69233424

asked 4 hours ago

Tim

24.2k69233424

24.2k69233424

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Why don't you write a small C program, and test what happens?
  – dirkt
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  It doesn't matter if it's "implicitly" or "explicitly" initialized -- If it starts out as 0, it's in the bss.
  – mosvy
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  @mosvy .data is only for Initialized global and static C variables initialized to nonzero?
  – Tim
  4 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  that's the way it is by default. but you can force a variable into the .data section with eg. int var __attribute((section (".data")));.
  – mosvy
  1 hour ago
  

  
  
  
  

add a comment | 

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Why don't you write a small C program, and test what happens?
  – dirkt
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  It doesn't matter if it's "implicitly" or "explicitly" initialized -- If it starts out as 0, it's in the bss.
  – mosvy
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  @mosvy .data is only for Initialized global and static C variables initialized to nonzero?
  – Tim
  4 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  that's the way it is by default. but you can force a variable into the .data section with eg. int var __attribute((section (".data")));.
  – mosvy
  1 hour ago
  

  
  
  
  

Why don't you write a small C program, and test what happens?
– dirkt
4 hours ago

Why don't you write a small C program, and test what happens?
– dirkt
4 hours ago

1

1

It doesn't matter if it's "implicitly" or "explicitly" initialized -- If it starts out as 0, it's in the bss.
– mosvy
4 hours ago

It doesn't matter if it's "implicitly" or "explicitly" initialized -- If it starts out as 0, it's in the bss.
– mosvy
4 hours ago

@mosvy .data is only for Initialized global and static C variables initialized to nonzero?
– Tim
4 hours ago

@mosvy .data is only for Initialized global and static C variables initialized to nonzero?
– Tim
4 hours ago

that's the way it is by default. but you can force a variable into the .data section with eg. int var __attribute((section (".data")));.
– mosvy
1 hour ago

that's the way it is by default. but you can force a variable into the .data section with eg. int var __attribute((section (".data")));.
– mosvy
1 hour ago

add a comment | 

1 Answer
1

active

oldest

votes

up vote
2
down vote

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

“Initialised” means that the variable is given an explicit value (which might be zero) alongside its declaration in the code. “Uninitialised” means that the variable isn’t explicitly initialised at declaration time, which in your terminology corresponds to “implicitly initialised”; if it’s a global variable, its value is zero.

If a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables

Next:

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

Uninitialised global variables are zero-valued by default.

So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero.

Note that none of this is a requirement; in particular, it assumes that uninitialised data segments are initialised to values corresponding to zero in C on the platform. This is typically the case on systems using ELF, but C allows zero to be represented by a bit pattern other than all zeroes.

In practice however, global or static variables initialised to zero, explicitly or not, are part of .bss in ELF binaries.

share|improve this answer

edited 3 hours ago

answered 3 hours ago

Stephen Kitt

152k23337405

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Thanks. Is .data only for Initialized global and static C variables initialized to nonzero?
  – Tim
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  Yes, your quote itself says “any global or static variables that are initialized to zero” go in .bss, not .data. See the Wikipedia page on .bss for more background.
  – Stephen Kitt
  3 hours ago
  
  

  
  
  
  

add a comment | 

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1 Answer
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1 Answer
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oldest

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up vote
2
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Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

“Initialised” means that the variable is given an explicit value (which might be zero) alongside its declaration in the code. “Uninitialised” means that the variable isn’t explicitly initialised at declaration time, which in your terminology corresponds to “implicitly initialised”; if it’s a global variable, its value is zero.

If a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables

Next:

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

Uninitialised global variables are zero-valued by default.

So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero.

Note that none of this is a requirement; in particular, it assumes that uninitialised data segments are initialised to values corresponding to zero in C on the platform. This is typically the case on systems using ELF, but C allows zero to be represented by a bit pattern other than all zeroes.

In practice however, global or static variables initialised to zero, explicitly or not, are part of .bss in ELF binaries.

share|improve this answer

edited 3 hours ago

answered 3 hours ago

Stephen Kitt

152k23337405

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Thanks. Is .data only for Initialized global and static C variables initialized to nonzero?
  – Tim
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  Yes, your quote itself says “any global or static variables that are initialized to zero” go in .bss, not .data. See the Wikipedia page on .bss for more background.
  – Stephen Kitt
  3 hours ago
  
  

  
  
  
  

add a comment | 

up vote
2
down vote

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

“Initialised” means that the variable is given an explicit value (which might be zero) alongside its declaration in the code. “Uninitialised” means that the variable isn’t explicitly initialised at declaration time, which in your terminology corresponds to “implicitly initialised”; if it’s a global variable, its value is zero.

If a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables

Next:

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

Uninitialised global variables are zero-valued by default.

So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero.

Note that none of this is a requirement; in particular, it assumes that uninitialised data segments are initialised to values corresponding to zero in C on the platform. This is typically the case on systems using ELF, but C allows zero to be represented by a bit pattern other than all zeroes.

In practice however, global or static variables initialised to zero, explicitly or not, are part of .bss in ELF binaries.

share|improve this answer

edited 3 hours ago

answered 3 hours ago

Stephen Kitt

152k23337405

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Thanks. Is .data only for Initialized global and static C variables initialized to nonzero?
  – Tim
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  Yes, your quote itself says “any global or static variables that are initialized to zero” go in .bss, not .data. See the Wikipedia page on .bss for more background.
  – Stephen Kitt
  3 hours ago
  
  

  
  
  
  

add a comment | 

up vote
2
down vote

up vote
2
down vote

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

“Initialised” means that the variable is given an explicit value (which might be zero) alongside its declaration in the code. “Uninitialised” means that the variable isn’t explicitly initialised at declaration time, which in your terminology corresponds to “implicitly initialised”; if it’s a global variable, its value is zero.

If a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables

Next:

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

Uninitialised global variables are zero-valued by default.

So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero.

Note that none of this is a requirement; in particular, it assumes that uninitialised data segments are initialised to values corresponding to zero in C on the platform. This is typically the case on systems using ELF, but C allows zero to be represented by a bit pattern other than all zeroes.

In practice however, global or static variables initialised to zero, explicitly or not, are part of .bss in ELF binaries.

share|improve this answer

edited 3 hours ago

answered 3 hours ago

Stephen Kitt

152k23337405

Do "initialized" and "unitiailized" in the quote mean explicitly or implicitly or either?

“Initialised” means that the variable is given an explicit value (which might be zero) alongside its declaration in the code. “Uninitialised” means that the variable isn’t explicitly initialised at declaration time, which in your terminology corresponds to “implicitly initialised”; if it’s a global variable, its value is zero.

If a global or static C variable is not explicitly initialized, but implicitly initialized, is it in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables

Next:

Does it matter whether the global or static C variable is implicitly initialized to zero or nonzero?

Uninitialised global variables are zero-valued by default.

So if a global or static C variable is initialized to zero, should it be in .data or .bss section?

As per your quote:

.bss Uninitialized global and static C variables, along with any global or static variables that are initialized to zero.

Note that none of this is a requirement; in particular, it assumes that uninitialised data segments are initialised to values corresponding to zero in C on the platform. This is typically the case on systems using ELF, but C allows zero to be represented by a bit pattern other than all zeroes.

In practice however, global or static variables initialised to zero, explicitly or not, are part of .bss in ELF binaries.

share|improve this answer

edited 3 hours ago

answered 3 hours ago

Stephen Kitt

152k23337405

share|improve this answer

share|improve this answer

edited 3 hours ago

edited 3 hours ago

edited 3 hours ago

answered 3 hours ago

Stephen Kitt

152k23337405

answered 3 hours ago

Stephen Kitt

152k23337405

answered 3 hours ago

Stephen Kitt

152k23337405

152k23337405

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Thanks. Is .data only for Initialized global and static C variables initialized to nonzero?
  – Tim
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  Yes, your quote itself says “any global or static variables that are initialized to zero” go in .bss, not .data. See the Wikipedia page on .bss for more background.
  – Stephen Kitt
  3 hours ago
  
  

  
  
  
  

add a comment | 

• 
  
  
  

  
  

  
  
  
  
  
  

  
  Thanks. Is .data only for Initialized global and static C variables initialized to nonzero?
  – Tim
  3 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  Yes, your quote itself says “any global or static variables that are initialized to zero” go in .bss, not .data. See the Wikipedia page on .bss for more background.
  – Stephen Kitt
  3 hours ago
  
  

  
  
  
  

Thanks. Is .data only for Initialized global and static C variables initialized to nonzero?
– Tim
3 hours ago

Thanks. Is .data only for Initialized global and static C variables initialized to nonzero?
– Tim
3 hours ago

Yes, your quote itself says “any global or static variables that are initialized to zero” go in .bss, not .data. See the Wikipedia page on .bss for more background.
– Stephen Kitt
3 hours ago

Yes, your quote itself says “any global or static variables that are initialized to zero” go in .bss, not .data. See the Wikipedia page on .bss for more background.
– Stephen Kitt
3 hours ago

add a comment | 

 

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