differences in array initialization(char, string, other) regarding storage duration

Question

In this question it was said in the comments:

char arr[10] = { 'H', 'e', 'l', 'l', 'o', '\0'}; and char arr[10] = "Hello"; are strictly the same thing. – Michael Walz

This got me thinking.

I know that "Hello" is string literal. String literals are stored with static storage duraction and are immutable.

But if both are are really the same then char arr[10] = { 'H', 'e', 'l', 'l', 'o', '\0'}; would also create a similar string literal with.

Does char b[10]= {72, 101, 108, 108, 111, 0}; also create a "string" literal with static storage duration? Because theoretically it is the same thing.

char a = 'a'; is the same thing as char a; ...; a = 'a';, so your thoughts are correct 'a' is simply written to a

Are there differences between:

• char a = 'a';
• char a = {'a'};

How/where are the differences defined?

EDIT: I see that I haven't made it clear enough that I am particularly interested in the memory usage/storage duration of the literals. I will leave the question as it is, but would like to make the emphasis of the question more clear in this edit.

Answer 1

I know that "Hello" is string literal. String literals are stored with static storage duraction and are immutable.

Yes, but string literals are also a grammatical item in the C language. char arr[10] = { 'H', 'e', 'l', 'l', 'o', '\0'}; is not a string literal, it is an initializer list. The initializer list does however behave as if it has static storage duration, remaining elements after the explicit \0 are set to zero etc.

The initializer list itself is stored in some manner of ROM memory. If your variable arr has static storage duration too, it will get allocated in the .data segment and initialized from the ROM init list before the program is started. If arr has automatic storage duration (local), then it is initialized from ROM in run-time, when the function containing arr is called.

The ROM memory where the initializer list is stored may or may not be the same ROM memory as used for string literals. Often there's a segment called .rodata where these things end up, but they may as well end up in some other segment, such as the code segment .text.

Compilers like to store string literals in a particular memory segment, because that means that they can perform an optimization called "string pooling". Meaning that if you have the string literal "Hello" several times in your program, the compiler will use the same memory location for it. It may not necessarily do this same optimization for initializer lists.

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Regarding 'a' versus {'a'} in an initializer list, that's just a syntax hiccup in the C language. C11 6.7.6/11:

The initializer for a scalar shall be a single expression, optionally enclosed in braces. The initial value of the object is that of the expression (after conversion); the same type constraints and conversions as for simple assignment apply,

In plain English, this means that a "non-array" (scalar) can be either initialized with or without braces, it has the same meaning. Apart from that, the same rules as for regular assignment apply.

Answer 2

I know that "Hello" is string literal. String literals are stored with static storage duraction and are immutable.

Yes. But with char arr[10] = "Hello";, you are copying the string literal to an array arr and there's no need to "keep" the string literal. So if an implementation chooses to do remove the string literal altogether after copying it to arr and that's totally valid.

But if both are are really the same then char arr[10] = { 'H', 'e', 'l', 'l', 'o', '\0'}; would also create a similar string literal.

Again there's no need to make/store a string literal for this.

Only if you directly have a pointer to a string literal, it'd be usually stored somewhere such as: char *string = "Hello, world!\n";

Even then an implementation can choose not to do so under the "as-if" rule. E.g.,

#include <stdio.h>
#include <string.h>

static const char *str = "Hi";
int main(void)
{
   char arr[10];
   strcpy(arr, str);
   puts(arr);
}

"Hi" can be eliminated because it's used only for copying it into arr and isn't accessed directly anywhere. So eliminating the string literal (and the strcpy call too) as if you had "char arr[10] = "Hi"; and wouldn't affect the observable behaviour.

Basically the C standard doesn't necessitate a string literal has to be stored anywhere as long as the properties associated with a string literal are satisfied.

Are there differences between: char a = 'a'; char a = {'a'}; How/where are the differences defined?

Yes. C11, 6.7.9 says:

The initializer for a scalar shall be a single expression, optionally enclosed in braces. [..]

Per the syntax, even: char c = {'a',}; is valid and equivalent too (though I wouldn't recommend this :).

Answer 3

In the abstract machine, char arr[10] = "Hello"; means that arr is initialized by copying data from the string literal "Hello" which has its own existence elsewhere; whereas the other version just has initial values like any other variable -- there is no string literal involved.

However, the observable behaviour of both versions is identical: there is created arr with values set as specified. This is what the other poster meant by the code being identical; according to the Standard, two programs are the same if they have the same observable behaviour. Compilers are allowed to generate the same assembly for both versions.

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Your second question is entirely separate to the first; but char a = 'a'; and char a = {'a'}; are identical. A single initializer may optionally be enclosed in braces.

Answer 4

I belive your question is highly implementation dependant (HW and compiler wise). However, in general: arrays are placed in RAM, let it be global or not.

I know that "Hello" is string literal. String literals are stored with static storage duraction and are immutable.

Yes this saves the string "Hello" in ROM (read only memory). Your array is loaded the literal in runtime.

But if both are are really the same then char arr[10] = { 'H', 'e', 'l', 'l', 'o', '\0'}; would also create a similar string literal.

Yes but in this case the single characters are placed in ROM. The array you are initialized is loaded with character literals in runtime.

Does char b[10]= {72, 101, 108, 108, 111, 0}; also create a "string" literal with static storage duration? Because theoretically it is the same thing.

If you use UTF-8, then yes, since char == uint8_t and those are the values.

Are there differences between: char a = 'a'; char a = {'a'}; How/where are the differences defined?

I believe not.

In reply to edit

Do you mean the lifetime of storage of string literals? Have a look at this.

So a string literal has static storage duration. It remains throughout the lifetime of the program, hardcoded in memory.