#include <stdio.h>
#include <stdlib.h>
// Global variable
int globalVar = 10;
// Static global variable
static int staticGlobalVar = 20;
int main() {
// Local variable
int localVar = 5;
// Static local variable
static int staticLocalVar = 15;
// Dynamic memory allocation using pointers
int *dynamicVar = (int *)malloc(sizeof(int));
if (dynamicVar == NULL) {
printf("Memory allocation failed.\n");
return 1;
}
*dynamicVar = 25;
// Extern variable (defined in another source file)
extern int externVar;
// Pointer variable
int *pointerVar;
pointerVar = &globalVar;
printf("Global variable: %d\n", globalVar);
printf("Static global variable: %d\n", staticGlobalVar);
printf("Local variable: %d\n", localVar);
printf("Static local variable: %d\n", staticLocalVar);
printf("Dynamic variable: %d\n", *dynamicVar);
printf("Extern variable: %d\n", externVar);
printf("Pointer variable (points to globalVar): %d\n", *pointerVar);
// Free dynamically allocated memory
free(dynamicVar);
return 0;
}I'm trying to wrap my head around the concept of memory allocation at compile time versus run time. My understanding is that memory for variables isn't allocated until the program runs on a PC. I belive that memory allocation happens at runtime. However, I'm confused about how memory allocation can occur at compile time when the program doesn't actually run on hardware(PC)
...
Could you please clarify this concept of memory allocation in C programs?
I'm trying to wrap my head around the concept of memory allocation at compile time versus run time. My understanding is that memory for variables isn't allocated until the program runs on a PC. I belive that memory allocation happens at runtime. However, I'm confused about how memory allocation can occur at compile time when the program doesn't actually run on hardware(PC)
...
Could you please clarify this concept of memory allocation in C programs?
Yet another poor question :( See https://entertaininghacks.wordpress.com/library-2/good-questions-pique-our-interest-and-dont-waste-our-time-2/
Your code example is useless, because you have just copied it from somewhere. You need to explain what you think will happen; only after that can anybody here help you correct your misunderstanding.
Yes, we can help: this is the same for every computer language, and is well described in textbooks. Textbooks will explain this subject better than quickly scribbled responses on a forum.
Textbooks about C are widely available in India, in many of the languages used in India.
However, I'm confused about how memory allocation can occur at compile time when the program doesn't actually run on hardware(PC)https://devconnected.com/understanding-processes-on-linux/ (https://devconnected.com/understanding-processes-on-linux/)
The C language supports two kinds of memory allocation through the variables in C programs:
- Static allocation is what happens when you declare a static or global variable. Each static or global variable defines one block of space, of a fixed size. The space is allocated once, when your program is started (part of the exec operation), and is never freed.
- Automatic allocation happens when you declare an automatic variable, such as a function argument or a local variable. The space for an automatic variable is allocated when the compound statement containing the declaration is entered, and is freed when that compound statement is exited.
In GNU C, the size of the automatic storage can be an expression that varies. In other C implementations, it must be a constant.
A third important kind of memory allocation, dynamic allocation, is not supported by C variables but is available via GNU C Library functions.
I'm trying to wrap my head around the concept of memory allocation at compile time versus run time. My understanding is that memory for variables isn't allocated until the program runs on a PC. I belive that memory allocation happens at runtime. However, I'm confused about how memory allocation can occur at compile time when the program doesn't actually run on hardware(PC)
...
Could you please clarify this concept of memory allocation in C programs?
Yet another poor question :( See https://entertaininghacks.wordpress.com/library-2/good-questions-pique-our-interest-and-dont-waste-our-time-2/
Your code example is useless, because you have just copied it from somewhere. You need to explain what you think will happen; only after that can anybody here help you correct your misunderstanding.
Yes, we can help: this is the same for every computer language, and is well described in textbooks. Textbooks will explain this subject better than quickly scribbled responses on a forum.
Textbooks about C are widely available in India, in many of the languages used in India.
Thank you for taking the time to respond to my question, and I appreciate your feedback. In my studies of memory allocation in C programs, I've come across the concept of memory allocation happening both at compile time and run time. My confusion arises from the fact that during compilation, the program isn't actually executed on hardware. So, I'm trying to undrstand how memory allocation can occur at compile time when there's no physical execution of the program.
The code I shared was meant to serve as an initial reference point to frame my question regarding memory allocation. Specifically, I intended to gain a better understanding of how memory allocation works in Compile Time vs. Run Time
#include <stdio.h>
#include <stdlib.h>
/* Global initialized variable in the Data Segment */
int global_initialized_var = 42;
/* Global uninitialized variable in the Data Segment (BSS) */
int global_uninitialized_var;
/* Static variable in the Data Segment */
static int static_var = 30;
void stack_example() {
/* Local variable in the Stack */
int local_var = 10;
printf("Local Variable in Stack: %d\n", local_var);
}
int main() {
/* Text Segment: CPU-executable code */
printf("Hello from the Text Segment!\n");
/* Accessing Initialized Data in Data Segment */
printf("Initialized Global Variable: %d\n", global_initialized_var);
/* Accessing Uninitialized Data in Data Segment (BSS) */
printf("Uninitialized Global Variable: %d\n", global_uninitialized_var);
/* Accessing Static Data in Data Segment */
printf("Static Global Variable: %d\n", static_var);
/* Calling a function to demonstrate Stack usage */
stack_example();
/* Dynamic memory allocation in the Heap */
int *dynamic_var = (int *)malloc(sizeof(int));
*dynamic_var = 20;
printf("Dynamic Variable in Heap: %d\n", *dynamic_var);
free(dynamic_var);
return 0;
}
I have a basic understanding that memory allocation occurs when a program both compiles and runs on a PC. However, I'm trying to understand the concept of "compile-time allocation" and whether memory allocation actually happens when a program is compiled but not run.
Simple and correct answer: you have told the compiler to allocate memory for a variable, and the compiler has chosen where to allocate it.
You need to understand what a compiler (any compiler) is and what a compiler does. Concentrate on the correspondence between simple language statements and machine instructions.
Simple and correct answer: you have told the compiler to allocate memory for a variable, and the compiler has chosen where to allocate it.
You need to understand what a compiler (any compiler) is and what a compiler does. Concentrate on the correspondence between simple language statements and machine instructions.
correct me if i am wrong in my understanding. When we compile a C program, no actual memory is allocated for variables or functions. Instead, what happens is the compiler generates instructions for the program regarding how much memory should be allocated and where it should be allocated when the program is run.
This means that the allocation of memory for variables and functions occurs during the runtime of the program, not during the compilation phase. The role of the compiler during compilation is to provide instructions and information, which manages memory allocation on the hardware, about how to allocate memory for the various variables of program when it is executed
Simple and correct answer: you have told the compiler to allocate memory for a variable, and the compiler has chosen where to allocate it.
You need to understand what a compiler (any compiler) is and what a compiler does. Concentrate on the correspondence between simple language statements and machine instructions.
correct me if i am wrong in my understanding. When we compile a C program, no actual memory is allocated for variables or functions. Instead, what happens is the compiler generates instructions for the program regarding how much memory should be allocated and where it should be allocated when the program is run.
This means that the allocation of memory for variables and functions occurs during the runtime of the program, not during the compilation phase. The role of the compiler during compilation is to provide instructions and information, which manages memory allocation on the hardware, about how to allocate memory for the various variables of program when it is executed
extern int put( int );
int aStatic = 12;
int foo( int x ) {
int aLocal = 34;
return x + aLocal - aStatic;
}
void baz() {
int result = foo( 56 );
put(result & 0xff);
}
# Compilation provided by Compiler Explorer at https://godbolt.org/
foo: # @foo
push rbp
mov rbp, rsp
mov dword ptr [rbp - 4], edi
mov dword ptr [rbp - 8], 34
mov eax, dword ptr [rbp - 4]
add eax, dword ptr [rbp - 8]
sub eax, dword ptr [rip + aStatic]
pop rbp
ret
baz: # @baz
push rbp
mov rbp, rsp
sub rsp, 16
mov edi, 56
call foo
mov dword ptr [rbp - 4], eax
mov edi, dword ptr [rbp - 4]
and edi, 255
call put@PLT
add rsp, 16
pop rbp
ret
aStatic:
.long 12 # 0xc
# Compilation provided by Compiler Explorer at https://godbolt.org/
foo: # @foo
sub edi, dword ptr [rip + aStatic]
lea eax, [rdi + 34]
ret
baz: # @baz
mov eax, 90
sub eax, dword ptr [rip + aStatic]
movzx edi, al
jmp put@PLT # TAILCALL
aStatic:
.long 12 # 0xc
Why don't you have a look at the output of a compiler at godbolt.org? There you will see what the compiler has done.
Why don't you have a look at the output of a compiler at godbolt.org? There you will see what the compiler has done.
This is cool. Thanks.
I'm trying to wrap my head around the concept of memory allocation at compile time versus run time.
My understanding is that memory for variables isn't allocated until the program runs on a PC.
I believe that memory allocation happens at runtime.
However, I'm confused about how memory allocation can occur at compile time when the program doesn't actually run on hardware(PC)
I wonder if the OP will?
static vs. dynamic allocation would make more sense than "compile time" vs. "run time" allocation IMHO.
I tried the link, but it produced assembly code. I'm not familiar with assembly language, so I didn't comment on it. I know it's good to learn assembly language, but at this time, I am focusing on learning C language only
This link say https://www.geeksforgeeks.org/difference-between-static-and-dynamic-memory-allocation-in-c/ (https://www.geeksforgeeks.org/difference-between-static-and-dynamic-memory-allocation-in-c/) that static memory is allocated at compile time.
Suppose we've written a program that includes a global variable. When we compile this code on Windows PC, does the compiler allocate memory for the global variable.?
Or Instead, does it write instructions into the program's executable file that allocate memory for the global variable when it run on PC?
This link say https://www.geeksforgeeks.org/difference-between-static-and-dynamic-memory-allocation-in-c/ (https://www.geeksforgeeks.org/difference-between-static-and-dynamic-memory-allocation-in-c/) that static memory is allocated at compile time.
Suppose we've written a program that includes a global variable. When we compile this code on Windows PC, does the compiler allocate memory for the global variable.?
Yes.
It really depends on what you mean by those words.QuoteMy understanding is that memory for variables isn't allocated until the program runs on a PC.This understanding is wrong. You are mistaken.
This is semantics, and it depends on the meaning of the word allocate. To allocate can mean to select in advance, or to earmark, how something will be used. Hence my post in Reply #2 above. The rules of Monopoly allocate, or earmark, a certain amount of money to each of the players at the start of the game. This happens in the abstract, before any game has actually started.
In the same way, the compiler allocates, or earmarks, a certain amount of memory to store static variables. This happens before the program has even been loaded.
I wonder if the OP will?
I tried the link, but it produced assembly code. I'm not familiar with assembly language, so I didn't comment on it. I know it's good to learn assembly language, but at this time, I am focusing on learning C language only
You will fail to understand C or C++ unless you understand (1) how a processor operates and (2) interacts with its memory.
Technically, what thecomplierlinker allocates statically are addresses.
The only case of truly static allocation of memory is when you design a computer which always runs one particular software :D
You will fail to understand C or C++ unless you understand (1) how a processor operates and (2) interacts with its memory.
My understanding is that Computer hardware reads binary instructions from program memory to perform tasks. Program memory stores the instructions that make up a program, and these instructions guide the hardware in executing specific tasks. Data memory, on the other hand, is where the values of variables are stored.
When we declare a global or static variable in code, the compiler assigns a memory address to that variable, which serves as the location in data memory where the variable's (global or static ) value will be stored during program execution.
Once you understand how a processor and memory works, then it will be much easier for you to understand
Once you understand how a processor and memory works, then it will be much easier for you to understand
The CPU is responsible for executing program instructions stored in program memory. It fetches, decodes, and executes these instructions (refrence 8051 archtecture), using registers and other components to manipulate data and perform operations. Program memory holds the program instructions,
Your question mainly asks the difference between static, stack and dynamic memory allocation.
I understand some concepts are hard to get by, by others like this one only take some reading!
There're thousands of sites explaining this. Have you even tried to google it?
I've asked questions because I'm genuinely curious and eager to understand a particular concept. English is not my first language, so I used AI to format my question. However, I know that ChatGPT can't replace the real-time work experience and expertise that individuals in the industry have earned through their careers.
I'd like to clarify that DavidAlfa mention what i need to ask. Just to clarify, Now I don't use AI to draft questions; I simply ask questions in my own version of English.Your question mainly asks the difference between static, stack and dynamic memory allocation.
I understand some concepts are hard to get by, by others like this one only take some reading!
There're thousands of sites explaining this. Have you even tried to google it?
Yes, unlikely, and it has already been suggested :(
The OP has admitted to using ChatGPT to write his questions.
There're thousands of sites explaining this. Have you even tried to google it?I've done quite a bit of research on this topic, and I've also included a reference link in my previous post (#17) FYI
I'd like to clarify that DavidAlfa mention what i need to ask. Just to clarify, Now I don't use AI to draft questions; I simply ask questions in my own version of English.Your question mainly asks the difference between static, stack and dynamic memory allocation.
I understand some concepts are hard to get by, by others like this one only take some reading!
There're thousands of sites explaining this. Have you even tried to google it?
Yes, unlikely, and it has already been suggested :(
The OP has admitted to using ChatGPT to write his questions.There're thousands of sites explaining this. Have you even tried to google it?I've done quite a bit of research on this topic, and I've also included a reference link in my previous post (#17) FYI
I wonder if the OP will?
I tried the link, but it produced assembly code. I'm not familiar with assembly language, so I didn't comment on it. I know it's good to learn assembly language, but at this time, I am focusing on learning C language only
I wonder if the OP will?
I tried the link, but it produced assembly code. I'm not familiar with assembly language, so I didn't comment on it. I know it's good to learn assembly language, but at this time, I am focusing on learning C language only
I'm going to go somewhat against what people have said that you "need" to understand how processors work at a machine level to understand C. However you are asking a question that goes beyond the scope of just the language. All C requires is that the implementation make global and static variable lifetime for the entire duration of the program. How and where that happens is fundamentally a machine question and if you won't take "it just does" for an answer you are going to need to learn about how actual machines operate.
Intel 51 is one of the architecture that you d best program in assembly.
Some aspects of C really cannot be understood without understanding how processors+memory work. Even "experts" have had problems with that concept, as demonstrated by Hans Boehm (of the conservative GC fame) needing to write "Threads cannot be implemented as a library" in exhaustive detail http://hboehm.info/misc_slides/pldi05_threads.pdf (http://hboehm.info/misc_slides/pldi05_threads.pdf)
The OP is targeting the 8051. Given how hostile that is to plain-vanilla C without extensions, good luck getting C code to work on that without understanding the 8051's arhitectural features.
Some aspects of C really cannot be understood without understanding how processors+memory work. Even "experts" have had problems with that concept, as demonstrated by Hans Boehm (of the conservative GC fame) needing to write "Threads cannot be implemented as a library" in exhaustive detail http://hboehm.info/misc_slides/pldi05_threads.pdf (http://hboehm.info/misc_slides/pldi05_threads.pdf)
I look at that in exactly the opposite way. Understanding the machine doesn't really help you here. Actually it can and does lead you astray,
Intel 51 is one of the architecture that you d best program in assembly.
Intel 51 is one of the architecture that you d best program in assembly.
It's not pretty but I've used C for 8051-based MCUs (like the Cypress FX stuff) using SDCC and it was perfectly usable once you got familiar with the limitations.
Some aspects of C really cannot be understood without understanding how processors+memory work. Even "experts" have had problems with that concept, as demonstrated by Hans Boehm (of the conservative GC fame) needing to write "Threads cannot be implemented as a library" in exhaustive detail http://hboehm.info/misc_slides/pldi05_threads.pdf (http://hboehm.info/misc_slides/pldi05_threads.pdf)
I look at that in exactly the opposite way. Understanding the machine doesn't really help you here. Actually it can and does lead you astray, since the guarantees that the hardware makes don't matter if the compiler doesn't expose them to the programmer in a usable fashion. For instance, even recently I have argued against to people still using volatile for multi-thread synchronization that they shouldn't do that because it is incorrect. They usually claim something like "I only care about x86 and this code is correct under x86 memory order guarantees." That would be fine if they were using assembly, but it's not correct when writing C (or C++) because the C memory model is not the same as the architectural memory model.
C and C++ needed a memory model to enable multi-threaded programming, and knowledge of the underlying architecture and it's memory model *was not* sufficient to produce correct multi-threaded code without it.
QuoteThe OP is targeting the 8051. Given how hostile that is to plain-vanilla C without extensions, good luck getting C code to work on that without understanding the 8051's arhitectural features.
That's a fair point, although at that point you have to debate whether you are programming C or "a C-like language"
If you want much lower level than C, use assembly. Conversely, if the high-level aspects of C are not high-level enough to you or for implementing a given piece of software - at least according to your standards - use another language as well.