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Special Forums UNIX Desktop Questions & Answers Knowing the size and location of variables in a C program Post 302833651 by Cambria on Wednesday 17th of July 2013 02:39:12 PM
Old 07-17-2013
Question Knowing the size and location of variables in a C program

So I need some help with this. Pardon me if I'm posting in the wrong forum, after some googling for my answer and finding nothing I found this forum. It seemed appropriate for what I was seeking. I just didnt find a forum that concerned the use of GDB. I'm learning to use the C language and GDB. What I don't understand is how the computer knows how big each piece of a program is in memory, and how I could find my variable's in memory using GDB.

For example how does the computer know that the disassembled instructions from main() are <main+##>? Is there a flag between each variable in memory on the stack? Or does the CPU reference the text segment with the variable in memory to know where a variable begins and ends?

I mean if all memory is numbered how can anyone including the CPU know where a word or giant or w/e starts and ends?

If I wanted to find my variable in memory after setting a break point in it and accessing the $esp register how would I know where my variables began and ended?

When I use the examine command "x" I don't know how to know where my variable begins and ends. Would it be the $ESP register on the stack minus the word size of my variable? $EIP shows how many bytes from main and the previous instruction when you disassemble something but everything on the stack is just numbers.

Any help would be much appreciated!
 

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pthread_default_rsestacksize_np(3T)									       pthread_default_rsestacksize_np(3T)

NAME
pthread_default_rsestacksize_np() - change the default stacksize. SYNOPSIS
PARAMETERS
new_size The new default register stack size. old_size Pointer to where the old default register stack size is returned. DESCRIPTION
The function allows an application to change the default value for the rsestacksize attribute. This function must be called before any threads have been created. The new default stack size is passed in the new_size parameter. If not NULL, the previous default stack size is returned in old_size. If new_size is zero, this function can be used (at any time) to query the current default stack size. On HP-UX, threads with default stack sizes are cached after they terminate. The next time a thread is created with a default stack size, a cached thread (and its stack) are reused. This can result in significant performance improvements for However, if the default register stack size is not appropriate for an application, it cannot take advantage of this performance enhance- ment. By using the function, the threads library will change the default stack size so that it matches the applications needs. This allows the application to utilize the performance benefit of cached threads. If either of the set register stack size APIs is called the register stack portion of the stack space will be that large instead of size determined by the implementation. The memory stack portion will be the remainder. RETURN VALUE
Upon successful completion, returns zero. Otherwise, an error number is returned to indicate the error (the variable is not set). ERRORS
If any of the following occur, the function returns the corresponding error number: The value specified by new_size is less than register stack required for a thread. The calling process has already created threads (this must be called before any threads are created). AUTHOR
was developed by HP. SEE ALSO
pthread_attr_getstacksize(3T), pthread_attr_setstacksize(3T), pthread_default_stacksize_np(3T). STANDARDS CONFORMANCE
Pthread Library pthread_default_rsestacksize_np(3T)
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