FAQ question (1.13)

[Eric]

Dear All:
In the process of reading the FAQ, I have some questions here is the
question from 1.13(in the new edition maybe 1.32).

quot:
================================================== ================
What is the difference between these initializations?

char a[] = "string literal";
char *p = "string literal";

My program crashes if I try to assign a new value to p[i].
================================================== ===============

Here I know the difference of the two initialize, what I want know is
why the ROM data can not modify?
Who assign the "ROM" area for our program?
Who load the program to memory and how load const to the "ROM" area?

Is the OS protect the "ROM" area?

Thanks a lot~

[Eric Sosman]

Eric wrote:

Dear All:
In the process of reading the FAQ, I have some questions here is the
question from 1.13(in the new edition maybe 1.32).

quot:
================================================== ================
What is the difference between these initializations?

char a[] = "string literal";
char *p = "string literal";

My program crashes if I try to assign a new value to p[i].
================================================== ===============

Here I know the difference of the two initialize, what I want know is
why the ROM data can not modify?

Because "ROM" means "Read-Only Memory," that is,
memory for which only reads and not writes are valid.
If you cannot write to the memory containing an object,
you cannot modify that object.

Who assign the "ROM" area for our program?
Who load the program to memory and how load const to the "ROM" area?

Is the OS protect the "ROM" area?

All of this is implementation-specific, and varies
from one implementation to the next. Here are a few
possibilities:

- There is no ROM at all, and all objects can be
written even if writing to them is a bad idea.

- The O/S can control the read/write permissions
for different sections of memory, often called
"pages" of memory. While loading the program, the
O/S makes all pages writeable so it can store the
initial contents in them. Then before calling the
main() function, the O/S changes some of the pages
to read-only status.

- There is no O/S, or only a minimal O/S. The
program is loaded "at the factory," and some
items are allocated to memory chips that cannot
be written after manufacture. (Or perhaps they
can be rewritten or "re-flashed" afterwards, but
not by way of ordinary memory reads and writes.)

The C programmer should not be concerned with such
details, because the C programmer tries to write code
that will run on many different machines, and the details
will differ from one machine to the next. Here's what
the C programmer should think: "It is always possible
and safe to change the contents of a[], but it might
be impossible to change the contents of *p and is not
safe to do so even if it happens to be possible."

Note that "the C programmer" has a different point
of view than that of "the C implementor," who must deal
with the picky details of his own machine.

--
Eric Sosman
es*****@ieee-dot-org.invalid

[Jack Klein]

On Fri, 06 Jun 2008 10:05:02 +0800, Eric <fr********@gmail.comwrote
in comp.lang.c:

Dear All:
In the process of reading the FAQ, I have some questions here is the
question from 1.13(in the new edition maybe 1.32).

quot:
================================================== ================
What is the difference between these initializations?

char a[] = "string literal";
char *p = "string literal";

My program crashes if I try to assign a new value to p[i].
================================================== ===============

Here I know the difference of the two initialize, what I want know is
why the ROM data can not modify?

What's the ROM data?

Who assign the "ROM" area for our program?

What is the ROM data area? C does not define such a thing. On some
implementations, usually embedded systems, there might be ROM,
although it is more likely flash these days.

Who load the program to memory and how load const to the "ROM" area?

The C standard does not specify how a program gets loaded into memory,
if it needs to be loaded into memory, and how its execution starts.
This depends on the operating system, if any, or perhaps what they
call the BSP these days.

Is the OS protect the "ROM" area?

What OS? Which OS?

Thanks a lot~

The FAQ does not use the term ROM, it merely says "may be stored in
read-only memory". Note the use of "may".

What the C standard says if basically what the FAQ says.

A string literal in C is an unnamed array of characters. Its members
have the type char, and not the type const char. Attempting to modify
a string literal causes undefined behavior. Undefined behavior means
that C does not know or care what happens.

If you want to know how certain low-level things are done on a
specific compiler/platform combination, you need to ask in a group for
that specific combination. The language does not specify these
details, they are up to the implementation, and there are many
different methods used by different implementations.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.club.cc.cmu.edu/~ajo/docs/FAQ-acllc.html

[Dan]

Here I know the difference of the two initialize, what I want know is

why the ROM data can not modify?
Who assign the "ROM" area for our program?
Who load the program to memory and how load const to the "ROM" area?

Is the OS protect the "ROM" area?

Thanks a lot~

Usually char * one would be used on embedded microcontroller systems, to
avoid the string from taking up RAM space, instead the string is only stored
in ROM, which can't be wrote to. Obviously you cannot write to ROM. The OS
may protect any memory it sees fit from writing, which is usually done to
protect the program and the system in general in a multitasking enviroment.
if you need to modify the string, there is nothing stopping you allocating
new memory, copying in the string, and reassigning the same pointer to that
string. As the pointer may change its value.

[Antoninus Twink]

On 6 Jun 2008 at 2:05, Eric wrote:

Here I know the difference of the two initialize, what I want know is
why the ROM data can not modify?
Who assign the "ROM" area for our program?
Who load the program to memory and how load const to the "ROM" area?

I believe you may have been confused by the terminology. Often when
people speak of ROM, they're talking about hardware that physically
cannot be written to. The FAQ answer is talking about an area of memory
that is designated read-only in software (usually by the OS). Even if
there is a hardware switch to mark a piece of memory as read-only, it
will be the OS that is responsible for setting that switch.

How exactly programs are loaded into memory and how they are started
varies with different OSes, but the basics are usually the same. There
are basically three parts to a program: uninitialized data, which goes
in the bss area; initialized data, which goes in the data area; and the
program instructions themselves, which go in the text area.

Within the data part, some executable formats (e.g. ELF) make a
distinction between read-only data (like string literals and perhaps
variables declared as const) and writable data. For example, in ELF
there are separate .data and .rodata sections. When you start up a
program, eventually a kernel function gets called (e.g. if you start a
program at your shell in Linux, probably the shell calls one of the
exec*() functions, which in turn invokes the kernel function
load_elf_binary()). This kernel function will put the .bss data from
your program into the bss segment of your program's address space, and
similarly for data and text. It will mark the text area as executable,
and it doesn't mark the read-only part of the data area as writable.

[Flash Gordon]

Antoninus Twink wrote, On 06/06/08 12:02:

On 6 Jun 2008 at 2:05, Eric wrote:

>Here I know the difference of the two initialize, what I want know is
why the ROM data can not modify?
Who assign the "ROM" area for our program?
Who load the program to memory and how load const to the "ROM" area?

I believe you may have been confused by the terminology. Often when
people speak of ROM, they're talking about hardware that physically
cannot be written to. The FAQ answer is talking about an area of memory
that is designated read-only in software (usually by the OS). Even if
there is a hardware switch to mark a piece of memory as read-only, it
will be the OS that is responsible for setting that switch.

Even when you need need to use ultra-violet light to erase the old data
and a 21V supply to write a new value and neither of those is present?
The FAQ is also talking about memory like that (which I've used) and all
the other variants of read-only memory *including* the types you are
talking about.

How exactly programs are loaded into memory and how they are started
varies with different OSes, but the basics are usually the same.

There are a *lot* of systems where the way the data and program gets in
to read-only memory is by being programmed there by some other device,
and this is an important consideration for *why* it is as it is.

There
are basically three parts to a program: uninitialized data, which goes
in the bss area; initialized data, which goes in the data area; and the
program instructions themselves, which go in the text area.

<snip>

A common but not universal model.

similarly for data and text. It will mark the text area as executable,
and it doesn't mark the read-only part of the data area as writable.

All very implementation specific.
--
Flash Gordon