In i386 I'm trying to initialize a string in data. This is stupid, but I can't get it to work.
sentence:
.char 'h',0
says .char is an illegal pseudo op. Clearly I'm not doing it right, though
sentence: db 'h',0
gives nothing either.
As lurker said, each assembler has its own syntax. db is used by nasm, for example. gas provides .byte, .string, .asciz and a bunch of other directives. See the manual. Your code could look like:
sentence: .string "h"
Related
I wrote this code in emu8086 and it goes well in the emulator but when I'm trying to compile it with NASM it's throwing me up the error: "operation size not specified", help someone?
add bx,[3565]
sub bx,0xcc
mov [bx],0CCh
NASM can't figure out what you meant by a line like mov [bx],0CCh. Clearly,
this sets something to 0CCh. But do you want to have bx pointing to a single byte
, short, long, ...? This will manifest itself as the fairly self-explanatory
error: operation size not specified in NASM. You could avoid the error specifying the type, as shown below:
SECTION .text
global start
start:
add bx,[3565]
sub bx,0xcc
mov byte [bx],0CCh
That'd assemble it ok... of course, don't try to run it as it is, it'll produce EXCEPTION_ACCESS_VIOLATION. Just open it with a debugger and you'll understand why.
I would like to know hot to parse a string like this "hello world" into "helloworld" using the strstrip kernel function. I am developing a Linux Kernel char device and this functions causes me a Kernel Panic (or Kernel Opss).
The way I'm using this function is the following:
char result[100];
strcpy(result, "hello world");
strstrip(result);
strstrip(&result); //Also tried this
strstrip("100+200"); //Also tried this
The Kernel error is caused as soon as the strstrip line gets executed. What is the proper way to call this function?
Actually strstrip helps to remove the white spaces at the front. It does not remove all the white spaces with in the string.
Please look at the below example.
char result[100];
strcpy(result, " hello world from stack exchange");
printk("\n before: %s",result);
strcpy(result, strstrip((char*)result));
printk("\n after: %s",result);
Hope it helps.
srtstrip() is a wrapper function for strim() (http://lxr.linux.no/linux+v3.11.2/lib/string.c#L361) in modern kernels. As it will attempt to modify the string itself, you cannot call it with a static string as you have in the third attempt.
The second attempt you have is passing a pointer to an array variable which is also a pointer. So you are passing a char** which if you look at the link above you can see is not correct.
The first attempt should not cause a kernel error, but you do not appear to be receiving the return value in a a local variable. What kind of error are you receiving? I will update this answer if you can provide that information.
In the end though as Balamurugan A points out, this function does not do what you seem to think it does. strsep() (http://lxr.linux.no/linux+v3.11.2/lib/string.c#L485) may help you out here but it will only be a stepping stone to removing all spaces. You will actually have to copy the string into a new buffer word by word as there is not way to simply "shift memory contents", as it were.
What does .string do in assembly?
In an inline asm call in c, I wrote
.string \"Hello World\"
1) Can somebody give me an overview of how .string works?
2) Somehow that value gets saved in %esi. Why?
3) How can I append a return carriage on the end? \n doesn't work
.string is an assembler directive in GAS similar to .long, .int, or .byte. All it does is create and initialize memory for you to use in your program. The .string directive will automatically null-terminate the string with [\0] for you. In this case, the assembler is allocating space for 14 characters in 14 contiguous bytes of memory. If you were to look at the memory, you would see this:
["][H][e][l][l][o][ ][W][o][r][l][d]["][\0]
Except in hexadecimal rather than characters. I'm not sure how or why %esi points to the string (it's either an assembler setting I'm not familiar with or has been set that way on a line of code you're not showing us). What it means is that %esi "points" to the first byte of the string - in this case the first ["]. You can use the %esi register to loop through the bytes using an offset (%esi + 5 would be [o]) or by incrementing the register itself.
To add a newline you might want to try \x0D\x0A instead of \n.
It just emits a sequence of characters into the appropriate code/data section. See this and this (they use .ascii, though).
Show us the code.
Try \\n or \12 or \xa.
I'm having issues grasping the concept of string and characters in MIPS. If I'm trying to loop through two strings, and concatenate two characters at a time, how would I do this?
Lets say I have
String1 = "St"
String2 = "ack"
How could I loop through to get
Sa, Sc, Sk, ta, tc, tk
Thanks in advance!
the most easy way to do this:
1)write the program with C.
2)compile it with GCC with flags "-S", and use "> xx.S" to store the assembly code to a file.
3)extract the assembly codes from "xx.S".
Wasn't able to figure it out, the MIPS environment I used had something built in apparently.
The Linux Programming Interface has an exercise in Chapter 3 that goes like this:
When using the Linux-specific reboot()
system call to reboot the system, the
second argument, magic2, must be
specified as one of a set of magic
numbers (e.g., LINUX_REBOOT_MAGIC2).
What is the significance of these
numbers? (Converting them to
hexadecimal provides a clue.)
The man page tells us magic2 can be one of LINUX_REBOOT_MAGIC2 (672274793), LINUX_REBOOT_MAGIC2A (85072278), LINUX_REBOOT_MAGIC2B (369367448), or LINUX_REBOOT_MAGIC2C (537993216). I failed to decipher their meaning in hex. I also looked at /usr/include/linux/reboot.h, which didn't give any helpful comment either.
I then searched in the kernel's source code for sys_reboot's definition. All I found was a declaration in a header file.
Therefore, my first question is, what is the significance of these numbers? My second question is, where's sys_reboot's definition, and how did you find it?
EDIT: I found the definition in kernel/sys.c. I only grepped for sys_reboot, and forgot to grep for the MAGIC numbers. I figured the definition must be hidden behind some macro trick, so I looked at the System.map file under /boot, and found it next to ctrl_alt_del. I then grepped for that symbol, which led me to the correct file. If I had compiled the kernel from source code, I could try to find which object file defined the symbol, and go from there.
Just a guess, but those numbers look more interesting in hex:
672274793 = 0x28121969
85072278 = 0x05121996
369367448 = 0x16041998
537993216 = 0x20112000
Developers' or developers' children's birthdays?
Regarding finding the syscall implementation, I did a git grep -n LINUX_REBOOT_MAGIC2 and found the definition in kernel/sys.c. The symbol sys_reboot is generated by the SYSCALL_DEFINE4(reboot, ... gubbins, I suspect.
It's the birthday of Linus Torvalds (The developer of the Linux kernel and the Git version control) and his 3 daughters. works as magic numbers to reboot the system.
http://en.wikipedia.org/wiki/Linus_Torvalds