gcc seems to ignore my inline assembly
Author
Hi,
I've been playing with inline assembly in C, using the gcc compiler v3.3.5 under gentoo linux. Unfortunately, gcc seems to ignore my assembly code. I wrote a function which should crash the program by writing junk into the stack, but code execution continues normally after calling it. Here's the code:
<code>
asm (
"movl $1221,(%esp)\n",
"movl $1221,4(%esp\n)",
"movl $1221,8(%esp)\n",
"movl $1221,12(%esp)\n"
"movl $1221,16(%esp)\n"
"movl $1221,20(%esp)\n"
"movl $1221,24(%esp)\n"
"movl $1221,28(%esp)\n"
"movl $1221,32(%esp)\n"
"movl $1221,36(%esp)\n"
"movl $1221,40(%esp)\n"
"movl $1221,44(%esp)\n"
);
</code>
Thanks in advance,
ga
Author
In the meantime I've found that I should just omit the commas between the assembly command strings.. argh;)
ga
ga
There's a difference between the compiler ignoring your code, and the program not crashing.
If you want to know if the compiler ignores the asm, try getting it to print out the compiled asm file, and check for yourself. Or just run the program in a debugger.
But writing junk on the stack doesn't neccesarily mean your program will crash.
If you want to know if the compiler ignores the asm, try getting it to print out the compiled asm file, and check for yourself. Or just run the program in a debugger.
But writing junk on the stack doesn't neccesarily mean your program will crash.
Author
Thank you for the replies.
Spoonbender: That's true, but I've now looked at the asm output of gcc and the code was not there when the commas where still there. The code is inside a function with only 1 pointer argument and no local variables, so it's quite safe to assume that the code overwrites the return address (and the program did indeed crash after i removed the commas).
Spoonbender: That's true, but I've now looked at the asm output of gcc and the code was not there when the commas where still there. The code is inside a function with only 1 pointer argument and no local variables, so it's quite safe to assume that the code overwrites the return address (and the program did indeed crash after i removed the commas).
As a previous poster implied, you also need to tell gcc why the inline assembly needs to be there, or the optimizer will remove it when optimizations are enabled. One way to do this is to add the volatile keyword, but there are far more elegant solutions, especially ones that honour gcc's register allocation. Consider reading one of the relevant documents.
cu,
Prefect
cu,
Prefect
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