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Full Discussion: Why does this example C code run and yet SHOULD either not compile or give a segmentation fault?
Top Forums Programming Why does this example C code run and yet SHOULD either not compile or give a segmentation fault? Post 303032978 by jim mcnamara on Wednesday 27th of March 2019 06:33:32 PM
Old 03-27-2019
This is basically another way to do the undefined operation thing.

asm:
Not part of standard C, so whatever asm does is implementation defined, i.e., the people who wrote gcc
Since it's not mentioned in ISO C standard (n1570 draft-- C2011), but mentioned in annex J (common extensions):

Quote:
J.5.10 The asm keyword

1 The asm keyword may be used to insert assembly language directly into the translator output (6.8). The most common implementation is via a statement of the form:

asm ( character-string-literal );
Annex J is informative, not normative, so an implementation need not provide inline assembly, and if it does it's not prescribed in which form. But it's a widespread extension, though not portable since compilers do indeed implement it differently.

In the C++ standard (n3376 draft of the C++11 standard), it is NOT mentioned in the body of the standard.

Quote:
7.4 The asm declaration [dcl.asm]

1 An asm declaration has the form

asm-definition:

asm ( string-literal ) ;

The asm declaration is conditionally-supported; its meaning is implementation-defined. [ Note: Typically it is used to pass information through the implementation to an assembler. - end note ]
I think Rudi correct. Try using return statements. That return values used by the calling code. Other than learning what not to do for reasonable code, what does this do for you? If you had not asked here your might have accidentally created a horrible bug in a piece of code that you thought was okay. It's okay with me, but it does not seem all that helpful....
Last edited by jim mcnamara; 03-28-2019 at 12:06 PM..
This User Gave Thanks to jim mcnamara For This Post:
wisecracker
 

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LEARN ABOUT LINUX

s390_guarded_storage

S390_GUARDED_STORAGE(2) 					System Calls Manual					   S390_GUARDED_STORAGE(2)

NAME

       s390_guarded_storage - operations with z/Architecture guarded storage facility

SYNOPSIS

       #include <asm/guarded_storage.h>

       int s390_guarded_storage(int command, struct gs_cb *gs_cb);

DESCRIPTION

       The  s390_guarded_storage()  system call enables the use of the Guarded Storage Facility (a z/Architecture-specific feature) for user-space
       processes.

       The guarded storage facility is a hardware feature that allows marking up to 64 memory regions (as of z14) as guarded;  reading	a  pointer
       with  a	newly  introduced  "Load  Guarded" (LGG) or "Load Logical and Shift Guarded" (LLGFSG) instructions will cause a range check on the
       loaded value and invoke a (previously set up) user-space handler if one of the guarded regions is affected.

       The command argument indicates which function to perform.  The following commands are supported:

       GS_ENABLE
	      Enable the guarded storage facility for the calling task.  The initial content of the guarded storage  control  block  will  be  all
	      zeros.   After enablement, user-space code can use the "Load Guarded Storage Controls" (LGSC) instruction (or the load_gs_cb() func-
	      tion wrapper provided in the asm/guarded_storage.h header) to load an arbitrary control block.  While a task is enabled, the  kernel
	      will save and restore the calling content of the guarded storage registers on context switch.

       GS_DISABLE
	      Disables the use of the guarded storage facility for the calling task.  The kernel will cease to save and restore the content of the
	      guarded storage registers, the task-specific content of these registers is lost.

       GS_SET_BC_CB
	      Set a broadcast guarded storage control block to the one provided in the gs_cb argument.	This is called per thread and associates a
	      specific	guarded  storage  control block with the calling task.	This control block will be used in the broadcast command GS_BROAD-
	      CAST.

       GS_CLEAR_BC_CB
	      Clears the broadcast guarded storage control block.  The guarded storage control block will no longer have  the  association  estab-
	      lished by the GS_SET_BC_CB command.

       GS_BROADCAST
	      Sends  a	broadcast to all thread siblings of the calling task.  Every sibling that has established a broadcast guarded storage con-
	      trol block will load this control block and will be enabled for guarded storage.	The broadcast guarded  storage	control  block	is
	      consumed; a second broadcast without a refresh of the stored control block with GS_SET_BC_CB will not have any effect.

       The  gs_cb  argument specifies the address of a guarded storage control block structure and is currently used only by the GS_SET_BC_CB com-
       mand; all other aforementioned commands ignore this argument.

RETURN VALUE

       On success, the return value of s390_guarded_storage() is 0.

       On error, -1 is returned, and errno is set appropriately.

ERRORS

       EFAULT command was GS_SET_BC_CB and the copying of the guarded storage control block structure pointed by the gs_cb argument has failed.

       EINVAL The value provided in the command argument was not valid.

       ENOMEM command was one of GS_ENABLE or GS_SET_BC_CB, and the allocation of a new guarded storage control block has failed.

       EOPNOTSUPP
	      The guarded storage facility is not supported by the hardware.

VERSIONS

       This system call is available since Linux 4.12.

CONFORMING TO

       This Linux-specific system call is available only on the s390 architecture.

       The guarded storage facility is available beginning with System z14.

NOTES

       Glibc does not provide a wrapper for this system call, use syscall(2) to call it.

       The description of the guarded storage facility along with related instructions and Guarded Storage Control Block and Guarded Storage Event
       Parameter List structure layouts is available in "z/Architecture Principles of Operations" beginning from the twelfth edition.

       The  gs_cb  structure has a field gsepla (Guarded Storage Event Parameter List Address), which is a user-space pointer to a Guarded Storage
       Event Parameter List structure (that contains the address of the aforementioned event handler in the gseha field), and its layout is avail-
       able as a gs_epl structure type definition in the asm/guarded_storage.h header.

SEE ALSO

       syscall(2)

Linux Programmer's Manual					    2019-03-06						   S390_GUARDED_STORAGE(2)
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