trying to understand headers

This is a discussion on trying to understand headers within the C Programming forums, part of the General Programming Boards category; extern int ioctl (int __fd, unsigned long int __request, ...) __THROW; how does one read above? I was thinking global ...

  1. #1
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    trying to understand headers

    extern int ioctl (int __fd, unsigned long int __request, ...) __THROW;

    how does one read above?

    I was thinking global int type function name ioctl has these (....).. but what is
    __THROW mean???

    btw, where is best resource to understand headers??

    thanks in advance~

  2. #2
    Registered User C_ntua's Avatar
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    __THROW can mean anything. It is not defined by the standard

  3. #3
    Just a pushpin. bernt's Avatar
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    This is a system-specific issue, and does not apply to C.

    For reference, __THROW is meant to declare the function as capable of
    throwing exceptions (a C++ feature). In C, the macro does nothing.
    I'm not really sure what you mean by "understand headers" but if you're trying to understand <stdio.h> then I truly wish you the best of luck.

  4. #4
    and the hat of int overfl Salem's Avatar
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    Well given that it's written in UPPERCASE with leading underscores, it's probably defined as a macro by some other header file.

    In particular, it might be defined as nothing at all for C, and the usual 'throw' for C++.
    Then again, it might not be - you would have to find the actual macros in your header files.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  5. #5
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    that's what i don't understand.
    I have gone through other included headers and did not see the __THROW defined anywhere.



    Code:
    #ifndef _SYS_IOCTL_H
    #define _SYS_IOCTL_H    1
    
    #include <features.h>
    
    __BEGIN_DECLS
    
    /* Get the list of `ioctl' requests and related constants.  */
    #include <bits/ioctls.h>
    
    /* Define some types used by `ioctl' requests.  */
    #include <bits/ioctl-types.h>
    
    /* On a Unix system, the system <sys/ioctl.h> probably defines some of
       the symbols we define in <sys/ttydefaults.h> (usually with the same
       values).  The code to generate <bits/ioctls.h> has omitted these
       symbols to avoid the conflict, but a Unix program expects <sys/ioctl.h>
       to define them, so we must include <sys/ttydefaults.h> here.  */
    #include <sys/ttydefaults.h>
    
    /* Perform the I/O control operation specified by REQUEST on FD.
       One argument may follow; its presence and type depend on REQUEST.
       Return value depends on REQUEST.  Usually -1 indicates error.  */
    extern int ioctl (int __fd, unsigned long int __request, ...) __THROW;
    Last edited by Salem; 03-22-2010 at 01:42 PM. Reason: [code] tags have [ ], not <>

  6. #6
    spurious conceit MK27's Avatar
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    Quote Originally Posted by convenientstore View Post
    that's what i don't understand.
    I have gone through other included headers and did not see the __THROW defined anywhere.
    If it is not intended for external use you may not find it declared in a header, you may need to examine the entire source (if available). Kind of irritating, but I have seen this before.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  7. #7
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    It seems to be defined on several places. One of them is sys/cdefs.h for me. How it is actually defined depends on some conditions. Here's part of the headerfile that is used for GCC:
    Code:
    # if !defined __cplusplus && __GNUC_PREREQ (3, 3)
    #  define __THROW       __attribute__ ((__nothrow__))
    #  define __NTH(fct)    __attribute__ ((__nothrow__)) fct
    # else
    #  if defined __cplusplus && __GNUC_PREREQ (2,8)
    #   define __THROW      throw ()
    #   define __NTH(fct)   fct throw ()
    #  else
    #   define __THROW
    #   define __NTH(fct)   fct
    #  endif
    # endif
    It seems to specify for C++ that the function won't throw anything, if supported. If not supported, it won't mean anything.

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