Hi everybody,
I'm trying to solve a very complex issue (complex at least for me!).
I have to pass a pointer to a struct's member as a function parameter. I can't pass a pointer to the whole structure, but only to a structure's member (I'm trying to use an API already done).
Is it possible to access the whole structure and to write values in other struct members location from inside the function?
Till now, my attempts gave me "segmentation fault" results.
Best regards
Beppe
Last edited by satsriakal; 12-20-2017 at 08:54 AM.
"...a computer is a stupid machine with the ability to do incredibly smart things, while computer programmers are smart people with the ability to do incredibly stupid things. They are,in short, a perfect match.." Bill Bryson
For example:
Code:typedef struct INDATA { int fld1; char *buf; } INDATA; void anotherFunc(int *x) { *x = 10; ........ // code } void aFunc(INDATA *idPtr) { idPtr->fld1 = 10; ..... } int main() { INDATA id; aFunc(&id); anotherFunc(&(id.fld1)); .... return(0); }
Actually I have this problem: I'm sure I'm writing in the right position; I verified the address is correct, but when I write, I get "segmentation fault".
I got the right address, adding offset to struct member address.
You need to post your actual test case.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
It is easy to make your own offsetof(),
for example:
Code:#include <stdio.h> #include <string.h> // gcc tst.c -o tst typedef struct INDATA { int fld1; int fld2, fld3, fld4; char buf1[8]; char buf2[8]; } INDATA; int main() { INDATA id; void *vptr1, *vptr2; unsigned long offset; vptr1 = &id; vptr2 = &(id.buf2); offset = vptr2 - vptr1; printf("offset field buf2 = %ld\n", offset); strcpy((char *) vptr2, "Hallo"); printf("id.buf2 = %s\n", id.buf2); return(0); }
>offset = vptr2 - vptr1;
You need to cast the pointers to char* in order to do arithmetic.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
Why ?>offset = vptr2 - vptr1;
You need to cast the pointers to char* in order to do arithmetic.
Because subtracting two pointers gives you the distance in elements, not bytes. If they're arrays of ints, it'll be the distance in ints, and so on. You can't use void pointers because the compiler doesn't know what size their elements are.Originally Posted by ddutch
Devoted my life to programming...
Is it not so that void-pointers are (single) byte pointers
No, never. void pointers literally have no type. You can't do anything with them except casting/assigning them to other types.
Devoted my life to programming...
offsetof() is already defined in stddef.h and has been since C89. Use that instead of rolling your own.
Yes indeed, one can not do pointer arithmetic with void pointer and it is not allowed according to the Ansi C standard.
But .... in my example above I only use void-pointers in order to obtain 2 memory addresses and find out the difference
between them. True, statements like vptr++ would have been illegal....
Required for my example were pointers which align on 1 byte boundaries and void pointer do so. But indeed, it
migth have been better for portability (my GNU compiler did not complain though) to have used unsigned char pointers.
Gcc allows it unless you use the options to restrict it to standard behaviour.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
This compiles for me without warning on gcc 5.4 with the same lack of warning options in the compile options as you used:
so if you find it acceptable to subtract pointers to void, then you must also find it acceptable to increment pointers to void, contrary to your claim that "statements like vptr++ would have been illegal". If you agree that the increment is illegal, then you are following the C standard, so likewise you should agree that subtracting pointers to void is illegal, and hence you should pass -pedantic as an option to gcc to warn you about such illegal things.Code:int main(void) { char str[] = "hello"; void *vptr = str; vptr++; return 0; }
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