I am trying to illustrate the reverse order of parameters on the stack when passed to a function in C:
Result is:
This is as expected on the 64 bit system (Ubuntu 19.04)
When I pass a structure as a parameter the stack looks puzzling to me:
Now result is:
Parameter 3 is close to 1 and parameter 2 is not between 1 and 3? offsets look wrong to me.Please shed some light here, thanks in advance.
The compiler is gcc 7.4.0
The standards don't specify how parameters are placed on the stack when a function is invoked and any code that assumes that parameters are placed on the stack in the order given (or in the reverse of the order given) in the function declaration is highly likely to fail on some operating systems on some hardware.
When an operating system is ported to or designed from scratch, the compiler group, the link editor group, and the operating system group will study the hardware design documents and agree on a scheme that they believe will be fast and conserve space on that hardware for the operating system being designed. Note that the scheme may well have exceptions that apply alternative behaviors when a function is invoked that takes a variable number of parameters (e.g., printf()). The only portable way to write functions like printf() is to use the macros defined in the Standard C header <stdarg.h> (i.e., va_start, va_arg, va_copy, and va_end).
These 3 Users Gave Thanks to Don Cragun For This Post:
Probably, placement has to do with word boundaries. Which vary with different OS and hardware. As Don mentioned clearly. gcc has options for packing objects in memory. try gcc -Q -v inputfilename.c - assuming that is what you used. Be prepared for a lot of information on your screen.
Last edited by jim mcnamara; 06-15-2019 at 07:46 PM..
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