Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>Page 150 of PC Assembly Tutorial on <a href="http://www.drpaulcarter.com/pcasm/" rel="nofollow">http://www.drpaulcarter.com/pcasm/</a> has a clear explanation about how C allows a function to return a struct:</p> <blockquote> <p>C also allows a structure type to be used as the return value of a func- tion. Obviously a structure can not be returned in the EAX register. Different compilers handle this situation differently. A common solution that compilers use is to internally rewrite the function as one that takes a structure pointer as a parameter. The pointer is used to put the return value into a structure defined outside of the routine called.</p> </blockquote> <p>I use the following C code to verify the above statement:</p> <pre><code>struct person { int no; int age; }; struct person create() { struct person jingguo = { .no = 1, .age = 2}; return jingguo; } int main(int argc, const char *argv[]) { struct person result; result = create(); return 0; } </code></pre> <p>Use "gcc -S" to generate assembly for this piece of C code:</p> <pre><code> .file "foo.c" .text .globl create .type create, @function create: pushl %ebp movl %esp, %ebp subl $16, %esp movl 8(%ebp), %ecx movl $1, -8(%ebp) movl $2, -4(%ebp) movl -8(%ebp), %eax movl -4(%ebp), %edx movl %eax, (%ecx) movl %edx, 4(%ecx) movl %ecx, %eax leave ret $4 .size create, .-create .globl main .type main, @function main: pushl %ebp movl %esp, %ebp subl $20, %esp leal -8(%ebp), %eax movl %eax, (%esp) call create subl $4, %esp movl $0, %eax leave ret .size main, .-main .ident "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3" .section .note.GNU-stack,"",@progbits </code></pre> <p>The stack before call create:</p> <pre><code> +---------------------------+ ebp | saved ebp | +---------------------------+ ebp-4 | age part of struct person | +---------------------------+ ebp-8 | no part of struct person | +---------------------------+ ebp-12 | | +---------------------------+ ebp-16 | | +---------------------------+ ebp-20 | ebp-8 (address) | +---------------------------+ </code></pre> <p>The stack right after calling create:</p> <pre><code> +---------------------------+ | ebp-8 (address) | +---------------------------+ | return address | +---------------------------+ ebp,esp | saved ebp | +---------------------------+ </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POAre there any downsides to passing structs by value in C, rather than passing a pointer?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USJingguo Yao
    1. USJingguo Yao
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTDownMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThere are two problems here. The most obvious one is that this does not at all describe "how C allows a function to return a struct". This only describes how it can be done on 32-bit x86 hardware, which happens to be one of the most limited architectures when you look at the number of registers etc. The second problem is that the way that C compilers generate code for returning values is dictated by the ABI (except for non-exported or inlined functions). And by the way, inlined functions are probably one of the places where returning structs are most useful.
      singulars
      1. PO
      1. USdkagedal
    2. COThanks for the corrections. For a complete detailed of calling convention, http://en.wikipedia.org/wiki/Calling_convention is a good reference.
      singulars
      1. PO
      1. USJingguo Yao
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload