Wrong output in a c program

Question

#include<stdio.h>
int main()
{
int a=1;
printf("%d%d%d",a,++a,a++);

}

Here why is the output 331 and not 122.

The reason i found on internet was that arguements are passed grom right to left . First a++ then ++a then a gets evaluated and then get printed in the reverse order. Is it the right reason.

@ cool guybut my book says it has something to do with calling convention — Ishan Bansal, Sep 30 '14 at 10:10
Calling convention indicates the order in which arguments are passed to a function when a function call is encountered. There are two possibilities here: (a) (b) Arguments might be passed from left to right. Arguments might be passed from right to left. C language follows the second order. — Ishan Bansal, Sep 30 '14 at 10:15
more over i find it right bcoz if u go from right to left output will be 331 — Ishan Bansal, Sep 30 '14 at 10:16
Read the link posted as the second answer to the question n.m. linked to: http://c-faq.com/expr/seqpoints.html It's the only valid answer here. — Will, Sep 30 '14 at 10:23

Ivan Ivanovich · Answer 1 · 2014-09-30T10:36:07.537

gcc -S your_source.c explain your question

    movl    $1, -4(%rbp)    #, a
    movl    -4(%rbp), %ecx  # a, a.0
    addl    $1, -4(%rbp)    #, a
    addl    $1, -4(%rbp)    #, a
    movl    -4(%rbp), %edx  # a, tmp61
    movl    -4(%rbp), %eax  # a, tmp62
    movl    %eax, %esi      # tmp62,
    movl    $.LC0, %edi     #, offset to "%d%d%d" sttring
    movl    $0, %eax        #,
    call    printf  #

compiler developer must not expect that someone will try to use it in such an ugly way

if you really need 122: then change asm code as follow:

    movl    $1, -4(%rbp)    #, a
    movl    -4(%rbp), %eax  # a, a.0
    addl    $1, -4(%rbp)    #, a
    movl    -4(%rbp), %edx  # a, tmp61
    movl    -4(%rbp), %ecx  # a, tmp62
    movl    %eax, %esi      # tmp62,
    movl    $.LC0, %edi     #,
    movl    $0, %eax        #,
    call    printf  #
    addl    $1, -4(%rbp)    #, a
    leave

Wrong output in a c program

1 Answers1