Thread: DIfference between , and ;

Hi,

I had this minor question -

What is the difference between using a , and ; to seperate expressions ?

for example-lets say i have a rectangle class and it has a constructor with 4 arguments -

Code:

IntRect(int left, int right , int bottom, int top) { l = left; r = right; b = bottom; t = top; }

What is the difference if I write it like this ->

Code:

IntRect(int left, int right , int bottom, int top) { l = left, r = right, b = bottom, t = top; }

I know a ; is used to finish an expression, but what is the internal(assembly level) difference between , and ; if there is any ?

here is the source code spinet which i wrote to give your answer i can't find any difference strange in assembly with objdump

Code:

#include <stdio.h>

int set_value_1(const int a, const int b) {
  int x = 0;
  int y = 0;
  x = a, y = b;
  printf("%d %d\n", x, y);
}

int set_value_2(const int a, const int b) {
  int x = 0;
  int y = 0;
  x = a;
  y = b;
  printf("%d %d\n", x, y);
}

int main() {
  set_value_1(10, 20);
  set_value_1(10, 20);

  return 0;
}

Objdump of exe

08048374 <set_value_1>:
8048374: 55 push %ebp
8048375: 89 e5 mov %esp,%ebp
8048377: 83 ec 28 sub $0x28,%esp
804837a: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%ebp)
8048381: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)
8048388: 8b 45 08 mov 0x8(%ebp),%eax
804838b: 89 45 f8 mov %eax,-0x8(%ebp)
804838e: 8b 45 0c mov 0xc(%ebp),%eax
8048391: 89 45 fc mov %eax,-0x4(%ebp)
8048394: 8b 45 fc mov -0x4(%ebp),%eax
8048397: 89 44 24 08 mov %eax,0x8(%esp)
804839b: 8b 45 f8 mov -0x8(%ebp),%eax
804839e: 89 44 24 04 mov %eax,0x4(%esp)
80483a2: c7 04 24 00 85 04 08 movl $0x8048500,(%esp)
80483a9: e8 2a ff ff ff call 80482d8 <printf@plt>
80483ae: c9 leave
80483af: c3 ret

080483b0 <set_value_2>:
80483b0: 55 push %ebp
80483b1: 89 e5 mov %esp,%ebp
80483b3: 83 ec 28 sub $0x28,%esp
80483b6: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%ebp)
80483bd: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)
80483c4: 8b 45 08 mov 0x8(%ebp),%eax
80483c7: 89 45 f8 mov %eax,-0x8(%ebp)
80483ca: 8b 45 0c mov 0xc(%ebp),%eax
80483cd: 89 45 fc mov %eax,-0x4(%ebp)
80483d0: 8b 45 fc mov -0x4(%ebp),%eax
80483d3: 89 44 24 08 mov %eax,0x8(%esp)
80483d7: 8b 45 f8 mov -0x8(%ebp),%eax
80483da: 89 44 24 04 mov %eax,0x4(%esp)
80483de: c7 04 24 00 85 04 08 movl $0x8048500,(%esp)
80483e5: e8 ee fe ff ff call 80482d8 <printf@plt>
80483ea: c9 leave
80483eb: c3 ret

The whole assembly instruction set is here

test: file format elf32-i386


Disassembly of section .init:

08048278 <_init>:
8048278: 55 push %ebp
8048279: 89 e5 mov %esp,%ebp
804827b: 53 push %ebx
804827c: 83 ec 04 sub $0x4,%esp
804827f: e8 00 00 00 00 call 8048284 <_init+0xc>
8048284: 5b pop %ebx
8048285: 81 c3 d8 13 00 00 add $0x13d8,%ebx
804828b: 8b 93 fc ff ff ff mov -0x4(%ebx),%edx
8048291: 85 d2 test %edx,%edx
8048293: 74 05 je 804829a <_init+0x22>
8048295: e8 1e 00 00 00 call 80482b8 <__gmon_start__@plt>
804829a: e8 b1 00 00 00 call 8048350 <frame_dummy>
804829f: e8 0c 02 00 00 call 80484b0 <__do_global_ctors_aux>
80482a4: 58 pop %eax
80482a5: 5b pop %ebx
80482a6: c9 leave
80482a7: c3 ret

Disassembly of section .plt:

080482a8 <__gmon_start__@plt-0x10>:
80482a8: ff 35 60 96 04 08 pushl 0x8049660
80482ae: ff 25 64 96 04 08 jmp *0x8049664
80482b4: 00 00 add %al,(%eax)
...

080482b8 <__gmon_start__@plt>:
80482b8: ff 25 68 96 04 08 jmp *0x8049668
80482be: 68 00 00 00 00 push $0x0
80482c3: e9 e0 ff ff ff jmp 80482a8 <_init+0x30>

080482c8 <__libc_start_main@plt>:
80482c8: ff 25 6c 96 04 08 jmp *0x804966c
80482ce: 68 08 00 00 00 push $0x8
80482d3: e9 d0 ff ff ff jmp 80482a8 <_init+0x30>

080482d8 <printf@plt>:
80482d8: ff 25 70 96 04 08 jmp *0x8049670
80482de: 68 10 00 00 00 push $0x10
80482e3: e9 c0 ff ff ff jmp 80482a8 <_init+0x30>

Disassembly of section .text:

080482f0 <_start>:
80482f0: 31 ed xor %ebp,%ebp
80482f2: 5e pop %esi
80482f3: 89 e1 mov %esp,%ecx
80482f5: 83 e4 f0 and $0xfffffff0,%esp
80482f8: 50 push %eax
80482f9: 54 push %esp
80482fa: 52 push %edx
80482fb: 68 40 84 04 08 push $0x8048440
8048300: 68 50 84 04 08 push $0x8048450
8048305: 51 push %ecx
8048306: 56 push %esi
8048307: 68 ec 83 04 08 push $0x80483ec
804830c: e8 b7 ff ff ff call 80482c8 <__libc_start_main@plt>
8048311: f4 hlt
8048312: 90 nop
8048313: 90 nop
8048314: 90 nop
8048315: 90 nop
8048316: 90 nop
8048317: 90 nop
8048318: 90 nop
8048319: 90 nop
804831a: 90 nop
804831b: 90 nop
804831c: 90 nop
804831d: 90 nop
804831e: 90 nop
804831f: 90 nop

08048320 <__do_global_dtors_aux>:
8048320: 55 push %ebp
8048321: 89 e5 mov %esp,%ebp
8048323: 83 ec 08 sub $0x8,%esp
8048326: 80 3d 80 96 04 08 00 cmpb $0x0,0x8049680
804832d: 74 0c je 804833b <__do_global_dtors_aux+0x1b>
804832f: eb 1c jmp 804834d <__do_global_dtors_aux+0x2d>
8048331: 83 c0 04 add $0x4,%eax
8048334: a3 7c 96 04 08 mov %eax,0x804967c
8048339: ff d2 call *%edx
804833b: a1 7c 96 04 08 mov 0x804967c,%eax
8048340: 8b 10 mov (%eax),%edx
8048342: 85 d2 test %edx,%edx
8048344: 75 eb jne 8048331 <__do_global_dtors_aux+0x11>
8048346: c6 05 80 96 04 08 01 movb $0x1,0x8049680
804834d: c9 leave
804834e: c3 ret
804834f: 90 nop

08048350 <frame_dummy>:
8048350: 55 push %ebp
8048351: 89 e5 mov %esp,%ebp
8048353: 83 ec 08 sub $0x8,%esp
8048356: a1 8c 95 04 08 mov 0x804958c,%eax
804835b: 85 c0 test %eax,%eax
804835d: 74 12 je 8048371 <frame_dummy+0x21>
804835f: b8 00 00 00 00 mov $0x0,%eax
8048364: 85 c0 test %eax,%eax
8048366: 74 09 je 8048371 <frame_dummy+0x21>
8048368: c7 04 24 8c 95 04 08 movl $0x804958c,(%esp)
804836f: ff d0 call *%eax
8048371: c9 leave
8048372: c3 ret
8048373: 90 nop

08048374 <set_value_1>:
8048374: 55 push %ebp
8048375: 89 e5 mov %esp,%ebp
8048377: 83 ec 28 sub $0x28,%esp
804837a: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%ebp)
8048381: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)
8048388: 8b 45 08 mov 0x8(%ebp),%eax
804838b: 89 45 f8 mov %eax,-0x8(%ebp)
804838e: 8b 45 0c mov 0xc(%ebp),%eax
8048391: 89 45 fc mov %eax,-0x4(%ebp)
8048394: 8b 45 fc mov -0x4(%ebp),%eax
8048397: 89 44 24 08 mov %eax,0x8(%esp)
804839b: 8b 45 f8 mov -0x8(%ebp),%eax
804839e: 89 44 24 04 mov %eax,0x4(%esp)
80483a2: c7 04 24 00 85 04 08 movl $0x8048500,(%esp)
80483a9: e8 2a ff ff ff call 80482d8 <printf@plt>
80483ae: c9 leave
80483af: c3 ret

080483b0 <set_value_2>:
80483b0: 55 push %ebp
80483b1: 89 e5 mov %esp,%ebp
80483b3: 83 ec 28 sub $0x28,%esp
80483b6: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%ebp)
80483bd: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)
80483c4: 8b 45 08 mov 0x8(%ebp),%eax
80483c7: 89 45 f8 mov %eax,-0x8(%ebp)
80483ca: 8b 45 0c mov 0xc(%ebp),%eax
80483cd: 89 45 fc mov %eax,-0x4(%ebp)
80483d0: 8b 45 fc mov -0x4(%ebp),%eax
80483d3: 89 44 24 08 mov %eax,0x8(%esp)
80483d7: 8b 45 f8 mov -0x8(%ebp),%eax
80483da: 89 44 24 04 mov %eax,0x4(%esp)
80483de: c7 04 24 00 85 04 08 movl $0x8048500,(%esp)
80483e5: e8 ee fe ff ff call 80482d8 <printf@plt>
80483ea: c9 leave
80483eb: c3 ret

080483ec <main>:
80483ec: 8d 4c 24 04 lea 0x4(%esp),%ecx
80483f0: 83 e4 f0 and $0xfffffff0,%esp
80483f3: ff 71 fc pushl -0x4(%ecx)
80483f6: 55 push %ebp
80483f7: 89 e5 mov %esp,%ebp
80483f9: 51 push %ecx
80483fa: 83 ec 14 sub $0x14,%esp
80483fd: c7 44 24 04 14 00 00 movl $0x14,0x4(%esp)
8048404: 00
8048405: c7 04 24 0a 00 00 00 movl $0xa,(%esp)
804840c: e8 63 ff ff ff call 8048374 <set_value_1>
8048411: c7 44 24 04 14 00 00 movl $0x14,0x4(%esp)
8048418: 00
8048419: c7 04 24 0a 00 00 00 movl $0xa,(%esp)
8048420: e8 4f ff ff ff call 8048374 <set_value_1>
8048425: b8 00 00 00 00 mov $0x0,%eax
804842a: 83 c4 14 add $0x14,%esp
804842d: 59 pop %ecx
804842e: 5d pop %ebp
804842f: 8d 61 fc lea -0x4(%ecx),%esp
8048432: c3 ret
8048433: 90 nop
8048434: 90 nop
8048435: 90 nop
8048436: 90 nop
8048437: 90 nop
8048438: 90 nop
8048439: 90 nop
804843a: 90 nop
804843b: 90 nop
804843c: 90 nop
804843d: 90 nop
804843e: 90 nop
804843f: 90 nop

08048440 <__libc_csu_fini>:
8048440: 55 push %ebp
8048441: 89 e5 mov %esp,%ebp
8048443: 5d pop %ebp
8048444: c3 ret
8048445: 8d 74 26 00 lea 0x0(%esi,%eiz,1),%esi
8048449: 8d bc 27 00 00 00 00 lea 0x0(%edi,%eiz,1),%edi

08048450 <__libc_csu_init>:
8048450: 55 push %ebp
8048451: 89 e5 mov %esp,%ebp
8048453: 57 push %edi
8048454: 56 push %esi
8048455: 53 push %ebx
8048456: e8 4f 00 00 00 call 80484aa <__i686.get_pc_thunk.bx>
804845b: 81 c3 01 12 00 00 add $0x1201,%ebx
8048461: 83 ec 1c sub $0x1c,%esp
8048464: e8 0f fe ff ff call 8048278 <_init>
8048469: 8d bb 20 ff ff ff lea -0xe0(%ebx),%edi
804846f: 8d 83 20 ff ff ff lea -0xe0(%ebx),%eax
8048475: 29 c7 sub %eax,%edi
8048477: c1 ff 02 sar $0x2,%edi
804847a: 85 ff test %edi,%edi
804847c: 74 24 je 80484a2 <__libc_csu_init+0x52>
804847e: 31 f6 xor %esi,%esi
8048480: 8b 45 10 mov 0x10(%ebp),%eax
8048483: 89 44 24 08 mov %eax,0x8(%esp)
8048487: 8b 45 0c mov 0xc(%ebp),%eax
804848a: 89 44 24 04 mov %eax,0x4(%esp)
804848e: 8b 45 08 mov 0x8(%ebp),%eax
8048491: 89 04 24 mov %eax,(%esp)
8048494: ff 94 b3 20 ff ff ff call *-0xe0(%ebx,%esi,4)
804849b: 83 c6 01 add $0x1,%esi
804849e: 39 fe cmp %edi,%esi
80484a0: 72 de jb 8048480 <__libc_csu_init+0x30>
80484a2: 83 c4 1c add $0x1c,%esp
80484a5: 5b pop %ebx
80484a6: 5e pop %esi
80484a7: 5f pop %edi
80484a8: 5d pop %ebp
80484a9: c3 ret

080484aa <__i686.get_pc_thunk.bx>:
80484aa: 8b 1c 24 mov (%esp),%ebx
80484ad: c3 ret
80484ae: 90 nop
80484af: 90 nop

080484b0 <__do_global_ctors_aux>:
80484b0: 55 push %ebp
80484b1: 89 e5 mov %esp,%ebp
80484b3: 53 push %ebx
80484b4: 83 ec 04 sub $0x4,%esp
80484b7: a1 7c 95 04 08 mov 0x804957c,%eax
80484bc: 83 f8 ff cmp $0xffffffff,%eax
80484bf: 74 12 je 80484d3 <__do_global_ctors_aux+0x23>
80484c1: 31 db xor %ebx,%ebx
80484c3: ff d0 call *%eax
80484c5: 8b 83 78 95 04 08 mov 0x8049578(%ebx),%eax
80484cb: 83 eb 04 sub $0x4,%ebx
80484ce: 83 f8 ff cmp $0xffffffff,%eax
80484d1: 75 f0 jne 80484c3 <__do_global_ctors_aux+0x13>
80484d3: 83 c4 04 add $0x4,%esp
80484d6: 5b pop %ebx
80484d7: 5d pop %ebp
80484d8: c3 ret
80484d9: 90 nop
80484da: 90 nop
80484db: 90 nop

Disassembly of section .fini:

080484dc <_fini>:
80484dc: 55 push %ebp
80484dd: 89 e5 mov %esp,%ebp
80484df: 53 push %ebx
80484e0: 83 ec 04 sub $0x4,%esp
80484e3: e8 00 00 00 00 call 80484e8 <_fini+0xc>
80484e8: 5b pop %ebx
80484e9: 81 c3 74 11 00 00 add $0x1174,%ebx
80484ef: e8 2c fe ff ff call 8048320 <__do_global_dtors_aux>
80484f4: 59 pop %ecx
80484f5: 5b pop %ebx
80484f6: c9 leave
80484f7: c3 ret

The first example is four statements; the second example is one statement. In this case it doesn't make much difference to the actual execution of the program. (Commas are necessary when you can only have one statement, such as in the opening parentheses of a for-loop.)

Originally Posted by tabstop

The first example is four statements; the second example is one statement. In this case it doesn't make much difference to the actual execution of the program. (Commas are necessary when you can only have one statement, such as in the opening parentheses of a for-loop.)

Hey TabStop assembly dump with objdum on linux 2.6.29
doesn't show any difference with 4 arguments also strange

08048374 <set_value_1>:
8048374: 55 push %ebp
8048375: 89 e5 mov %esp,%ebp
8048377: 83 ec 28 sub $0x28,%esp
804837a: c7 45 f0 00 00 00 00 movl $0x0,-0x10(%ebp)
8048381: c7 45 f4 00 00 00 00 movl $0x0,-0xc(%ebp)
8048388: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%ebp)
804838f: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)
8048396: 8b 45 08 mov 0x8(%ebp),%eax
8048399: 89 45 f0 mov %eax,-0x10(%ebp)
804839c: 8b 45 0c mov 0xc(%ebp),%eax
804839f: 89 45 f4 mov %eax,-0xc(%ebp)
80483a2: 8b 45 10 mov 0x10(%ebp),%eax
80483a5: 89 45 f8 mov %eax,-0x8(%ebp)
80483a8: 8b 45 14 mov 0x14(%ebp),%eax
80483ab: 89 45 fc mov %eax,-0x4(%ebp)
80483ae: 8b 45 fc mov -0x4(%ebp),%eax
80483b1: 89 44 24 10 mov %eax,0x10(%esp)
80483b5: 8b 45 f8 mov -0x8(%ebp),%eax
80483b8: 89 44 24 0c mov %eax,0xc(%esp)
80483bc: 8b 45 f4 mov -0xc(%ebp),%eax
80483bf: 89 44 24 08 mov %eax,0x8(%esp)
80483c3: 8b 45 f0 mov -0x10(%ebp),%eax
80483c6: 89 44 24 04 mov %eax,0x4(%esp)
80483ca: c7 04 24 70 85 04 08 movl $0x8048570,(%esp)
80483d1: e8 02 ff ff ff call 80482d8 <printf@plt>
80483d6: c9 leave
80483d7: c3 ret

080483d8 <set_value_2>:
80483d8: 55 push %ebp
80483d9: 89 e5 mov %esp,%ebp
80483db: 83 ec 28 sub $0x28,%esp
80483de: c7 45 f0 00 00 00 00 movl $0x0,-0x10(%ebp)
80483e5: c7 45 f4 00 00 00 00 movl $0x0,-0xc(%ebp)
80483ec: c7 45 f8 00 00 00 00 movl $0x0,-0x8(%ebp)
80483f3: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%ebp)
80483fa: 8b 45 08 mov 0x8(%ebp),%eax
80483fd: 89 45 f0 mov %eax,-0x10(%ebp)
8048400: 8b 45 0c mov 0xc(%ebp),%eax
8048403: 89 45 f4 mov %eax,-0xc(%ebp)
8048406: 8b 45 10 mov 0x10(%ebp),%eax
8048409: 89 45 f8 mov %eax,-0x8(%ebp)
804840c: 8b 45 14 mov 0x14(%ebp),%eax
804840f: 89 45 fc mov %eax,-0x4(%ebp)
8048412: 8b 45 fc mov -0x4(%ebp),%eax
8048415: 89 44 24 10 mov %eax,0x10(%esp)
8048419: 8b 45 f8 mov -0x8(%ebp),%eax
804841c: 89 44 24 0c mov %eax,0xc(%esp)
8048420: 8b 45 f4 mov -0xc(%ebp),%eax
8048423: 89 44 24 08 mov %eax,0x8(%esp)
8048427: 8b 45 f0 mov -0x10(%ebp),%eax
804842a: 89 44 24 04 mov %eax,0x4(%esp)
804842e: c7 04 24 70 85 04 08 movl $0x8048570,(%esp)
8048435: e8 9e fe ff ff call 80482d8 <printf@plt>
804843a: c9 leave
804843b: c3 ret

Cheers

Code:

#include <stdio.h>

int set_value_1(const int a, const int b,
                const int c, const int d) {
  int x = 0;
  int y = 0;
  int z = 0;
  int w = 0;
  x = a, y = b, z = c, w = d;
  printf("%d %d %d %d\n", x, y, z, w);
}

int set_value_2(const int a, const int b,
                const int c, const int d) {
  int x = 0;
  int y = 0;
  int z = 0;
  int w = 0;
  x = a;
  y = b;
  z = c;
  w  = d;
  printf("%d %d %d %d\n", x, y, z, w);
}

int main() {
  set_value_1(10, 20, 30, 40);
  set_value_1(10, 20, 30, 40);

  return 0;
}

Originally Posted by RockyMarrone

Hey TabStop assembly dump with objdum on linux 2.6.29
doesn't show any difference with 4 arguments also strange





Cheers

I wasn't referring to assembly, but to C syntax. One example had 4 C statements in it, the other had one C statement.

one big difference is that , is an overloadable operator.