Parameter evaluation order before a function calling in C [duplicate]

Question

Can it be assumed a evaluation order of the function parameters when calling it in C ? According to the following program, it seems that there is not a particular order when I executed it.

#include <stdio.h>  int main() {    int a[] = {1, 2, 3};    int * pa;      pa = &a[0];    printf("a[0] = %d\ta[1] = %d\ta[2] = %d\n",*(pa), *(pa++),*(++pa));    /* Result: a[0] = 3  a[1] = 2    a[2] = 2 */     pa = &a[0];    printf("a[0] = %d\ta[1] = %d\ta[2] = %d\n",*(pa++),*(pa),*(++pa));    /* Result: a[0] = 2  a[1] = 2     a[2] = 2 */     pa = &a[0];    printf("a[0] = %d\ta[1] = %d\ta[2] = %d\n",*(pa++),*(++pa), *(pa));    /* a[0] = 2  a[1] = 2 a[2] = 1 */  } 

Answer 1

No, function parameters are not evaluated in a defined order in C.

See Martin York's answers to What are all the common undefined behaviour that c++ programmer should know about?.

Answer 2

Order of evaluation of function arguments is unspecified, from C99 §6.5.2.2p10:

The order of evaluation of the function designator, the actual arguments, and subexpressions within the actual arguments is unspecified, but there is a sequence point before the actual call.

Similar wording exists in C89.

Additionally, you are modifying pa multiple times without intervening sequence points which invokes undefined behavior (the comma operator introduces a sequence point but the commas delimiting the function arguments do not). If you turn up the warnings on your compiler it should warn you about this:

$ gcc -Wall -W -ansi -pedantic test.c -o test test.c: In function ‘main’: test.c:9: warning: operation on ‘pa’ may be undefined test.c:9: warning: operation on ‘pa’ may be undefined test.c:13: warning: operation on ‘pa’ may be undefined test.c:13: warning: operation on ‘pa’ may be undefined test.c:17: warning: operation on ‘pa’ may be undefined test.c:17: warning: operation on ‘pa’ may be undefined test.c:20: warning: control reaches end of non-void function