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    <p>I previously replied with the below answer. But then I saw that I misinterpreted several information and gave an incorrect answer. Just out of curiosity, I did the same test with VS2008 and I got different results. This is just brain exercise...</p> <pre> Why do you need the second one ? Both headers say the same thing. And it does not even matter if you write 0 or NULL or ((void *)0) All of them will take 8 bytes. </pre> <p>I did a quick test on a 64-bit platform with GCC 4.1.3</p> <pre><code>#include &lt;string.h&gt; void str_concat(char *po_buf, int pi_max, ...) { strcpy(po_buf, "Malkocoglu"); /* bogus */ } int main() { char buf[100]; str_concat(buf, 100, "abc", 1234LL, "def", 5678LL, "ghi", 2345LL, "jkl", 6789LL, "mno", 3456LL, 0, "pqx", 0); return 1; } </code></pre> <p>And this is the assembly generated by the compiler...</p> <pre><code>main: .LFB3: pushq %rbp .LCFI3: movq %rsp, %rbp .LCFI4: subq $192, %rsp .LCFI5: leaq -112(%rbp), %rdi movl $0, 64(%rsp) 0 movq $.LC2, 56(%rsp) "pqx" movl $0, 48(%rsp) 0 movq $3456, 40(%rsp) 3456LL movq $.LC3, 32(%rsp) "mno" movq $6789, 24(%rsp) 6789LL movq $.LC4, 16(%rsp) "jkl" movq $2345, 8(%rsp) 2345LL movq $.LC5, (%rsp) "ghi" movl $5678, %r9d 5678LL movl $.LC0, %r8d "def" movl $1234, %ecx 1234LL movl $.LC1, %edx "abc" movl $100, %esi 100 movl $0, %eax call str_concat movl $1, %eax leave ret </code></pre> <p>Notice all the stack displacements are 8 byte...</p> <pre> Compiler treats 0 as it was a 32-bit data-type. Although it does the correct displacement on the stack pointer, the value pushed should not be 32-bit ! </pre> <p>I did the same test with VS2008 , the assembly output is as follows :</p> <pre><code>mov QWORD PTR [rsp+112], 0 lea rax, OFFSET FLAT:$SG3597 mov QWORD PTR [rsp+104], rax mov QWORD PTR [rsp+96], 0 mov QWORD PTR [rsp+88], 3456 ; 00000d80H lea rax, OFFSET FLAT:$SG3598 mov QWORD PTR [rsp+80], rax mov QWORD PTR [rsp+72], 6789 ; 00001a85H lea rax, OFFSET FLAT:$SG3599 mov QWORD PTR [rsp+64], rax mov QWORD PTR [rsp+56], 2345 ; 00000929H lea rax, OFFSET FLAT:$SG3600 mov QWORD PTR [rsp+48], rax mov QWORD PTR [rsp+40], 5678 ; 0000162eH lea rax, OFFSET FLAT:$SG3601 mov QWORD PTR [rsp+32], rax mov r9d, 1234 ; 000004d2H lea r8, OFFSET FLAT:$SG3602 mov edx, 100 ; 00000064H lea rcx, QWORD PTR buf$[rsp] call ?str_concat@@YAXPEADHZZ ; str_concat </code></pre> <p>This time compiler generates different code and it treats 0 as an 64-bit data-type (notice the QWORD keyword). Both the value and stack displacement is correct. VS and GCC behaves differently.</p>
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    1. PONULL definition problem on 64 bit system
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    1. COActually I just perked up, is `int` 4 bytes on a 64-bits platform right ?
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      1. USMatthieu M.
    2. COIt depends on system/compiler/ABI but yes generally int is 4 bytes but 0 is not. 0 is a number, it is not a type (like int/long/char) and it is promoted to whatever compiler thinks it should be...
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    3. CODownvoted because 0 is *not* 8 bytes. I ran into this with some Gnome software in the very early days of Gentoo AMD64. In a variadic argument list, C does not know to convert 0 to a pointer so you must cast to void*.
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      1. USZan Lynx
 

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