Why does the statement "2i;" NOT cause a compiler error?

Question

Instead of 2*i, I carelessly wrote 2i:

int foo(int i) {     2i;     return 2i; } 

I expected the compiler to catch the error. But it did not. So is 2i a valid statement in C? If so what does it do? Puzzled!

I compiled using gcc version 5.3.0 and here is the assembly output:

    .file   "strange.c"     .text     .globl  foo     .type   foo, @function foo: .LFB0:     .cfi_startproc     pushq   %rbp     .cfi_def_cfa_offset 16     .cfi_offset 6, -16     movq    %rsp, %rbp     .cfi_def_cfa_register 6     movl    %edi, -4(%rbp)     nop     popq    %rbp     .cfi_def_cfa 7, 8     ret     .cfi_endproc .LFE0:     .size   foo, .-foo     .ident  "GCC: (GNU) 5.3.0"     .section    .note.GNU-stack,"",@progbits 

Answer 1

This is a gcc extension, and 2i is the imaginary constant . So you can write a complex number like so:

#include <complex.h>  _Complex x = 4 + 5i; 

Answer 2

2i is a gcc extension for a complex integer literal, a pure imaginary number twice the square root of -1. This extension is supported by clang as well.

It is somewhat surprising that your compiling with gcc 5.4.0 produces the posted assembly output:

• Compiling on http://gcc.godbolt.org/# I get a compilation error from gcc 5.3.0: http://gcc.godbolt.org/#: error: cannot convert '__complex__ int' to 'int' in return.
• The posted assembly code for function foo is incorrect: it does not return 0. Converting the complex integer constant 2i to int should return its real part 0.

Conversely, with clang 3.7, it compiles without a warning and generates optimum code, but of course not what you expect:

foo(int):                       # @foo(int)     xorl    %eax, %eax     retq 

This syntax can be combined with other suffixes in any order. Compiling the code below with clang -Weverything gives me appropriate warnings warning: imaginary constants are a GNU extension [-Wgnu-imaginary-constant]:

#include <stdio.h>  int main() {     /* complex integer literals */     printf("sizeof(2i) = %zd\n", sizeof(2i));     printf("sizeof(2ui) = %zd\n", sizeof(2ui));     printf("sizeof(2li) = %zd\n", sizeof(2li));     printf("sizeof(2lli) = %zd\n", sizeof(2lli));     /* complex floating point literals */     printf("sizeof(2.i) = %zd\n", sizeof(2.i));     printf("sizeof(2.fi) = %zd\n", sizeof(2.fi));     printf("sizeof(2e0fi) = %zd\n", sizeof(2e0fi));     printf("sizeof(2e0i) = %zd\n", sizeof(2e0i));     /* alternate order */     printf("sizeof(2il) = %zd\n", sizeof(2il));     printf("sizeof(2ill) = %zd\n", sizeof(2ill));     printf("sizeof(2.if) = %zd\n", sizeof(2.if));      return 0; } 

It produces this output in my environment:

sizeof(2i) = 8 sizeof(2ui) = 8 sizeof(2li) = 16 sizeof(2lli) = 16 sizeof(2.i) = 16 sizeof(2.fi) = 8 sizeof(2e0fi) = 8 sizeof(2e0i) = 16 sizeof(2il) = 16 sizeof(2ill) = 16 sizeof(2.if) = 8 

Try the last one with your syntax coloring editor ;-)