Why does not execute printf inside function? - c

Question

Following code compile and run on GCC compiler.
#include <stdio.h>
int arr[10];
int func()
{
printf("In func\n");
return 0;
}
int main()
{
if (&arr[func()])
printf("In main\n");
return 0;
}
Output:
In main
Why does not execute printf("In func\n"); ?

Answer 1

There seems to be a subtle issue, either intended, or unintended with various combinations of the latest gcc. ver 7.3 on the latest kernel 4.15.8 on Archlinux. For whatever reason the call to func() is omitted for the code generated for main(). e.g.
$ gcc -S -masm=intel -o infunc2.asm infunc2.c
The generated assembly is:
$ cat infunc2.asm
.file "infunc2.c"
.intel_syntax noprefix
.text
.comm arr,40,32
.section .rodata
.LC0:
.string "In func"
.text
.globl func
.type func, #function
func:
.LFB0:
.cfi_startproc
push rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
mov rbp, rsp
.cfi_def_cfa_register 6
lea rdi, .LC0[rip]
call puts#PLT
mov eax, 0
pop rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size func, .-func
.section .rodata
.LC1:
.string "In main"
.text
.globl main
.type main, #function
main:
.LFB1:
.cfi_startproc
push rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
mov rbp, rsp
.cfi_def_cfa_register 6
lea rdi, .LC1[rip]
call puts#PLT
mov eax, 0
pop rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size main, .-main
.ident "GCC: (GNU) 7.3.0"
.section .note.GNU-stack,"",#progbits
Note the call to func() is labeled .LFB0: above. The procedure for main: does not call func or .LFB0: at all, despite it being present, and despite the "In func" string being present in .LC0:. I suspect this is not intended behavior.
For example, simple compilation without optimization -O0 the function is not called, e.g.:
$ gcc -g -O0 -o bin/if2 infunc2.c
$ ./bin/if2
In main
Changing the code to store the address of arr[func()] does force func() to be called, e.g.
#include <stdio.h>
int arr[10];
int func()
{
printf ("In func\n");
return 0;
}
int main (void)
{
int *p = &arr[func()];
if (p)
printf("In main\n");
return 0;
}
Then
$ gcc -Wall -Wextra -pedantic -std=gnu11 -Ofast -o bin/infunc infunc.c
$ ./bin/infunc
In func
In main
And the generated assembly supports the different behavior:
$ gcc -S -masm=intel -o infunc.asm infunc.c
$ cat infunc.asm
.file "infunc.c"
.intel_syntax noprefix
.text
.comm arr,40,32
.section .rodata
.LC0:
.string "In func"
.text
.globl func
.type func, #function
func:
.LFB0:
.cfi_startproc
push rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
mov rbp, rsp
.cfi_def_cfa_register 6
lea rdi, .LC0[rip]
call puts#PLT
mov eax, 0
pop rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size func, .-func
.section .rodata
.LC1:
.string "In main"
.text
.globl main
.type main, #function
main:
.LFB1:
.cfi_startproc
push rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
mov rbp, rsp
.cfi_def_cfa_register 6
sub rsp, 16
mov eax, 0
call func
cdqe
lea rdx, 0[0+rax*4]
lea rax, arr[rip]
add rax, rdx
mov QWORD PTR -8[rbp], rax
cmp QWORD PTR -8[rbp], 0
je .L4
lea rdi, .LC1[rip]
call puts#PLT
.L4:
mov eax, 0
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size main, .-main
.ident "GCC: (GNU) 7.3.0"
.section .note.GNU-stack,"",#progbits
I wish I could provide some logical explanation for the handling here, but I can only document it. Seems we need to talk with the guys on the gcc list.
Side effects discarded in address computation inside 'if'
This seems to be a regression in gcc that will appear depending on whether an individual distro applies enough patching to mask it. It is a gcc bug in work. Bug 84607

Answer 2

This is a gcc bug (#84607) and has been fixed in gcc 7.3.1 or later.

Answer 3

The problem is with your compilation. I use gcc to compile. I compiled your file like this:
gcc main.c -o prog
./prog
In func
In main
Seems good to me. Check the procedure on how to compile with you compiler if you use a different compiler than gcc. Also I use gcc 7.3