C GCC – Re-Declare Variable Inside For Loop

c++declarationgcc

I have compiled following program using gcc prog.c -Wall -Wextra -std=gnu11 -pedantic command on GCC compiler. I wondered, it is working fine without any warnings or errors.

#include <stdio.h>

int main(void)
{
    for (int i = 0; i == 0;  i++) 
    {        
        printf("%d\n", i);
        long int i = 1; // Why doesn't redeclaration error?
        printf("%ld\n", i);
    }
}

Why compiler doesn't generate redeclaration variable i error?

Best Answer

From standard §6.8.5.5 (N1570)

An iteration statement is a block whose scope is a strict subset of the scope of its enclosing block. The loop body is also a block whose scope is a strict subset of the scope of the iteration statement.

^{Emphasis added}

Related Solutions

C – For Loop Initial Declaration Syntax

for (int i = 0; ...)

is a syntax that was introduced in C99. In order to use it you must enable C99 mode by passing -std=c99 (or some later standard) to GCC. The C89 version is:

int i;
for (i = 0; ...)

EDIT

Historically, the C language always forced programmers to declare all the variables at the begin of a block. So something like:

{
   printf("%d", 42); 
   int c = 43;  /* <--- compile time error */

must be rewritten as:

{
   int c = 43;
   printf("%d", 42);

a block is defined as:

block := '{' declarations statements '}'

C99, C++, C#, and Java allow declaration of variables anywhere in a block.

The real reason (guessing) is about allocating internal structures (like calculating stack size) ASAP while parsing the C source, without go for another compiler pass.

C++ – Declaring and Initializing Variable in a For Loop

It's valid in C++.

It was not legal in the original version of C.
But was adopted as part of C in C99 (when some C++ features were sort of back ported to C)
Using gcc

gcc -std=c99 <file>.c

The variable is valid inside the for statement and the statement that is looped over. If this is a block statement then it is valid for the whole of the block.

for(int loop = 0; loop < 10; ++loop)
{
    // loop valid in here aswell
}

// loop NOT valid here.

Best Answer

Related Solutions

C – For Loop Initial Declaration Syntax

C++ – Declaring and Initializing Variable in a For Loop

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