ISO C90 forbids mixed declarations and code in C

Question

I declared a variable in this way:

int i = 0;

I get the warning:

ISO C90 forbids mixed declarations and code

Answer 1

-Wdeclaration-after-statement minimal reproducible example

main.c

#!/usr/bin/env bash

set -eux

cat << EOF > main.c
#include 

int main(void) {
    puts("hello");
    int a = 1;
    printf("%d\n", a);
    return 0;
}
EOF

Give warning:

gcc -std=c89 -Wdeclaration-after-statement -Werror main.c
gcc -std=c99 -Wdeclaration-after-statement -Werror main.c
gcc -std=c89 -pedantic -Werror main.c

Don't give warning:

gcc -std=c89 -pedantic -Wno-declaration-after-statement -Werror main.c
gcc -std=c89 -Wno-declaration-after-statement -Werror main.c
gcc -std=c99 -pedantic -Werror main.c
gcc -std=c89 -Wall -Wextra -Werror main.c
# https://stackoverflow.com/questions/14737104/what-is-the-default-c-mode-for-the-current-gcc-especially-on-ubuntu/53063656#53063656
gcc -pedantic -Werror main.c

The warning:

main.c: In function ‘main’:
main.c:5:5: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
     int a = 1;
     ^~~

Tested on Ubuntu 16.04, GCC 6.4.0.