如何解决“在C99模式外部使用的for循环初始声明” GCC错误？

Question

I'm trying to solve but I don't know where a mistake.

int main() {
    for (int i = 0; i < 3; i++) {
        pid_t pid = fork();
  }
    return 0;
}

Answer 1

You're declaring your i variable inside the for loop. This is common in C++, but was added (surprisingly recently) in the C99 specification.

Move the declaration of your i variable outside of the for loop:

int main() {
    int i;
    for (i = 0; i < 3; i++) {
        pid_t pid = fork();
  }
    return 0;
}

Alternatively, you can tell GCC to compile your code in C99 mode:

gcc -std=c99

Or if you want to retain GCC-specific features, use:

gcc -std=gnu99