How do I fix “for loop initial declaration used outside C99 mode” GCC error?

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;
}

Tell GCC to use the C99 standard (or later). Read the GCC online documentation for your version of GCC for the flags to use. — Some programmer dude
– Some programmer dude, Commented Oct 28, 2016 at 11:02

Jonathon Reinhart · Accepted Answer · 2016-10-28 11:01:58Z

4

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

answered Oct 28, 2016 at 11:01

Jonathon Reinhart

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5 Comments

Jens Gustedt Over a year ago

1999 is recent for you ?)

Jens Gustedt Over a year ago

Best would just be to pass to a more recent version of gcc which has even C11 as a default.

Jonathon Reinhart Over a year ago

@JensGustedt Considering Visual Studio 2013 only implemented some of C99 features, I'd say C99 support is a recent event (not the spec being recent).

Fady Hany Over a year ago

you said " (not the spec being recent) ". is " spec " an abbreviation for " specification " ? @Jonathon Reinhart

Jonathon Reinhart Over a year ago

@FadyHany Yes, "spec" is a clipping of "specification". When I said "surprisingly recently in the C99 specification" I meant that compiler support for C99 is fairly recent, even though the spec(ification) is much older.

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