Why is my FILE pointer causing an undefined symbol error on mac?

I don't understand why is marking as identifier "FILE" is undefined. Firstly I thought it was because of the includes, but in my code I include <stdio.h>. Then I thought that it was just a "marking" squiggle, but when I execute in the terminal shows segmentation fault, so I don't know what I can do. Here is my program:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main() {
    FILE *fp;
    fp = fopen("taula.txt", "W");
    for (double i = -0.001; i < 0.001; i += 0.00001) {
        fprintf(fp, "%lf %.14lf \n", i, -pow(i,4)*1/(840)+(i*i)*1/(30)-1/3);
    }
    fclose(fp);
    return 0;
}

I'm using Visual Studio Code 1.71.2 (Universal), clang as compiler and the OS I use is macOS Monterey 12.6 in MacBook Pro with M1 Pro. I hope someone can solve my problem.

Solution

The error reported for FILE seems unwarranted. Check these possibilities:

maybe the compiler cannot find the standard header files, but it should report this as an error too.
is there is an empty file called stdio.h somewhere in your include path?

Note also these problems:

to open the file for writing, you should use "w", not "W".
you should test for fopen failure, which probably happens because of the above mistake.
the number of iterations in the loop may not be exactly 200 because of cumulative errors adding 0.00001, which cannot be represented exactly using binary floating point representation.
the expression -pow(i,4)*1/(840)+(i*i)*1/(30)-1/3 seems incorrect: 1/3 evaluates to 0 because it uses integer arithmetics.

Here is a modified version:

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
    FILE *fp = fopen("taula.txt", "w");
    if (fp == NULL) {
        fprintf(stderr, "cannot open taula.txt: %s\n", strerror(errno));
        return 1;
    }
    for (double j = -100; j < 100; j += 1) {
        double i = j / 100000.;
        double i2 = i * i;
        fprintf(fp, "%f %.14f\n", i, -i2*i2/840. + i2/30. - 1./3.);
    }
    fclose(fp);
    return 0;
}