Question

Im stuck when trying to write to my file within my subprogram.

void new_page(float *a, float *b, float *c, int *d){
    fprintf(results,"\nPage Totals: %f\t%f\t%f\t%d", *a,*b,*c,*d);
}

I get a warning saying

Warning: incompatible implicit declaration of built-in function 'fprinf' [enabled by default]

“error: ‘results’ undeclared (first use in this function)”

in main fprintf works fine, its just when it comes to the subprogram/function it wont work. from my understanding it thinks that results is undeclared, so do i have to pass the name or location of the file to make it work?

Answer 1

Unless results is global, it is undefined.

From the looks of it, you have opened a file (stream) as results in main. If so it only exists in the scope of main.

Pass it along to the function. Do not recommend to make it global.

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It would look like:

void new_page(FILE *results,  float *a, float *b, float *c, int *d){
    fprintf(results,"\nPage Totals: %f\t%f\t%f\t%d", *a,*b,*c,*d);
}

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Also; if you have this function in a separate file then main(), where, as you say fprintf works – you have to include stdio.h in that file as well.

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Edit: Sounds like you are doing something wrong with that function signature and prototype etc.

As a simple example (without needed checks on fopen etc):

#include <stdio.h>

void foo(FILE * fp)
{
    fprintf(fp, "HELLO\n");
}

int main(void)
{
    FILE *fp;

    fp = fopen("blabla", "w");
    foo(fp);
    fclose(fp);

    return 0;
}

Also; you say your compiler is not complaining. Are you using extensive warnings? If not you should. Ie GCC:

$ gcc -Wall -Wextra -pedantic -o my_prog my_source.c

I usually also add -std=c89, or if needed, -std=c99.

Then run it wit valgrind:

$ valgrind ./my_prog

These two steps will give you lots of hints and clues to where things could be wrong.