Variable declaration

Question

I am working a piece of code which reads data from a file and manipulates it. The idea is to load the data in a global manner and then use several functions on the data to perform calculations. The problem I am having is that when compile I get the following error :

' vertices' undeclared (first use in this function).

Header file contains the following :

typedef struct
{
    double x;
    double y;
    double z;
} variable;

In the main I call malloc and a function which will use this array of 'variable' called 'vertices':

int main (void)
{   
    variable *vertices = (variable*) malloc( 5000 * sizeof (variable) ) ; 
    load_file();
    free(vertices);
    return 0;
}

The function load_file ():

    FILE *fp1 ;
    fp1 = fopen( 'file',"r");
    if (fp1==NULL)
    {
        printf("File couldn't be opened or read!");
        return 1;
    }   

    int j = 0;
    while(fscanf(fp1, "%lf %lf %lf ", &vertices[j].x, &vertices[j].y, &vertices[j].z ) == 3 )
    {
        j++;
    }
    fclose(fp1);

In reality when I put the malloc in load_file, it compiles and works but the problem is that I have various other functions which will use the data and if I free it in load_file, I lose everything. If I redefine the typedef above main, I get a 'previous definition was here' and if I add variable vertices; before main, a get a load of errors.

How would I solve such an issue?

Are you sure it doesn't say 'vertices' undeclared in load_file ??? that at least would make some sense. — Roddy
– Roddy, Commented Mar 21, 2013 at 19:53
@Roddy You are absolutely right it does give that error. I just mixed them up. — user1873500
– user1873500, Commented Mar 21, 2013 at 20:00
I've edited your post. @KScottPiel's answer looks right to me. In future it's a great idea to have a naming convention that makes distinguishing types from variables easy. Most people either give types a leading capital (Variable) or a trailing '_t' variable_t — Roddy
– Roddy, Commented Mar 21, 2013 at 20:09

K Scott Piel · Accepted Answer · 2013-03-22 12:10:04Z

3

The problem is that you're declaring "vertices" inside main which makes its scope local to main and load_file() can't see it.

Change the declaration of load_file() as follows...

void load_file( variable* vertices )
{
    /// blah blah
}

and then in main, pass your vertices variable in to it...

int main (void)
{   
    variable *vertices = malloc( 5000 * sizeof (variable) ) ; 
    load_file( vertices );
    free(vertices);
    return 0;
}

EDIT: I would recommend against just making vertices a global variable... that means that anything can access it and/or modify it... even unintentionally. It's almost always wiser to pass arguments in and out of the functions that need them rather than to just make them globally available to the world... scope is your friend.

edited Mar 22, 2013 at 12:10

answered Mar 21, 2013 at 19:52

K Scott Piel

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2 Comments

Randy Howard Over a year ago

If this is indeed C and not C++, then don't cast malloc.

K Scott Piel Over a year ago

True... I should have edited that out when I copy/pasted his code. Good catch.

Christopher Neylan · Accepted Answer · 2013-03-21 19:50:28Z

1

If you want the vertices to be accessible by all of the functions in the file, then move its declaration outside of the main() function (i.e., give it global scope) and only initialize it inside of the main() function:

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

typedef struct {
    double x;
    double y;
    double z;
} variable;

static variable *vertices = NULL;

int load_file() {
    FILE *fp1 ;
    fp1 = fopen( "file","r");
    if (fp1==NULL){
        printf("File couldn't be opened or read!");
        return 1;
    }

    int j = 0;
    while(fscanf(fp1, "%lf %lf %lf ", &vertices[j].x, &vertices[j].y, &vertices[j].z ) == 3 ){
        j++;
    }
    fclose(fp1);
    return 0;
}

int main (void){
    vertices = (variable*) malloc( 5000 * sizeof (variable) ) ;
    load_file();
    free(vertices);
    return 0;
}

If you want the vertices pointer to accessible across all files in your program, then give it external linkage by removing the static keyword from its declaration.

answered Mar 21, 2013 at 19:50

Christopher Neylan

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2 Comments

user1873500 Over a year ago

I had the exact same code as above apart from the null pointer definition : variable *vertices = NULL;. Now it appears to be running (with a segmentation fault...). Thank you very much Sir.

Christopher Neylan Over a year ago

i'd recommend the suggestion of @K Scott Piel. global variables should be reserved for constants. any function that needs the vertices variable should be explicitly given it as a parameter.

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