C: Define global array variable that user can declare its size

Question

I want to set a global reference of an int array, in C language, but I want to initialize it inside main function (actually the user is going to declare its size). Anyone knows how is this done?

Thanks in advance!

Using global variables is usually not a good solution, why don't you pass the array and its size to the function from the other modules? — chqrlie
– chqrlie, Commented Jan 17, 2016 at 1:10
Formally, you can't do that. The size of a global array is fixed at compilation time. You can achieve the equivalent effect by using a global pointer and allocating the correct space before any code uses it. But it is usually a good idea to avoid using global variables. It is not always a good idea: stdin, stdout and stderr are global variables, and it would be a confounded nuisance if they weren't. OTOH, errno is global and manages to present problems, though the standard has now partially neutralized the worst of them. — Jonathan Leffler
– Jonathan Leffler, Commented Jan 17, 2016 at 1:19

chqrlie · Accepted Answer · 2016-01-17 01:18:02Z

7

Declare a pointer to an int as a global variable and initialize it in main using malloc.

/* outside any function, so it's a global variable: */
int *array;
size_t array_size;

/* inside main(): */
array_size = user_defined_size;
array = malloc( sizeof(int)*array_size);
if ( array == NULL) {
    /* exit - memory allocation failed. */
}
/* do stuff with array */
free(array);

If you need to access the global variable from another module (source file), declare it there again using

extern int *array;
extern size_t array_size;

or, preferably, declare them extern in a header file included in any source file that uses the array, including the source where they are defined (to ensure type consistency).

edited Jan 17, 2016 at 1:18

chqrlie

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answered Jan 17, 2016 at 1:04

Jochen Müller

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1 Comment

Jonathan Leffler Over a year ago

The header should be used in the file where the pointer (array) is defined as well as in the source files that use the array. That ensures (or does the most that can be done to ensure) that the definitions and uses of the array and its size are consistent.

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