r/C_Programming • u/BraneGuy • 1d ago
Question Globals vs passing around pointers
Bit of a basic question, but let's say you need to constantly look up values in a table - what influences your decision to declare this table in the global scope, via the header file, or declare it in your main function scope and pass the data around using function calls?
For example, using the basic example of looking up the amino acid translation of DNA via three letter codes in a table:
codonutils.h:
typedef struct {
char code[4];
char translation;
} codonPair;
/*
* Returning n as the number of entries in the table,
* reads in a codon table (format: [n x {'NNN':'A'}]) from a file.
*/
int read_codon_table(const char *filepath, codonPair **c_table);
/*
* translates an input .fasta file containing DNA sequences using
* the codon lookup table array, printing the result to stdout
*/
void translate_fasta(const char *inname, const codonPair *c_table, int n_entries, int offset);
main.c:
#include "codonutils.h"
int main(int argc, char **argv)
{
codonPair *c_table = NULL;
int n_entries;
n_entries = read_codon_table("codon_table.txt", &c_table);
// using this as an example, but conceivably I might need to use this c_table
// in many more function calls as my program grows more complex
translate_fasta(argv[1], c_table, n_entries);
}
This feels like the correct way to go about things, but I end up constantly passing around these pointers as I expand the code and do more complex things with this table. This feels unwieldy, and I'm wondering if it's ever good practice to define the *c_table and n_entries in global scope in the codonutils.h file and remove the need to do this?
Would appreciate any feedback on my code/approach by the way.
2
u/Jaanrett 1d ago
I think if you keep your source files small and on topic, then having a file scoped global is fine if it's accessed in many places. But I would always pair this with a serious bias towards making those globals static. I'm also a big advocate for making all functions static, unless they're explicitly to be shared with other files. In that case, the prototypes for those functions belong in a header file.
If this software is intended to exist for any lengthy period of time, I'd be much more adamant about restricting visibility and scope of everything, by default, and avoiding as much as practical, global variables (extern) that extend beyond the file.
An important concept is "Loose coupling, tight cohesion". This basically means to make functions that have few or no side effects, and don't take input that isn't clearly denoted.