Returning structures from functions. C - c

Question

I am new to C and programming as a whole. I am trying to create a simple file validation program that reads in records from a file and sorts valid from invalid records. I have managed to achieve this but have run into problems when trying to return the structs for use in the main program.
#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>
#define MAX 20
typedef struct data{
int ref;
int serial;
char string[MAX+1];
}vaild,invalid;
int g = 1;
int e = 1;
void read(FILE *file);
void val(struct data* t);
void create_record(struct data* temp);
void create_error(struct data* temp);
void export_data(struct data* record, struct data* error);
int main(){
FILE *file = fopen("file.txt", "r");
if (file != NULL){
read (file);
I want to be able to call the structures 'valid and 'invalid' from here inside the main function. Like the commented out printf function below.
//printf("%i", valid[1].ref); <<<<<<<<<< I need to be able to play around with the structures from inside the main function!
}
return 0;
}
void read(FILE *file){
struct data* t = malloc(sizeof(struct data));
char buf[1000];
while(!feof(file)){
fgets(buf, sizeof buf, file);
sscanf(buf, "%d.%d.%s", &t->ref, &t->serial, t->string);
val(t);
}
}
void val (struct data* t){
if((t->ref < 30)){
struct data* valid = (struct data*) malloc(sizeof(struct data));
valid = (struct data*)realloc(valid, g * sizeof(struct data));
valid[g-1] = *t;
if (valid == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nGOOD:%i.%i.%s\n", valid[g-1].ref, valid[g-1].serial, valid[g-1].string);
g++;
}
else{
struct data* invalid = (struct data*) malloc(sizeof(struct data));
invalid = (struct data*)realloc(invalid, e * sizeof(struct data));
invalid[e-1] = *t;
if ( invalid == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nBAD:%i.%i.%s\n", invalid[e-1].ref, invalid[e-1].serial, invalid[e-1].string);
e++;
}
}
I cant seem to be able to call the structures from within main by using the return functions, I feel I must be doing something really simple wrong. Getting pretty frustrated.
The input file is like so:
04.06.hello
09.65.test
88.55.string
27.12.qwerty
11.53.ytrewq
92.02.ecco
Everything else is working fine and there are no compiler errors. I have changed all the return types to void to make it less messy, I think I must be using them wrong.

Answer 1

Found your problem
void val (struct data* t){
if((t->ref < 30)){
struct data* valid = (struct data*) malloc(sizeof(struct data));
valid = (struct data*)realloc(valid, g * sizeof(struct data));
valid[g-1] = *t;
if (valid == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nGOOD:%i.%i.%s\n", valid[g-1].ref, valid[g-1].serial, valid[g-1].string);
g++;
}
else{
struct data* invalid = (struct data*) malloc(sizeof(struct data));
invalid = (struct data*)realloc(invalid, e * sizeof(struct data));
invalid[e-1] = *t;
if ( invalid == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nBAD:%i.%i.%s\n", invalid[e-1].ref, invalid[e-1].serial, invalid[e-1].string);
e++;
}
}
In above code when you define valid and invalid inside If and else if , scope of these variables are inside those if conditions. So you will get "valid not defined" error.
Try this
struct data* val(struct data* t){
if((t->ref < 30)){
struct data* valid_file = (struct data*) malloc(sizeof(struct data));
valid_file = (struct data*)realloc(valid_file, g * sizeof(struct data));
valid_file[g-1] = *t;
if (valid_file == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nGOOD:%i.%i.%s\n", valid_file[g-1].ref, valid_file[g-1].serial, valid_file[g-1].string);
g++;
return valid_file;
}
// Will come when result is invalid
struct data* invalid_file = (struct data*) malloc(sizeof(struct data));
invalid_file = (struct data*)realloc(invalid_file, e * sizeof(struct data));
invalid_file[e-1] = *t;
if ( invalid_file == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nBAD:%i.%i.%s\n", invalid_file[e-1].ref, invalid_file[e-1].serial, invalid_file[e-1].string);
e++;
return invalid_file;
}
Note - Changed valid and invalid to different variable names to separate struct name definitions.
Complete working example :)
#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>
#define MAX 20
typedef struct data{
int ref;
int serial;
char string[MAX+1];
}vaild,invalid;
int g = 1;
int e = 1;
struct data* read(FILE *file);
struct data* val(struct data* t);
void create_record(struct data* temp);
void create_error(struct data* temp);
void export_data(struct data* record, struct data* error);
int main(){
FILE *file = fopen("file.txt", "r");
if (file != NULL){
struct data* answ=read (file);
printf("%i \n", answ->ref);
}
return 0;
}
struct data* read(FILE *file){
struct data* t = malloc(sizeof(struct data));
char buf[1000];
while(!feof(file)){
fgets(buf, sizeof buf, file);
sscanf(buf, "%d.%d.%s", &t->ref, &t->serial, t->string);
val(t);
}
return t;
}
struct data* val(struct data* t){
if((t->ref < 30)){
struct data* valid_file = (struct data*) malloc(sizeof(struct data));
valid_file = (struct data*)realloc(valid_file, g * sizeof(struct data));
valid_file[g-1] = *t;
if (valid_file == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nGOOD:%i.%i.%s\n", valid_file[g-1].ref, valid_file[g-1].serial, valid_file[g-1].string);
g++;
return valid_file;
}
// Will come when result is invalid
struct data* invalid_file = (struct data*) malloc(sizeof(struct data));
invalid_file = (struct data*)realloc(invalid_file, e * sizeof(struct data));
invalid_file[e-1] = *t;
if ( invalid_file == NULL){
puts("Memory allocation error!");
exit(EXIT_FAILURE);
}
printf("\nBAD:%i.%i.%s\n", invalid_file[e-1].ref, invalid_file[e-1].serial, invalid_file[e-1].string);
e++;
return invalid_file;
}

Answer 2

Returning a struct in C is acceptable when the struct is small. Otherwise you'd be better off using pointers.
The snippet below is a simple example that shows you how to return a struct.
Same rules apply as returning any other data type (e.g., int, double, ...), return types need to match and the function needs to have a return statement.
#include <stdio.h>
#include <stdlib.h>
typedef struct {
int x;
int y;
} Point;
Point init_point(Point p, int x, int y) {
p.x = x;
p.y = y;
return p;
}
void main() {
Point p1, p2;
printf("%d, %d\n", p1.x, p1.y);
printf("%d, %d\n", p2.x, p2.y);
p1 = init_point(p1, 55, 10);
printf("%d, %d\n", p1.x, p1.y);
printf("%d, %d\n", p2.x, p2.y);
p2 = init_point(p2, 10000, 190);
printf("%d, %d\n", p1.x, p1.y);
printf("%d, %d\n", p2.x, p2.y);
}
The result is :
-1080464084, -1080464264
1, -1080464092
55, 10
1, -1080464092
55, 10
10000, 190