Dynamically allocating array of structs C - c

Question

I'm trying to make an array of structs in c, but I can't make it work. When I try to run it, the program crashes.
typedef struct{
char name[20];
char manufacturer[20];
unsigned int price;
} product;
unsigned int stringToNr(char *numbers){
unsigned int nr = 0;
unsigned int i;
for (i = 0; i < strlen(numbers); i ++)
{
nr *= 10; nr += numbers[i] - '0';
}
return nr;
}
I have a function that would print the list to a file, sometimes it reaches this function, sometimes it crashes before.
void printList(product *products, unsigned int nr){
unsigned int i;
FILE *f;
f = fopen("output.txt", "w");
for (i = 0; i < nr; i ++){
fprintf(f, "%s ", products[i].name);
fprintf(f, "%s ", products[i].manufacturer);
fprintf(f, "%d\n", products[i].price);
}
fclose(f);
}
I have to use a separate function to read the list from file.
void readList(product **products, unsigned int *nr){
FILE *f;
f = fopen("input.txt", "r");
char *row;
row = malloc(sizeof(char) * 45);
unsigned int rowLength;
fgets(row, 45, f);
rowLength = strlen(row);
if (row[rowLength - 1] == '\n'){
rowLength--;
row[rowLength ] = '\0';
}
*nr = stringToNr(row);
products = malloc((*nr) * sizeof(product*));
unsigned int i;
char *rowElement;
for (i = 0; i < *nr; i ++){
fgets(row, 45, f);
rowElement = strtok(row, " ");
strcpy((*products)[i].name, rowElement);
rowElement = strtok(NULL, " ");
strcpy((*products)[i].manufacturer, rowElement);
rowElement = strtok(NULL, " ");
rowLength = strlen(row);
if (row[rowLength- 1] == '\n'){
rowLength--;
row[rowLength] = '\0';
}
(*products)[i].price = stringToNr(rowElement);
}
free(row);
fclose(f);
}
Obviously the program has more features, but those work fine.
int main(){
product *products;
unsigned int nr;
readList(&products, &nr);
printList(products, nr);
free(products);
return 0;
}
My input file looks like this:
3
AAA FactoryA 300
BBB FactoryC 550
ZZZ Factory5 100

Answer 1

Code ignores value of products.
What ever readList() receives in products is overwritten with the malloc() call.
void readList(product **products, unsigned int *nr){
...
// bad
products = malloc((*nr) * sizeof(product*));
Instead, use *products. Also allocate by the size of the referenced variable, not by the size of the type. Easier to code, review and maintain.
*products = malloc(sizeof *(*products) * (*nr));
if (*products == NULL) Handle_OOM();
Minor: After fgets(row, ..., ...); , following is not safe from a hacker exploit of reading an initial null character.
rowLength = strlen(row);
// What happens when rowLength == 0
if (row[rowLength- 1] == '\n'){
...
Instead code could use below to rid the optional trailing '\n'.
row[strcspn(row, "\n")] = '\0';