将结构存储在数组中时遇到问题

Question

I want to create a database sort of, that stores a list of 5000 names and 5000 coresponding salaries into an array, I simply can't find the problem since the console either crashes or the compiler gives me the following error: "cannot convert 'char* ( )[30]' to 'char ' for argument '1' to 'char*' fgets(char*,int,*FILE)".

EDIT: I changed whatever I could figure out in the code, and I seem to have an issue with this line in particular:

person* TAB = calloc(N, sizeof(struct)); 

I can't spot other errors(lack of experience), and I don't know exactly what to use instead of fgets to put in a line.

#include <stdio.h>
#include <stdlib.h>
#define N 5000
typedef struct {
  char name[30]
  int  salary;
} person;
int main()
{
person* TAB = calloc(N, sizeof(struct));
FILE * input;
input = fopen("in.txt","r+");
int nr=0;
int r;
while(nr<5000)
    {
      fscanf(input,"%s",TAB[nr].name);
      fscanf(input,"%d",TAB[nr].salary);
      nr++;
    }
printf("%s %d",TAB[1].name,TAB[1].salary);
fclose(input);
return 0;
}

Answer 1

You really should avoid putting that much data on the stack, which is what a normal variable like that will generally do. It will occupy around 5000 * (30 + 4 * 5000) = 95 MB of stack space, which might be more than your operating system feels is reasonable.

Anyway, the fix is not to allocate this on the heap; the fix is to change the declaration. I believe there's a logic error, since you allocate space for 5000 salaries per person , which is probably not what you meant.

Also, the name field should be an array of characters, but you've declared it as an array of character pointers , which is what the warnings are all about.

I believe you should have:

struct person
{
    char name[30];
    int  salary;
};

This will drop the memory usage for struct person TAB[N]; down to around 5000 * (30 + 4) or around 166 KB which is way more reasonable. This assumes a 4-byte int which is a pretty common situation.

Finally, your file reading code is not very nicely designed, it will probably not work.

Look into using fgets() to read lines, stopping when it fails (ie never calling feof() ), and then parsing/tokenizing each line as read. Remember that names can contain whitespace, which will make %s in sscanf() stop.

Answer 2

The name struct member should not be declared as 30 char pointers and the salary is one per name so it should look something like this

typedef struct {
  char name[30]
  int  salary;
} person;

Now in order to have it as an array you are best off allocating on the heap

person* persons = calloc(N, sizeof(struct));

Now you can access the name and salary of one person

persons[3].name
persons[3].salary

...

fgets(persons[nr].name,30,input); // although you may want to remove \n

Answer 3

struct person
{
    char* name[30];

I think you mean char name[30] . That is if you want one name of at most 29 bytes.

Later on you do:

fgets(&TAB[nr].name,30,input);

Why are you taking the address? If you make the change above, the TAB[nr].name should be of type char[30] , which should degenerate into a char* as desired.