Passing array of structs from function

Question

Very new to C and I am trying to figure out (for an assignment) how to properly use structs and functions. Specifically, I am having trouble working out how to pass an array of structs from a function.

The function is supposed to take data from a file and input into an array. The input file has the following data: 1 Al 2 Bill 3 Clark 4 Dean 5 Ellen

After I call the function, I would like to be able to view the array values in the main function.

I don't think I'm passing the struct correctly but am not sure where I'm going wrong. Any suggestions? Thanks.

Here is my code attempt:

// Input file into array
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    int number;
    char name[30];
} Name;

#define SIZE 5

// function prototypes
int loadName( char *file, Name N[] );

// function main begins program execution
int main( void )
{ 
   char file[ 30 ]; // file name
   Name N[ SIZE ]; // array to store names
   size_t i, j=0; // counter

  // check read function opens file correctly
  if (-1 == ( i =  loadName ( file, N ) ) ) {
     puts( "Employee file could not be opened" );
   } // end load if

  printf("\n\nCheck for names in main function\n\n");

    for (j=0; j<i; ++j) {
        printf("%8d%18s\n", N[j].number, N[j].name );
    }
  return 0;

  free(N);
}


// load values from name file
int loadName( char *file, Name N[] )
{
    FILE *inPtr; // inPtr = input file pointer
    size_t i = 0; // counter

    // fopen opens file. Exit program and return -1 if unable to open file 
    if ( ( inPtr = fopen( "name.txt", "r" ) ) == NULL ) {
        return -1;
    } // end if
            else {
               printf("Employee Number   Employee Name\n" );
                // read name from file
        do {
         N = (Name*)malloc(100);
            fscanf( inPtr, "%d%s", &N[i].number, &N[i].name );
            printf("%8d%18s\n", N[i].number, N[i].name );
          i++;
        }  while (!feof( inPtr ));

          } // end else
          fclose( inPtr ); // fclose closes the file  
          return i; // return i after successful for loop
} // end function loadname

Some programmer dude · Accepted Answer · 2014-03-03 17:18:51Z

2

You have to remember that arrays decays to pointers, so in the loadName function the variable N is actually a pointer, and behaves like any other local variable. That means that if you reassign it you only reassign the local copy of the pointer. That's the first thing.

The second thing is that you pass in a pointer to already allocated memory, an array that hold five structures. No need to allocate again inside the function.

Which leads me to the third thing, that you allocate in every iteration of the loop, thereby loosing the previous allocation and leaking memory.

You should take care though, as the array you pass only have allocated space for five structures, if you try to read any more than that you will write beyond the limits of the array and will have a buffer overflow which leads to so called undefined behavior.

answered Mar 3, 2014 at 17:18

Some programmer dude

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