Reading a dynamic array from a file with function - c

Question

I have the following code:`
#include <stdio.h>
#include <stdlib.h>
void load_from_file(int A[], int *n);
int main()
{
int *A;
A = (int*)malloc(0);
int count = 0;
int i;
load_from_file(A, &count);
for(i = 0; i < count; i++)
{
printf("A[%d]=%d ", i, A[i]);
printf("\n");
printf("&A[%d]=%p \n\n", i, &A[i]);
}
return 0;
}
void load_from_file(int A[], int *n)
{
FILE* fp;
int temp, i;
fp = fopen("data.txt", "r");
if (fp == NULL)
{
printf("error!!");
exit (1);
}
fscanf(fp, "%d", &temp);
*n = temp;
A = (int*) realloc(A, temp * sizeof(int));
if (*A == NULL)
{
printf("error realloc!!");
exit(1);
}
for(i = 0; i < temp; i++)
{
fscanf(fp, "%d", &A[i]);
}
for(i = 0; i < temp; i++)
{
printf("A[%d]=%d ", i, A[i]);
printf("\n");
printf("&A[%d]=%p \n\n", i, &A[i]);
}
fclose(fp);
}
I'm trying to read a text file into an array.
First line of the file has the number of elements of the array, and second line the numbers-elements.
We create the array through realloc.
But something is going wrong.....
I have some patches, printing the address of array's elements.
But unfortunatelly they are different(not all the times) inside the function, and outside the function, although an array is passed by reference (as I think...)
Please, tell me where is the mistake, and how can I fix the problem.
Thanks in advance...
Dimitri

Answer 1

Call:
int *A = load_from_file(&count);
Function:
int *load_from_file(int *n)
{
FILE* fp;
int temp,i;
int *A = 0;
fp=fopen("data.txt","r");
if (fp==NULL){fprintf(stderr, "error!!\n");exit (1);}
fscanf(fp,"%d",&temp);
*n=temp;
A = (int*) realloc(A,temp * sizeof(int));
if (A == NULL) {fprintf(stderr, "error realloc!!\n");exit(1);}
for(i=0;i<temp;i++)
fscanf(fp, "%d",&A[i]);
for(i=0;i<temp;i++)
{
printf("A[%d]=%d ",i,A[i]);
printf("\n");
printf("&A[%d]=%p \n\n",i,&A[i]);
}
fclose(fp);
return A;
}
This is a totally minimal set of changes; the code needs a lot more work. In particular, the fscanf() calls should be error checked. There's a subtle but important change in the allocation test: if (A == NULL) rather than the original if (*A == NULL).
With somewhat more complete error checking:
int *load_from_file(int *n)
{
FILE *fp;
int temp, i;
int *A = 0;
const char file[] = "data.txt";
fp = fopen(file, "r");
if (fp == NULL)
{
fprintf(stderr, "Failed to open file %s for reading\n", file);
exit(1);
}
if (fscanf(fp, "%d", &temp) != 1)
{
fprintf(stderr, "Failed to read number of entries\n");
exit(1);
}
*n = temp;
A = (int *) realloc(A, temp * sizeof(int));
if (A == NULL)
{
fprintf(stderr, "Failed to reallocate %zu bytes of memory\n",
temp * sizeof(int));
exit(1);
}
for (i = 0; i < temp; i++)
{
if (fscanf(fp, "%d", &A[i]) != 1)
{
fprintf(stderr, "Failed to read entry number %d\n", i);
exit(1);
}
}
for (i = 0; i < temp; i++)
printf("A[%d]=%d: &A[%d]=%p\n", i, A[i], i, &A[i]);
fclose(fp);
return A;
}
I have a library of error reporting functions that I use which would reduce the error handling from 4 lines to 1 line per error. I strongly recommend creating and using such a library for yourself.