Read a file containing an array of long in C - c

Question

I am trying to get the data from an array of longs that I have just created but I got different data.
please see code below :
#include <string.h>
#include "readfile.h"
int main()
{
long wr_data [6] ;
wr_data[0] = 11;
wr_data[1] = 1100;
wr_data[2] = 1122323;
wr_data[3] = 11333;
wr_data[4] = 11434243;
wr_data[5] = 1166587;
writeFile(wr_data);
readFile();
return(0);
}
int readFile()
{
FILE *file;
long * data
printf("Error Reading File\n");;
/* Open file for both reading and writing */
file = fopen(fileName, "r");
if (file == NULL)
{
printf("Error Reading File\n");
return -1;
}
for (int i = 0; i < 5; i++)
{
fscanf(file, "%ld", &data[i] );
printf("data[%d]: %ld \n",i, data[i]);
}
fclose(file);
return 0;
}
int writeFile(long * data)
{
FILE *fp;
if (data != NULL)
{
if ((fp = fopen(fileName,"w")) == NULL)
return -1;
if (*data !=0 )
fwrite(data,sizeof(long),6,fp);
printf("Write data\n");
fclose(fp);
}
return 0;
}
the result I get is as follows :
Write data
data[0]: 140526045102081
data[1]: 47
data[2]: 197764
data[3]: 140526045102080
data[4]: 4096
I want to preserve the write function as it is as it comes from an existing code. I tried also the function fread but without success
fread(data, sizeof(long ), 6, file);
Thanks in advance for help.

Answer 1

It's working here. I made the following changes to your code:
//needed for malloc
#include <stdio.h>
//needed for output
#include <stdlib.h>
...
char *fileName = "so";
...
//allocate memory to store the values
long *data = (long *)malloc(sizeof(long)*6);
...
//read the stored longs
fread(data, sizeof(long ), 6, file);
int i;
for(i=0; i<6; i++)
printf("%ld\n", data[i]);
what do you think?
edit:
Well the main change was the memory allocation. When you want to store values of any kind, your program needs to be granted by the operating system a memory zone to store those values.
In this case we had two options, either create a staticly allocated array with a fixed size, or allocate the needed memory in a dynamic fashion with the malloc function or equivalent.
Don't forget, if you want to store something, first make sure you have a place for it to be stored (i.e. allocated memory). If you don't you will most likely get an error "Segmentation Fault" aka "SIGSEGV" which means that you tried to access memory that didn't belong to you.
Also, the "fscanf(file, "%ld", &data[i] );" will read "file" as text and will try to parse floats out of that same text. Since you're storing the longs as longs and not as text, this will not work, since you're writing and reading different things.

Answer 2

You are writing the binary content of the array to the file and afterwards try to interpret this as a long value which can obviously not work. If you want to store the numbers as text you must convert them to text before writing or print them to file by using the fprintf(FILE *, const char *, ...) function.
It is working as expected using the following code using a text file (you might want to change the filename). Otherwise you could just fwrite and fread the whole content, depending on your needs.
#include <stdio.h>
const char *filename = "yourfile";
int readFile()
{
FILE *file;
long data[6];
int i;
printf("Error Reading File\n");;
/* Open file for both reading and writing */
file = fopen(filename, "r");
if (file == NULL)
{
printf("Error Reading File\n");
return -1;
}
for (i = 0; i < 6; i++)
{
fscanf(file, "%ld", &data[i] );
printf("data[%d]: %ld \n",i, data[i]);
}
fclose(file);
return 0;
}
int writeFile(long * data)
{
FILE *fp;
int i;
if (data != NULL)
{
if ((fp = fopen(filename,"w")) == NULL)
return -1;
if (*data !=0 )
{
for(i = 0; i != 6; ++i)
fprintf(fp, "%ld ", data[i]);
}
printf("Write data\n");
fclose(fp);
}
return 0;
}
int main()
{
long wr_data [6] ;
wr_data[0] = 11;
wr_data[1] = 1100;
wr_data[2] = 1122323;
wr_data[3] = 11333;
wr_data[4] = 11434243;
wr_data[5] = 1166587;
writeFile(wr_data);
readFile();
return(0);
}