How to get the last line of a binary file in C? - c++

Question

I want to get the last record in a binary file, extract its ID and add one to it to mimic an auto-incrementing ID feature. So for example, if the previous user had an ID of 1, then the next user should have an ID of 2, and so on. The issue is I can't get the last record of the binary file using the code below. How can I go about doing this? Here's my code so far:
#include <stdio.h>
struct Applicant
{
int id;
};
void increment()
{
char fileName = "data.dat";
FILE *file;
// Instantiate the applicant struct
struct Applicant applicant;
// Will contain details of the last applicant in the file
struct Applicant previousApplicant;
// Open the file in 'append binary' mode
file = fopen(fileName, "ab");
// If file does not exist, print error message and exit
if (!file)
{
printf("\nSorry, that file does not exist");
exit(1);
}
// Read the last applicant to get their id
while(1)
{
fread(&previousApplicant, sizeof(previousApplicant), 1, file);
if (feof(file)) { break; }
}
// If there's no previous applicant, set the ID to 100
if (previousApplicant.id == 0)
{
applicant.id = 100;
}
// Otherwise just increment the previous applicant's id and assign it to the new applicant
else
{
applicant.id = previousApplicant.id + 1;
}
fwrite(&applicant, sizeof(applicant), 1, file);
fclose(file);
}
To achieve this, I'm using structs

Answer 1

The way I'd suggest doing it is to call fseek() to seek directly to the last record in the file, read it in to RAM, update it, and write it out again. That way you don't have to read in every record in the file just to update the last one, which would be really inefficient if the file is large and/or you have to perform this operation often.
Something like this:
#include <stdio.h>
#include <stdlib.h>
struct Applicant
{
int id;
};
void increment()
{
const char fileName[] = "data.dat";
// Open the file in 'append binary' mode
FILE *file = fopen(fileName, "a+b");
if (file == NULL)
{
printf("Couldn't open file [%s]\n", fileName);
return;
}
// Seek to the end of the file
if (fseek(file, 0, SEEK_END) != 0)
{
printf("Couldn't seek to the end of the file!\n");
fclose(file);
return;
}
// Will contain details of the last applicant in the file
struct Applicant previousApplicant;
// Check to make sure the file is big enough to hold at least one record
const long fileLengthBytes = ftell(file);
if (fileLengthBytes < sizeof(previousApplicant))
{
printf("File is to short to contain a full record!?\n");
fclose(file);
return;
}
// Check to make sure the file length is an even multiple of the record-length
if ((fileLengthBytes % sizeof(previousApplicant)) != 0)
{
printf("File length (%li) is not a multiple of record size (%zu)!?\n", fileLengthBytes, sizeof(previousApplicant));
fclose(file);
return;
}
// Seek backwards to the start of the last record in the file
if (fseek(file, -sizeof(previousApplicant), SEEK_CUR) != 0)
{
printf("Couldn't seek to the start of the last record in the file!\n");
fclose(file);
return;
}
// Read in the last record in the file
if (fread(&previousApplicant, sizeof(previousApplicant), 1, file) != 1)
{
printf("Couldn't read the last record in the file!\n");
fclose(file);
return;
}
// Increment the record
printf("Incrementing ID of last record in the file from %i to %i\n", previousApplicant.id, previousApplicant.id+1);
previousApplicant.id++;
// Finally, write out the new record in the file with incremented ID
if (fwrite(&previousApplicant, sizeof(previousApplicant), 1, file) != 1)
{
printf("Couldn't write the last record in the file!\n");
fclose(file);
return;
}
// success!
fclose(file);
}
int main(int, char **)
{
increment();
return 0;
}

Answer 2

There were a few problems with your code. You were reading the applicant ID into the applicant struct variable rather than the previousApplicant variable and when reading and writing files it is best to separate out the tasks: first open the file for reading, then when you're ready to write, close it and open it for writing.
Here's the modified working code which will do what you want:
#include <stdio.h>
#include <stdlib.h>
struct Applicant
{
int id;
};
void increment()
{
char* fileName = "data.dat";
FILE *file;
// Instantiate the applicant struct
struct Applicant applicant;
// Will contain details of the last applicant in the file
struct Applicant previousApplicant;
// Open the file in 'append binary' mode
file = fopen(fileName, "rb");
// If file does not exist, print error message and exit
if (!file)
{
printf("\nSorry, that file does not exist");
exit(1);
}
// Read the last applicant to get their id
while(1)
{
fread(&previousApplicant, sizeof(applicant), 1, file);
if( feof(file) ) break;
}
// If there's no previous applicant, set the ID to 100
if (previousApplicant.id == 0)
{
applicant.id = 100;
}
// Otherwise just increment the previous applicant's id and assign it to the new applicant
else
{
applicant.id = previousApplicant.id + 1;
}
fclose(file);
FILE* out = fopen(fileName, "ab");
fwrite(&applicant, sizeof(applicant), 1, out);
printf("%d\n", applicant.id);
fclose(out);
}
void main() {
increment();
}

Answer 3

I am not sure I got the logic of what you are trying to do. As pointed above, you can use just fseek() to move the current pointer for the file.
from the [documentation][1]
int fseek(
FILE *stream,
long offset,
int origin
);
int _fseeki64(
FILE *stream,
__int64 offset,
int origin
);
As for the offset: you can use this pre-defined constants: SEEK_CUR meaning the current position in file, SEEK_SET meaning the beginning and SEEK_END pointing, well, to the END of the file. The current offset from the beginning of file you can get by calling fseek()'s sister, ftell(), that returns a long with the position. First byte is 0. You can for sure seek for any position.
An example:
At first consider this code:
int create(const char* f_name, unsigned int n_rec)
{
Applicant one = {100};
FILE* f = fopen(f_name, "wb");
if (f == NULL) return -1;
for (unsigned i = 0; i < n_rec; i += 1, one.id+= 1)
if (fwrite(&one, sizeof(one), 1, f) != 1)
return 1 + i;
fclose(f);
return n_rec;
}; // create()
This function creates the file and writes down the supplied number of IDs. e.g.
create( "x.dat", 300 )
will go as expected and create x.dat and write 300 IDs starting at 100 --- inclusive --- in the file.
Then the following function in the example below reopens the file, get the last ID used and returns it, and you can use it as a prototype:
int get_last_id(const char* f_name)
{
Applicant one = {0};
int res = 0;
FILE* f = fopen(f_name, "rb+");
if (f == NULL) return -1;
res = fseek(f, SEEK_END, SEEK_SET);
if (res != 0) return -2;
res = fseek(f, -(long) (sizeof(one)), SEEK_END);
if (res != 0)
{
printf("res = %d\n", res);
perror("Error in seek()");
return -3;
}
printf("(file position: byte #%ld)\n", ftell(f));
if (fread(&one, sizeof(one), 1, f) != 1) return -4;
fclose(f);
return one.id;
}
The lines you want are these:
Applicant one = {0};
fseek(f, SEEK_END, SEEK_SET);
fseek(f, -(long) (sizeof(one)), SEEK_END);
fread(&one, sizeof(one), 1, f);
You go to the end if file, then goes back 1 record and read it.
sample output
5 Ids written to "sample.dat"
now open file and get the last ID used
(file position: byte #16)
Last ID on "sample.dat" is 104
For this
C code
#include <stdio.h>
typedef struct
{ int id;
} Applicant;
int create(const char*,unsigned);
int get_last_id(const char*);
int main(void)
{
const char* f_name = "sample.dat";
const unsigned initial_size = 5;
int res = 0;
// a few ids are written to file
if ( (res = create(f_name, initial_size)) < 0) return -1;
printf("\
%d Ids written to \"%s\"\n\
now open file and get the last ID used\n",
res, f_name);
// get the last id on file
res = get_last_id(f_name);
if (res < 0)
{
printf("Could not get id from file (%d)\n", res);
return -1;
}
else
printf("Last ID on \"%s\" is % d\n", f_name, res);
return 0;
}
int get_last_id(const char* f_name)
{
Applicant one = {0};
int res = 0;
FILE* f = fopen(f_name, "rb+");
if (f == NULL) return -1;
res = fseek(f, SEEK_END, SEEK_SET);
if (res != 0) return -2;
res = fseek(f, -(long) (sizeof(one)), SEEK_END);
if (res != 0)
{
printf("res = %d\n", res);
perror("Error in seek()");
return -3;
}
printf("(file position: byte #%ld)\n", ftell(f));
if (fread(&one, sizeof(one), 1, f) != 1) return -4;
fclose(f);
return one.id;
}
int create(const char* f_name, unsigned int n_rec)
{
Applicant one = {100};
FILE* f = fopen(f_name, "wb");
if (f == NULL) return -1;
for (unsigned i = 0; i < n_rec; i += 1, one.id+= 1)
if (fwrite(&one, sizeof(one), 1, f) != 1)
return 1 + i;
fclose(f);
return n_rec;
}; // create()
```
[1]: https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/fseek-fseeki64?view=msvc-160