Question

Hello every one this is basically extension to my previous question. i have write the short int value in file using

short int x= 254;
FILE * f1 = fopen("infile" ,"w");
fwrite (&x , 1 , sizeof(short int ) , f1 );

it working fine but when i tried to retrieve value like this

short int y ;
fread(&y , 2, 1 ,f1);
printf("%d"  , y);

it gave me answer 8180 and next time 12276 so on… what should i do

actually I want to store short integers in my file and then retrieve them one by one am i doing it wrong kindly guide me

Answer 1

If you want to read back from the file you just wrote to, you need to open it with “w+” options to allow you to read and write. You then need to seek back to the beginning of the file:

// Open the file
const char* fname = "infile";
FILE * f1 = fopen("infile" ,"wb+");
if( f1 != NULL )
{
    printf("opened file '%s' for writing / reading\n", fname);

    // Write two bytes
    short int x = 254;
    size_t bytes_written = fwrite(&x, 1, sizeof(short int), f1 );
    printf("%hd bytes written\n"  , bytes_written);

    // Seek to the start of the file
    int seek_result = fseek(f1, 0, 0);
    if( seek_result == 0 )
    {
        printf("found beginning of file\n");
    }
    else
    {
        printf("failed to seek to beginning of file\n");
    }

    // Read the two bytes back in
    short int y = -9999;
    size_t bytes_read = fread(&y, 1, sizeof(short int), f1);
    printf("%hd bytes read: %hd\n", bytes_read, y);

    // We're done with the file so close it
    fclose(f1);
}
else
{
    // Something went wrong and we failed to open the file
    printf("failed to open file '%s' for writing / reading\n", fname);
}

Alternatively, you could read / write as separate operations:

// Open the file
const char* fname = "infile";
FILE * f1 = fopen("infile" ,"wb");
if( f1 != NULL )
{
    printf("opened file '%s' for writing\n", fname);

    // Write two bytes
    short int x = 254;
    size_t bytes_written = fwrite(&x, 1, sizeof(short int), f1 );
    printf("%hd bytes written\n"  , bytes_written);

    // We're done with the file so close it
    fclose(f1);
}
else
{
    // Something went wrong and we failed to open the file
    printf("failed to open file '%s' for writing\n", fname);
}

// re-open the file
FILE * f2 = fopen("infile" ,"rb");
if( f2 != NULL )
{
    printf("opened file '%s' for reading\n", fname);

    // Read the two bytes back in
    short int y = -9999;
    size_t bytes_read = fread(&y, 1, sizeof(short int), f2);
    printf("%hd bytes read: %hd\n", bytes_read, y);

    // We're done with the file so close it
    fclose(f2);
}
else
{
    // Something went wrong and we failed to open the file
    printf("failed to open file '%s' for reading\n", fname);
}

Note that in your original code you weren’t checking the return value of fread to check whether you’d actually read anything from the file. If you’d done that you’d have seen it was returning 0 to indicate that it read 0 bytes. If you’re not reading anything from the file and you’re not initialising y then it’s likely to just be a random number.