update part of a binary file but dont overwrite whole file

squinchy

Hi,

Im wondering..is there a way to update a record in a binary file rather than overwriting the whole file?

For example, lets say I have a binary file containing 5 records - each record has a name and age.

what i want to do is to open the file, put the file pointer to the record that is to be updated (lets say the third record), read the new data into a record and write that record to the file - overwriting the third record in the file.

Is there a way I can do this?

So far, I have been able to display the contents of the record the user wants to update with this code:

fseek(pointer,sizeof(struct rec)*(record_pos), SEEK_SET);

then i read into a record structure
scanf(............);
scanf(..............);

and then write this record to the file:

fwrite(&myrecord, sizeof(struct rec), 1, pointer);



I have tried initially opening the file using the:

pointer=fopen("dfile.bin", "ab+");

and also

pointer=fopen("dfile.bin", "wb+");

and also

pointer=fopen("dfile.bin", "w");

But all with no luck

cheers

squinchy

srsly78

Make sure your program actually has permissions to write the file. If using windows, certain dirs like program files etc are specially protected.

Colonel Panic

a is append, you can't use that, w creates an empty file for writing and w+ an empty file that is read write.

Try rb+ and check if the pointer is NULL, it would be if you couldn't open the file with the desired access.