Help on fwrite

Hi guys,

I wrote a program to read DBF file header, content & do some search/delete on records. All others function work fine but delete function doesn't works or exactly it make DBF corrupted after writing deletion mark. The deletion code look like this:

assume that i want to mark delete record number 10

PHP Code:

---

FILE * f = fopen(myFile. "rb");
DBF_HEADER header;
DBF_FHEADER *fields;
int fcount;
int deleteRec = 10;
char *delMark = "*";
int bytesWritten;

//read header
fread(&header, sizeof(header), 1, f);
fcount = (header.headerlen - 33) / 32;  //get total fields

//init & read fields header
fields = new struct DBF_FHEADER[fcount];
fread(fields, sizeof(DBF_FHEADER), fcount, f);

//move to record 10 & take a look at it.
fseek(f, (10-1) * header.RecordLen, SEEK_CUR);
char p;
p = fread(&p, sizeof(char), 1, f);
if(p == "*"){
     cout << "Cannot delete a deleted record." << endl;
     return 0;
}
fclose(f);

//now start to mark delete given record
f = fopen(myFile, "wb");

//move cursor over main header & field header
fseek(f, 32 + (32 * fcount) + 1, SEEK_CUR); 

//move to given record
fseek(f, (10 - 1) * header.RecordLen, SEEK_CUR);

//now write * to the first byte of record string.
bytesWritten = fwrite(delMark, 1, 1, f);
//bytesWritten now has value of 1 indicate that write function completed successfully.

fclose(f); 


---

the code above successfully write a byte to dbf file but it also destroys my DBF file (file size after writing task reduced from > 2 MB to ~520KB) :( anyone knows what happens to my file? please give me an advice. Thanks.

You're opening the file with the "w" flag, which will truncate it to zero length. Use the "a" flag instead. (You will need to use SEEK_SET for your first fseek then, since the current position will be the end of the file after the open.)

yes, my mistake. I edit 'w' flag to 'a' flag then the try it again. but unfortunately, even if i use any move method, the new data keep writing to the end of the file ? That's weird!? I then open a new simple project like this:

PHP Code:

---

//first write some bytes into the file
FILE *f = fopen("test.bin", "wb");
if(f == NULL) return 0;
char *data = "Hello!";
fwrite(data, strlen(data), 1, f);

//close file
fclose(f);

//open it again
f = fopen("test.bin", "ab");

//reposition cursor to begin of file
rewind(f);

//byte to write
char *v = "*";

/set cursor pointer 2 bytes far from byte 0.
fseek(f,  2, SEEK_SET);      //then SEEK_CUR

//write *
fwrite(v, 1, 1, f);

//close file
fclose(f); 


---

the result is: Hello! *

in this sample, the asterisk byte always write into the end of the file (after eof byte) while cursor pointer are in the right place when i add a watch to view it? Do i miss something?

"a" always appends the data to the file. you should use "r+b" so that you can move the file pointer and write the data correctly.

Hope it will help you :)

thanks rxbgain,

after carefully read msdn, i've notice that r+ is use for "update" mode. Now everything works fine. Thank you for your help.