Q50793: Why After Edit, fopen with Mode "a" Doesn’t Appear to Append

Article: Q50793
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | S_QuickC S_QuickASM | mspl13_c
Last Modified: 30-NOV-1989

Problem:

If I use edlin or Wordstar to edit a file, then call the fopen
routine with the "a" option and append to the file, the appended
text doesn't show up when I type the file.

Response:

Edlin and Wordstar are examples of editors that place a CTRL+Z
character at the end of the file. Opening a file with the "a" option
writes to the file beginning after the last character in the file (in
this case, the CTRL+Z). If you then edit the file again with the same
editor, the editor ignores all characters after the CTRL+Z.

The DOS TYPE command will display all characters up to the first EOF.
However, if you bring the file up in the M editor or Word, which
display the CTRL+Z character as a normal character, you can see the
appended text and, if you want, delete the CTRL+Z character.

If you fopen a file with the "a+" mode, the CTRL+Z character is
deleted at the end of the file, and appended text can be seen in any
editor or by using the DOS TYPE command.

The following code demonstrates this behavior:

#include <stdio.h>
#include <time.h>

void main(void)
{
     FILE *fp;
     int num;
     time_t system_time;

     fp = fopen("out.dat","a");          /* change to "a+" to fix */
     time(&system_time);                 /* to demonstrate last append */
     num = fprintf(fp, "Writing to file at %s\n",
                         ctime(&system_time));
     num = fclose(fp);
}

Run this program once to write the current system time to the output
file. Then edlin "out.dat". You do not need to add any lines, just
open the file and then end the session. Then run the program again.
Type "out.dat", and notice that you still see only the first output
time.

Changing the fopen "a" to "a+" and rebuilding the program will
demonstrate the solution.

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