In fflush, reset the mark to account for flushing the buffer even on read-only files.

This is needed to keep the file mark consistent with the state of the buffer. Otherwise, subsequent reads may return data from the wrong place. (Using fflush on a read-only stream is undefined behavior, but other things in stdio call it that way, so it must work consistently with what they expect.)

Here is an example that was affected by this:

#include <stdio.h>
#include <string.h>

char buf[100];

int main(void) {
        FILE *f = fopen("somefile","r");
        if (!f) return 0;

        setvbuf(f, 0L, _IOFBF, 14);

        fgets(buf, 10, f);
        printf("first read : %s\n", buf);
        ftell(f);

        memset(buf, 0, sizeof(buf));
        fgets(buf, 10, f);
        printf("second read: %s\n", buf);

        fseek(f, 0, SEEK_CUR);
        memset(buf, 0, sizeof(buf));
        fgets(buf, 10, f);
        printf("third read : %s\n", buf);
}

stdio.asm

@@ -372,27 +372,27 @@ fa4      OSwrite wr                     write the info
          jsr   ~ioerror
          bra   rts
 
-fl1      ldy   #FILE_flag               if the file is open for read/write then
+fl1      ldy   #FILE_flag               get flags
          lda   [stream],Y
-fl1a     bit   #_IORW
-         beq   fl3
-         bit   #_IOREAD                   if the file is being read then
-         beq   fl2
-         ph4   <stream                      set the mark to current position
+fl1a     bit   #_IOREAD                 if the file is being read then
+         beq   fl2a
+         ph4   <stream                    set the mark to current position
          jsl   ftell
          cmp   #-1
-         bne   fl1b
+         bne   fl2b
          cpx   #-1
-         beq   fl1c
-fl1b     sta   smPosition
+         beq   fl2
+fl2b     sta   smPosition
          stx   smPosition+2
          ldy   #FILE_file
          lda   [stream],Y
          sta   smRefNum
          OSSet_Mark sm
-fl1c     ldy   #FILE_flag
+fl2      ldy   #FILE_flag               get flags
          lda   [stream],Y
-fl2      and   #$FFFF-_IOWRT-_IOREAD      turn off the reading and writing flags
+fl2a     bit   #_IORW                   if the file is open for read/write then
+         beq   fl3
+         and   #$FFFF-_IOWRT-_IOREAD      turn off the reading and writing flags
          sta   [stream],Y
 fl3      ph4   <stream                  prepare file for output
          jsl   ~InitBuffer