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llvm-mos / llvm-mos-sdk

SDK for developing with the llvm-mos compiler
https://www.llvm-mos.org
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fopen(filename, "a") truncates? #353

Closed pfusik closed 3 months ago

pfusik commented 3 months ago

Looking at the code:

static unsigned filemode(const char *const mode) {
  unsigned rc = 0;

  switch (mode[0]) {
  case 'r':
    rc |= FREAD;
    break;

  case 'w':
    rc |= FWRITE;
    break;

  case 'a':
    rc |= FAPPEND | FWRITE;
    break;
  }
static signed char stdio_open(const char *const filename, unsigned int mode) {
  int osmode;

  if (mode & FRW)
    osmode = O_RDWR;
  else if (mode & FREAD)
    osmode = O_RDONLY;
  else
    osmode = O_WRONLY;

  if (mode & (FWRITE | FAPPEND))
    osmode |= O_CREAT;

  if (mode & FWRITE)
    osmode |= mode & FEXCL ? O_EXCL : O_TRUNC;
  else if (mode & FAPPEND)
    osmode |= O_APPEND;

"a" will set O_TRUNC i.e. truncate an existing file. I think we should either not set FWRITE or reorganize the logic in stdio_open.