Reentrant Syscalls - Untitled
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### 12.2 Reentrant covers for OS subroutines

Since the system subroutines are used by other library routines that require reentrancy, `libc.a` provides cover routines (for example, the reentrant version of `fork` is `_fork_r`). These cover routines are consistent with the other reentrant subroutines in this library, and achieve reentrancy by using a reserved global data block (see Reentrancy).

`_open_r`
  A reentrant version of `open`. It takes a pointer to the global data block, which holds `errno`.

                int _open_r(void *reent,
                    const char *file, int flags, int mode);

`_close_r`
  A reentrant version of `close`. It takes a pointer to the global data block, which holds `errno`.

                int _close_r(void *reent, int fd);

`_lseek_r`
  A reentrant version of `lseek`. It takes a pointer to the global data block, which holds `errno`.

                off_t _lseek_r(void *reent,
                    int fd, off_t pos, int whence);

`_read_r`
  A reentrant version of `read`. It takes a pointer to the global data block, which holds `errno`.

                long _read_r(void *reent,
                    int fd, void *buf, size_t cnt);

`_write_r`
  A reentrant version of `write`. It takes a pointer to the global data block, which holds `errno`.

                long _write_r(void *reent,
                    int fd, const void *buf, size_t cnt);

`_fork_r`
  A reentrant version of `fork`. It takes a pointer to the global data block, which holds `errno`.

                int _fork_r(void *reent);

`_wait_r`
  A reentrant version of `wait`. It takes a pointer to the global data block, which holds `errno`.

                int _wait_r(void *reent, int *status);

`_stat_r`
  A reentrant version of `stat`. It takes a pointer to the global data block, which holds `errno`.

                int _stat_r(void *reent,
                    const char *file, struct stat *pstat);

`_fstat_r`
  A reentrant version of `fstat`. It takes a pointer to the global data block, which holds `errno`.

                int _fstat_r(void *reent,
                    int fd, struct stat *pstat);

`_link_r`
  A reentrant version of `link`. It takes a pointer to the global data block, which holds `errno`.

                int _link_r(void *reent,
                    const char *old, const char *new);

`_unlink_r`
  A reentrant version of `unlink`. It takes a pointer to the global data block, which holds `errno`.

                int _unlink_r(void *reent, const char *file);

`_sbrk_r`
  A reentrant version of `sbrk`. It takes a pointer to the global data block, which holds `errno`.

                char *_sbrk_r(void *reent, size_t incr);