当我们在调用pid_t waitpid(pid_t pid, int *stat_loc, int options)时，其中第二个参数stat_loc会提供子进程退出的详细信息，为此posix还提供了一组宏来解析这个status.
在\glibc\bits\waitstatus.h
/* If WIFEXITED(STATUS), the low-order 8 bits of the status. */
#define __WEXITSTATUS(status) (((status) & 0xff00) >> 8)
/* If WIFSIGNALED(STATUS), the terminating signal. */
#define __WTERMSIG(status) ((status) & 0x7f)
/* If WIFSTOPPED(STATUS), the signal that stopped the child. */
#define __WSTOPSIG(status) __WEXITSTATUS(status)
/* Nonzero if STATUS indicates normal termination. */
#define __WIFEXITED(status) (__WTERMSIG(status) == 0)
/* Nonzero if STATUS indicates termination by a signal. */
#define __WIFSIGNALED(status) \
(((signed char) (((status) & 0x7f) + 1) >> 1) > 0)
/* Nonzero if STATUS indicates the child is stopped. */
#define __WIFSTOPPED(status) (((status) & 0xff) == 0x7f)
/* Nonzero if STATUS indicates the child continued after a stop. We only
define this if <bits/waitflags.h> provides the WCONTINUED flag bit. */
#ifdef WCONTINUED
# define __WIFCONTINUED(status) ((status) == __W_CONTINUED)
#endif
/* Nonzero if STATUS indicates the child dumped core. */
#define __WCOREDUMP(status) ((status) & __WCOREFLAG)
/* Macros for constructing status values. */
#define __W_EXITCODE(ret, sig) ((ret) << 8 | (sig))
#define __W_STOPCODE(sig) ((sig) << 8 | 0x7f)
#define __W_CONTINUED 0xffff
#define __WCOREFLAG 0x80
我们重点看一下#define __W_EXITCODE(ret, sig) ((ret) << 8 | (sig))，其中：
ret是exit code，就是子进程在退出时，如果有调用exit()时，产生的exitcode. 比如子进程显示调用了exit(1)，那么exitcode就是1。
sig是signal，比如SIGTERM，这时可以认为子进程是被动kill掉的，所以需要记录此时的SIGNAL。
示例：
pclose return 30720 = 0x7800, 0x78 = 120,
sys — System-specific parameters and functions — Python 3.13.3 documentation
Changed in version 3.6: If an error occurs in the cleanup after the Python interpreter has caught SystemExit (such as an error flushing buffered data in the standard streams), the exit status is changed to 120.
)
