程序返回return与系统退出exit
return是语言级别的，它表示了调用堆栈的返回；
exit则是系统调用级别的，它表示了一个进程的结束。
return是返回函数调用，如果返回的是main函数，则为退出程序 。
exit是在调用处强行退出程序，运行一次程序就结束 。
exit(1)与 exit(0)区别
exit(1)表示异常退出。
exit(0)表示正常退出
该值返回给操作系统的,0是正常退出,其他值是异常退出,
该值会被写入环境变量ERRORLEVEL，其它程序可以由此判断该程序结束状态。
在退出前可以给出一些提示信息,便可以在调试程序中查看出错原因。
```c
#include <stdio.h>
void sys_err(const char *str)
{
perror(str);
exit(1);
}
int fd = socket(AF_INET, SOCK_STREAM, 0);
if(fd == -1){
sys_err("socket");
}
只要调用的 Linux 系统函数有异常（返回 - 1）, 错误对应的错误号就会被设置给这个全局变量errno 。
错误号：
全局变量 errno 和错误信息描述:
#define EPERM 1
#define ENOENT 2
#define ESRCH 3
#define EINTR 4
#define EIO 5
#define ENXIO 6
#define E2BIG 7
#define ENOEXEC 8
#define EBADF 9
#define ECHILD 10
#define EAGAIN 11
#define ENOMEM 12
#define EACCES 13
#define EFAULT 14
#define ENOTBLK 15
#define EBUSY 16
#define EEXIST 17
#define EXDEV 18
#define ENODEV 19
#define ENOTDIR 20
#define EISDIR 21
#define EINVAL 22
#define ENFILE 23
#define EMFILE 24
#define ENOTTY 25
#define ETXTBSY 26
#define EFBIG 27
#define ENOSPC 28
#define ESPIPE 29
#define EROFS 30
#define EMLINK 31
#define EPIPE 32
#define EDOM 33
#define ERANGE 34
#define EDEADLK 35
#define ENAMETOOLONG 36
#define ENOLCK 37
#define ENOSYS 38
#define ENOTEMPTY 39
#define ELOOP 40
#define EWOULDBLOCK EAGAIN
#define ENOMSG 42
#define EIDRM 43
#define ECHRNG 44
#define EL2NSYNC 45
#define EL3HLT 46
#define EL3RST 47
#define ELNRNG 48
#define EUNATCH 49
#define ENOCSI 50
#define EL2HLT 51
#define EBADE 52
#define EBADR 53
#define EXFULL 54
#define ENOANO 55
#define EBADRQC 56
#define EBADSLT 57
#define EDEADLOCK EDEADLK
#define EBFONT 59
#define ENOSTR 60
#define ENODATA 61
#define ETIME 62
#define ENOSR 63
#define ENONET 64
#define ENOPKG 65
#define EREMOTE 66
#define ENOLINK 67
#define EADV 68
#define ESRMNT 69
#define ECOMM 70
#define EPROTO 71
#define EMULTIHOP 72
#define EDOTDOT 73
#define EBADMSG 74
#define EOVERFLOW 75
#define ENOTUNIQ 76
#define EBADFD 77
#define EREMCHG 78
#define ELIBACC 79
#define ELIBBAD 80
#define ELIBSCN 81
#define ELIBMAX 82
#define ELIBEXEC 83
#define EILSEQ 84
#define ERESTART 85
#define ESTRPIPE 86
#define EUSERS 87
#define ENOTSOCK 88
#define EDESTADDRREQ 89
#define EMSGSIZE 90
#define EPROTOTYPE 91
#define ENOPROTOOPT 92
#define EPROTONOSUPPORT 93
#define ESOCKTNOSUPPORT 94
#define EOPNOTSUPP 95
#define EPFNOSUPPORT 96
#define EAFNOSUPPORT 97
#define EADDRINUSE 98
#define EADDRNOTAVAIL 99
#define ENETDOWN 100
#define ENETUNREACH 101
#define ENETRESET 102
#define ECONNABORTED 103
#define ECONNRESET 104
#define ENOBUFS 105
#define EISCONN 106
#define ENOTCONN 107
#define ESHUTDOWN 108
#define ETOOMANYREFS 109
#define ETIMEDOUT 110
#define ECONNREFUSED 111
#define EHOSTDOWN 112
#define EHOSTUNREACH 113
#define EALREADY 114
#define EINPROGRESS 115
#define ESTALE 116
#define EUCLEAN 117
#define ENOTNAM 118
#define ENAVAIL 119
#define EISNAM 120
#define EREMOTEIO 121
#define EDQUOT 122
#define ENOMEDIUM 123
#define EMEDIUMTYPE 124
#define ECANCELED 125
#define ENOKEY 126
#define EKEYEXPIRED 127
#define EKEYREVOKED 128
#define EKEYREJECTED 129
#define EOWNERDEAD 130
#define ENOTRECOVERABLE 131
#define ERFKILL 132
#define EHWPOISON 133
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