Linux 环境编程：errno的基本用法

背景

error是一个包含在<errno.h>中的预定义的外部int变量，用于表示最近一个函数调用是否产生了错误。若为0，则无错误，其它值均表示一类错误。
errno只在函数返回错误时才有意义，当函数执行正常返回时并不会重置errno，因此此时的errno是不能作为错误处理的。所以在有些时候需要清理error。

perror()和strerrot()函数可以把errno的值转化为有意义的字符输出。

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <errno.h>
#include <string.h>
 
int main(void){

    opendir("/tmp/no_exist");
    //用法1：直接读取errno的值
    printf("errno = %d
", errno);
 
    //用法2：使用perror函数，把error的值转化为有意义的字符提示。
    if(errno != 0){
        perror("opendir: ");
        // exit(EXIT_FAILURE);
    }
    // 补充一个可以 格式化打印的 
    fprintf(stderr, "[%s] %s
", "opendir", strerror(errno)); 

    //用法3：类似用法2，但使用strerror函数。
    if(errno != 0){
        printf("%s
", strerror(errno));
        exit(0);
    }
 
 
    exit(0);
}

输出结果为：

errno = 2
 opendir: : No such file or directory
 No such file or directory

以下程序用于显示所有的错误代码含义

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
 
int main(void){
 
    for(int i = 0; i < 140; i++){
        printf("%4d:%s
", i, strerror(i));
    }
 
    return 0;
}

输出结果如下：

   0:Success
   1:Operation not permitted
   2:No such file or directory
   3:No such process
   4:Interrupted system call
   5:Input/output error
   6:No such device or address
   7:Argument list too long
   8:Exec format error
   9:Bad file descriptor
  10:No child processes
  11:Resource temporarily unavailable
  12:Cannot allocate memory
  13:Permission denied
  14:Bad address
  15:Block device required
  16:Device or resource busy
  17:File exists
  18:Invalid cross-device link
  19:No such device
  20:Not a directory
  21:Is a directory
  22:Invalid argument
  23:Too many open files in system
  24:Too many open files
  25:Inappropriate ioctl for device
  26:Text file busy
  27:File too large
  28:No space left on device
  29:Illegal seek
  30:Read-only file system
  31:Too many links
  32:Broken pipe
  33:Numerical argument out of domain
  34:Numerical result out of range
  35:Resource deadlock avoided
  36:File name too long
  37:No locks available
  38:Function not implemented
  39:Directory not empty
  40:Too many levels of symbolic links
  41:Unknown error 41
  42:No message of desired type
  43:Identifier removed
  44:Channel number out of range
  45:Level 2 not synchronized
  46:Level 3 halted
  47:Level 3 reset
  48:Link number out of range
  49:Protocol driver not attached
  50:No CSI structure available
  51:Level 2 halted
  52:Invalid exchange
  53:Invalid request descriptor
  54:Exchange full
  55:No anode
  56:Invalid request code
  57:Invalid slot
  58:Unknown error 58
  59:Bad font file format
  60:Device not a stream
  61:No data available
  62:Timer expired
  63:Out of streams resources
  64:Machine is not on the network
  65:Package not installed
  66:Object is remote
  67:Link has been severed
  68:Advertise error
  69:Srmount error
  70:Communication error on send
  71:Protocol error
  72:Multihop attempted
  73:RFS specific error
  74:Bad message
  75:Value too large for defined data type
  76:Name not unique on network
  77:File descriptor in bad state
  78:Remote address changed
  79:Can not access a needed shared library
  80:Accessing a corrupted shared library
  81:.lib section in a.out corrupted
  82:Attempting to link in too many shared libraries
  83:Cannot exec a shared library directly
  84:Invalid or incomplete multibyte or wide character
  85:Interrupted system call should be restarted
  86:Streams pipe error
  87:Too many users
  88:Socket operation on non-socket
  89:Destination address required
  90:Message too long
  91:Protocol wrong type for socket
  92:Protocol not available
  93:Protocol not supported
  94:Socket type not supported
  95:Operation not supported
  96:Protocol family not supported
  97:Address family not supported by protocol
  98:Address already in use
  99:Cannot assign requested address
 100:Network is down
 101:Network is unreachable
 102:Network dropped connection on reset
 103:Software caused connection abort
 104:Connection reset by peer
 105:No buffer space available
 106:Transport endpoint is already connected
 107:Transport endpoint is not connected
 108:Cannot send after transport endpoint shutdown
 109:Too many references: cannot splice
 110:Connection timed out
 111:Connection refused
 112:Host is down
 113:No route to host
 114:Operation already in progress
 115:Operation now in progress
 116:Stale file handle
 117:Structure needs cleaning
 118:Not a XENIX named type file
 119:No XENIX semaphores available
 120:Is a named type file
 121:Remote I/O error
 122:Disk quota exceeded
 123:No medium found
 124:Wrong medium type
 125:Operation canceled
 126:Required key not available
 127:Key has expired
 128:Key has been revoked
 129:Key was rejected by service
 130:Owner died
 131:State not recoverable
 132:Operation not possible due to RF-kill
 133:Memory page has hardware error
 134:Unknown error 134
 135:Unknown error 135
 136:Unknown error 136
 137:Unknown error 137
 138:Unknown error 138
 139:Unknown error 139