简介
IO顾名思义就是input/output.俗话说linux一切皆文件,对文件的操作不外乎就读和写.这里我们来介绍read和write函数.
函数原型
read
SYNOPSIS
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
RETURN VALUE
On success, the number of bytes read is returned(zero indicates end of file)
On error, -1 is returned, and errno is set appropriately
write
SYNOPSIS
#include <unistd.h>
ssize_t write(int fd, const void *buf, size_t count);
RETURN VALUE
On success, the number of bytes written is returned(zero indicates nothing was written)
On error, -1 is returned, and errno is set appropriately
例子
这里以C语言为例.
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
int main(int argc __unused, char **argv __unused) {
int fd[2] = { 0 };
char *sendMsg = "Hello World";
char recvMsg[128] = { 0 }; //作为例子,这里buffer只用了128字节.实际应以需求为准
if (pipe(fd) < 0) { //管道,fd[0] 为读管道,fd[1] 为写管道
printf("Create pipe fail, reason : %s\n", strerror(errno));
return -1;
}
if (write(fd[1], sendMsg, strlen(sendMsg) + 1) < 0) {
printf("write fail, reason : %s\n", strerror(errno));
return -1;
}
if (read(fd[0], recvMsg, sizeof(recvMsg)/sizeof(*recvMsg)) < 0) {
printf("read fail, reason : %s\n", strerror(errno));
return -1;
}
printf("read message success : %s\n", recvMsg);
//注意打开fd 和关闭fd 应成对出现,防止fd泄露.
close(fd[0]);
close(fd[1]);
return 0;
}
运行结果
generic_arm64:/ # example
example
read message success : Hello World
## 关于errno
如果你对errno这个错误码值赋值流程感兴趣,可以继续往下看,需要一点汇编基础.否则本章完.
以下基于ARMv8 64bit体系架构分析
read.S
#include <private/bionic_asm.h>
ENTRY(read)
mov x8, __NR_read //read函数的系统调用号赋值给x8寄存器
svc #0 //svc中断,进入系统调用
cmn x0, #(MAX_ERRNO + 1)
cneg x0, x0, hi
b.hi __set_errno_internal
ret
END(read)
系统调用返回的错误码都是相反数,这里以权限不足的错误码EACCES(13)为例来分析该流程.
1.系统调用read返回错误码EACCES的相反数-13,并将该值保存在x0寄存器.
2.cmn比较指令做加法操作,会引起PSTATE寄存器C标志位发生变化
-13的16进制:0xFFFF FFFF FFFF FFF3,MAX_ERRNO + 1的16进制:0x1000.
两者相加,会导致-13的最高位符号位溢出,因此PSTATE寄存器的C标志位 置1.
3.hi条件码代表PSTATE的C标志位为1且Z标志位为0.因此此时hi条件code为true
4.cneg条件否定指令,cneg x0,x0,hi 等价于if hi == true,then x0 = -x0 else x0 = x0
因此此处指令执行完后x0 = -x0 也就是x0 = -(-13) = 13
5.b.hi条件跳转指令,当hi条件码成立,跳转执行__set_errno_internal函数,此处会跳转执行该函数
extern "C" __LIBC_HIDDEN__ long __set_errno_internal(int n) {
errno = n; //形参n的值为x0寄存器,也就是错误码13,此处完成对errno的赋值
return -1; //返回-1,该值会赋给x0寄存器,因此read函数发生错误时,得到的返回值为-1
}
———————————–本节完————————————-
参考 Linux基础知识之错误码表如下：
这里列出 常见的Linux错误码 ,方便核对系统调用发生err时,出错的原因是什么.
你也可以在代码中使用 strerror 打印错误描述.
#include <errno.h>
#include <string.h>
printf("err reason : %s\n", strerror(errno));
错误码表
// Errno-base.h (kernel-4.9\include\uapi\asm-generic)
#define EPERM 1 /* Operation not permitted */
#define ENOENT 2 /* No such file or directory */
#define ESRCH 3 /* No such process */
#define EINTR 4 /* Interrupted system call */
#define EIO 5 /* I/O error */
#define ENXIO 6 /* No such device or address */
#define E2BIG 7 /* Argument list too long */
#define ENOEXEC 8 /* Exec format error */
#define EBADF 9 /* Bad file number */
#define ECHILD 10 /* No child processes */
#define EAGAIN 11 /* Try again */
#define ENOMEM 12 /* Out of memory */
#define EACCES 13 /* Permission denied */
#define EFAULT 14 /* Bad address */
#define ENOTBLK 15 /* Block device required */
#define EBUSY 16 /* Device or resource busy */
#define EEXIST 17 /* File exists */
#define EXDEV 18 /* Cross-device link */
#define ENODEV 19 /* No such device */
#define ENOTDIR 20 /* Not a directory */
#define EISDIR 21 /* Is a directory */
#define EINVAL 22 /* Invalid argument */
#define ENFILE 23 /* File table overflow */
#define EMFILE 24 /* Too many open files */
#define ENOTTY 25 /* Not a typewriter */
#define ETXTBSY 26 /* Text file busy */
#define EFBIG 27 /* File too large */
#define ENOSPC 28 /* No space left on device */
#define ESPIPE 29 /* Illegal seek */
#define EROFS 30 /* Read-only file system */
#define EMLINK 31 /* Too many links */
#define EPIPE 32 /* Broken pipe */
#define EDOM 33 /* Math argument out of domain of func */
#define ERANGE 34 /* Math result not representable */
// Errno.h (kernel-4.9\include\uapi\asm-generic)
#define EDEADLK 35 /* Resource deadlock would occur */
#define ENAMETOOLONG 36 /* File name too long */
#define ENOLCK 37 /* No record locks available */
#define ENOSYS 38 /* Invalid system call number */
#define ENOTEMPTY 39 /* Directory not empty */
#define ELOOP 40 /* Too many symbolic links encountered */
#define EWOULDBLOCK EAGAIN /* Operation would block */
#define ENOMSG 42 /* No message of desired type */
#define EIDRM 43 /* Identifier removed */
#define ECHRNG 44 /* Channel number out of range */
#define EL2NSYNC 45 /* Level 2 not synchronized */
#define EL3HLT 46 /* Level 3 halted */
#define EL3RST 47 /* Level 3 reset */
#define ELNRNG 48 /* Link number out of range */
#define EUNATCH 49 /* Protocol driver not attached */
#define ENOCSI 50 /* No CSI structure available */
#define EL2HLT 51 /* Level 2 halted */
#define EBADE 52 /* Invalid exchange */
#define EBADR 53 /* Invalid request descriptor */
#define EXFULL 54 /* Exchange full */
#define ENOANO 55 /* No anode */
#define EBADRQC 56 /* Invalid request code */
#define EBADSLT 57 /* Invalid slot */
#define EDEADLOCK EDEADLK
#define EBFONT 59 /* Bad font file format */
#define ENOSTR 60 /* Device not a stream */
#define ENODATA 61 /* No data available */
#define ETIME 62 /* Timer expired */
#define ENOSR 63 /* Out of streams resources */
#define ENONET 64 /* Machine is not on the network */
#define ENOPKG 65 /* Package not installed */
#define EREMOTE 66 /* Object is remote */
#define ENOLINK 67 /* Link has been severed */
#define EADV 68 /* Advertise error */
#define ESRMNT 69 /* Srmount error */
#define ECOMM 70 /* Communication error on send */
#define EPROTO 71 /* Protocol error */
#define EMULTIHOP 72 /* Multihop attempted */
#define EDOTDOT 73 /* RFS specific error */
#define EBADMSG 74 /* Not a data message */
#define EOVERFLOW 75 /* Value too large for defined data type */
#define ENOTUNIQ 76 /* Name not unique on network */
#define EBADFD 77 /* File descriptor in bad state */
#define EREMCHG 78 /* Remote address changed */
#define ELIBACC 79 /* Can not access a needed shared library */
#define ELIBBAD 80 /* Accessing a corrupted shared library */
#define ELIBSCN 81 /* .lib section in a.out corrupted */
#define ELIBMAX 82 /* Attempting to link in too many shared libraries */
#define ELIBEXEC 83 /* Cannot exec a shared library directly */
#define EILSEQ 84 /* Illegal byte sequence */
#define ERESTART 85 /* Interrupted system call should be restarted */
#define ESTRPIPE 86 /* Streams pipe error */
#define EUSERS 87 /* Too many users */
#define ENOTSOCK 88 /* Socket operation on non-socket */
#define EDESTADDRREQ 89 /* Destination address required */
#define EMSGSIZE 90 /* Message too long */
#define EPROTOTYPE 91 /* Protocol wrong type for socket */
#define ENOPROTOOPT 92 /* Protocol not available */
#define EPROTONOSUPPORT 93 /* Protocol not supported */
#define ESOCKTNOSUPPORT 94 /* Socket type not supported */
#define EOPNOTSUPP 95 /* Operation not supported on transport endpoint */
#define EPFNOSUPPORT 96 /* Protocol family not supported */
#define EAFNOSUPPORT 97 /* Address family not supported by protocol */
#define EADDRINUSE 98 /* Address already in use */
#define EADDRNOTAVAIL 99 /* Cannot assign requested address */
#define ENETDOWN 100 /* Network is down */
#define ENETUNREACH 101 /* Network is unreachable */
#define ENETRESET 102 /* Network dropped connection because of reset */
#define ECONNABORTED 103 /* Software caused connection abort */
#define ECONNRESET 104 /* Connection reset by peer */
#define ENOBUFS 105 /* No buffer space available */
#define EISCONN 106 /* Transport endpoint is already connected */
#define ENOTCONN 107 /* Transport endpoint is not connected */
#define ESHUTDOWN 108 /* Cannot send after transport endpoint shutdown */
#define ETOOMANYREFS 109 /* Too many references: cannot splice */
#define ETIMEDOUT 110 /* Connection timed out */
#define ECONNREFUSED 111 /* Connection refused */
#define EHOSTDOWN 112 /* Host is down */
#define EHOSTUNREACH 113 /* No route to host */
#define EALREADY 114 /* Operation already in progress */
#define EINPROGRESS 115 /* Operation now in progress */
#define ESTALE 116 /* Stale file handle */
#define EUCLEAN 117 /* Structure needs cleaning */
#define ENOTNAM 118 /* Not a XENIX named type file */
#define ENAVAIL 119 /* No XENIX semaphores available */
#define EISNAM 120 /* Is a named type file */
#define EREMOTEIO 121 /* Remote I/O error */
#define EDQUOT 122 /* Quota exceeded */
#define ENOMEDIUM 123 /* No medium found */
#define EMEDIUMTYPE 124 /* Wrong medium type */
#define ECANCELED 125 /* Operation Canceled */
#define ENOKEY 126 /* Required key not available */
#define EKEYEXPIRED 127 /* Key has expired */
#define EKEYREVOKED 128 /* Key has been revoked */
#define EKEYREJECTED 129 /* Key was rejected by service */
#define EOWNERDEAD 130 /* Owner died */
#define ENOTRECOVERABLE 131 /* State not recoverable */
#define ERFKILL 132 /* Operation not possible due to RF-kill */
#define EHWPOISON 133 /* Memory page has hardware error */