Note for fopen and note for fopen

Note for fopen and note for fopen

Organized from https://msdn.microsoft.com/zh-cn/library/t3ayayh1 (VS.80). aspx

Errno, _ doserrno, _ sys_errlist, and _ sys_nerr

 

These global variables include the error code used by the perror and strerror functions to print error messages. Use the more secure feature versions listed in the following table to replace their global variables.

Global Variable	Functional Equivalents
_ Doserrno	_ Get_doserrno, _ set_doserrno
_ Errno	_ Get_errno, _ set_errno

These variables are declared in STDLIB. H

extern int _doserrno; extern int errno; extern char *_sys_errlist[ ];extern int _sys_nerr;

Remarks

Errno is set for errors in system-level calls. Because errno retains the value set for its last call, this value may be changed by subsequent calls. Always check errno before and after possible calls.
In the error, errno is not necessarily set to the same value as the error code returned by the system call. For I/O operations, use _ doserrno to access the operating system error code of errno code. For most non-I/O operations, the value of _ doserrno is undefined.
Each error value is associated with an error message that can be printed using perror or stored in a string using strerror. Perror and strerror use the _ sys_errlist array and _ sys_nerr (number of elements in _ sys_errlist) to process error information.
The library mathematical routine sets errno by calling _ matherr. To handle mathematical errors, write your own routine according to the _ matherr reference description and name it _ matherr.
All ERRNO values defined as pre-defined constants in errno. H are UNIX compatible and listed below. Only ERANGE, EILSEQ, And EDOM are specified in the ANSI standard.

Constant	System error message	Value
EPERM	Operation not permitted	1
ENOENT	No such file or directory	2
ESRCH	No such process	3
EINTR	Interrupted function	4
EIO	I/O error	5
ENXIO	No such device or address	6
E2BIG	Argument list too long	7
ENOEXEC	Exec format error	8
EBADF	Bad file number	9
ECHILD	No spawned processes	10
EAGAIN	No more processes or not enough memory or maximum nesting level reached	11
ENOMEM	Not enough memory	12
EACCES	Permission denied	13
EFAULT	Bad address	14
EBUSY	Device or resource busy	16
EEXIST	File exists	17
EXDEV	Cross-device link	18
ENODEV	No such device	19
ENOTDIR	Not a directory	20
EISDIR	Is a directory	21
EINVAL	Invalid argument	22
ENFILE	Too program files open in system	23
EMFILE	Too program open files	24
ENOTTY	Inappropriate I/O control operation	25
EFBIG	File too large	27
ENOSPC	No space left on device	28
ESPIPE	Invalid seek	29
EROFS	Read-only file system	30
EMLINK	Too many links	31
EPIPE	Broken pipe	32
EDOM	Math argument	33
ERANGE	Result too large	34
EDEADLK	Resource deadlock wocould occur	36
EDEADLOCK	Same as EDEADLK for compatibility with older Microsoft C versions	36
ENAMETOOLONG	Filename too long	38
ENOLCK	No locks available	39
ENOSYS	Function not supported	40
ENOTEMPTY	Directory not empty	41
EILSEQ	Illegal byte sequence	42
STRUNCATE	String was truncated	80

Note: fopen has a trap. Of course, it is only for beginners.

The fopen method is defined as follows:

FILE*fopen(const char * __restrict __filename, const char * __restrict __mode) __DARWIN_ALIAS_STARTING(__MAC_10_6, __IPHONE_2_0, __DARWIN_ALIAS(fopen));

Fopen has two parameters: one is the file name _ filename you want to operate, and the other is the operation _ mode you want to perform. There is nothing to say about the mode. The filename parameter, if you only upload the file name you want to operate, and your file is local, you may be prompted that the file cannot be found due to Path Problems. Error = 2 (No such file or directory) There is No relevant path or this file does not exist.

In this case, there are two solutions:

One is to use w's mode locally to create a file. to operate on this file, you can ensure that the file is in your current directory, but this is not necessarily applicable, for example, I want to read a bind file of a font, and then convert the Unicode font matrix in the font file into utf8, store it in an array, and write it into the file.

The second method is relatively simple. You can try the absolute path, find the absolute path of the file, and set the parameter _ filename to OK. Note that, paths generally contain "\" characters, which are translated in C/C ++;

All escape characters and their meanings:

Escape characters	Meaning	ASCII value (decimal)
\	Bell (BEL)	007
\ B	Return to BS to move the current position to the previous column.	008
\ F	Change page (FF), move the current position to the beginning of the next page	012
\ N	Line feed (LF), move the current position to the beginning of the next line	010
\ R	Press enter (CR) to move the current position to the beginning of the line	013
\ T	Horizontal tabulation (HT) (jump to the next TAB)	009
\ V	Vertical tabulation (VT)	011
\\	Represents a backslash character ''\'	092
\'	Represents a single quotation mark (apostrophes) character	039
\"	Represents a double quotation mark character	034
\?	Represents a question mark	063
\ 0	NULL)	000
\ Ooo	Any character from 1 to 3 octal digits	Three octal nodes
\ Xhh	Any character in hexadecimal notation between 1 and 2	Two hexadecimal

  Note: Yes. slashes: "/" and backslash: "\" are not interchangeable here.Therefore, if your path is c: \ demo \ abc.txt, You need to input the parameter _ filename = c: \ demo \ abc.txt

 1 #include <iostream> 2 #include <fstream> 3  4 int main () 5 { 6     std::cout << "hello world \n"; 7      8     FILE *fp; 9     char *ch = "";10     char buff[100];11 12     errno_t err = fopen_s(&fp,"c:\\Users\\zhi\\Desktop\\abc.txt","r");13 14     if(err == 0)15     {16         printf("the file open successful!");17         18         ch = fgets(buff,100,fp);19         printf("ch is :%s",ch);20         fclose(fp);21 22     }else{23         printf("err:%d",err);24         printf("the file open not OK!");25     }26     return 0;27 }

These are all problems encountered yesterday, but they have grown up.