Lei niukesi --- basis of embedded AI

Linux file IO( standard I/O< One >)

– One 、 standard IO

– Two 、 flow [FILE]

– 3、 ... and 、 Opening and closing of flow

– Four 、 Read write flow

↓

One 、 standard IO

• In a word, understand ： Using standard C Write out a set of with input and output API( Application programming interface ) function
• You have to know Two nouns ： Buffer mechanism 、 system call .
• standard IO adopt The buffer mechanism reduces the number of system calls So as to achieve more efficient processing

Two 、 flow [FILE]

• FILE： standard IO Use a structure type to store information about the open file [ One FILE Structure represents an open file ]

• flow （stream）：FILE Also known as flow

• In a word, understand ： standard IO All operations of are around FILE To carry out

• The flow is divided into Binary stream 、 Text stream
  - stay Windows Next ： Text stream ( Carriage returns :\r+\n) And binary streams (\n) There's a difference
  - stay Linux Next ： Text stream (\n) And binary streams (\n) No difference between
  - When the program needs Migration to Windows Run time , Consider the difference between the two , With binary stream

• The buffer type of the stream
  - Full buffer [ When opening a normal file , The default is full buffer ]： When the buffer of the stream has no data （ Write operations ）、 No space （ Read operations ） when , Conduct IO operation
  - The line buffer [ Standard input and output ( When a flow is associated with a terminal ) when ]： When line breaks are encountered in input and output （\n） when , Conduct IO operation
  - No buffer [ When outputting error stream data ]： Standard error stream output time data , Write directly to file , Stream is not buffered

• standard IO Predefined three flows ：stdin【 Standard input stream 】 / stdout 【 Standard output stream 】/ stderr【 Standard error flow 】

3、 ... and 、 Opening and closing of flow

open

• Open a standard IO flow
  **FILE *fopen(const char path, const char mode);
  Return stream pointer on success , Return... On error NULL;

  • path Is the path of the stream
  • mode Is the way to open a stream （r(b)/r+b w(b)/w+b a(b)/a+b）

#include<stdio.h>

int main(int argc, char *argv[])
{
    
	FILE *fp;
	if ((fp = fopen('demo.txt', "w+")) == NULL) {
    
		perror("fopen error");
		return -1;
	}
...
return 0;
}

- perror It's a standard. IO One way to handle error messages ： Output string first s, Output the error message corresponding to the error number  
-  If there is an error in the above program, it will output ：fopen error：No such file or directory

close

• Close a standard IO flow
  *int fclose(FILE stream);
  Successfully returns 0; Failure to return EOF, And set up errno（errno Store error number ）

  • Automatically flushes the data in the buffer and releases the buffer when the stream is closed
  • Once the stream is closed, no operation can be performed

Four 、 Read write flow

• Stream supports different reading and writing modes ：
  1. Read and write a character ：fgetc()/fputc() Once read / Write a character
  2. Read and write a line ：fgets()/fputs() Once read / Write a line

Press character input output

• Type... By character
  - int fgetc(FILE *stream);
  - Return the read characters when successful ; If it reaches the end of the file or if there is an error, it returns EOF（-1）

### Part of the code block 
FILE *fp;
int ch, count;
if ((fp = fopen(argv[1], “r”)) == NULL ) {
    
	perror(“fopen”);
	return -1;
}
while ((ch = fgetc(fp) != EOF) {
    
	count++;
}
printf(“total bytes %d \n”, count);

• Output by character
  - int fputc(int c, FILE *stream);
  - Returns the characters written on success ; Return... On error EOF（-1）

### Part of the code block 
FILE *fp;
int ch;
if ((fp = fopen(argv[1], “w”)) == NULL) {
    
	perror(“fopen”);
	return -1;
}
for (ch = ‘a’; ch <= ‘z’; ch++) {
    
	fputc(ch, fp);
}

Input and output by line

• Press the line to enter
  - char *fgets(char *s, int size, FILE *stream);
  - Return on success s, To the end of the file or in case of an error NULL
  - encounter ’\n’ Or entered size-1 Returns... Characters , It always ends with ’\0’

### Part of the code block 
#define N 6;
char buf[N]
fgets(buf, N, stdin);
printf(“%s”, buf);

 -  Suppose the keyboard input is ：abcd< enter >  abcdef< enter >,buf The content of is ？

• Press line to output
  - int fputs(conts char *s, FILE *stream);
  - The number of characters to be output when successful , Return... On error EOF(-1)

### Part of the code block 
FILE *fp;
char buf[] “hello world”;
if((fp = fopen(argv[1], “a”)) == NULL) {
    
	perror(“fopen”);
	return -1;
}
fputs(buf, fp);

OK 了 , Deere friends,this chapter is over, see you next