It's hard to hear C language? Why don't you take a look at my article (7) input and output

1. Write it at the front

The previous blog , I've already introduced myself C Some basic information about language . Including variable 、 control flow 、 function 、 The pointer 、 Structure and so on . We learn Java Of all know ,Java There are many frameworks in powerful places , There's a lot of API For us to use .C It's an old language , It has its powerful API, It is hidden in the corresponding header file , In this blog, let's introduce C Standard input and output of language .

2. The standard input / Output

The standard library implements simple text input / The output mode . A text stream consists of a series of lines , At the end of each line is a newline character . If the system does not follow this pattern , The standard library will adopt some measures to adapt to this mode .

The simplest input mechanism is to use getchar Function from standard input once Read A character ：

int getchar(void)

getchar The function returns the next input character each time it is called . If the end of the file is encountered , Then return to EOF. Symbolic constant EOF In the header file <stdio.h> In the definition of , Its value is generally -1, But the program should use EOF To test whether the file ends , Only in this way can we ensure that the procedure is the same as EOF Is independent of the specific value of .

function

int putchar(int)

be used for Output data .putchar The character c To the standard output , By default , Standard output is screen display . If nothing goes wrong , The function putchar The output character will be returned ; If something goes wrong , Then return to EOF. Again , usually , You can also use "> Output file name " Redirect output to a file in the format of .

Let's look at the following procedure , The details are as follows ：

#include <stdio.h>
#include <ctype.h>
main() /* lower: convert input to lower case*/
{
    
  int c;
  while ((c = getchar()) != EOF)
    putchar(tolower(c));
  return 0;
}

The above program is a simple file input and output function , By entering data from the console , Then output to the console , At the same time, these data have been converted to lowercase . The other letters are output as is .

3. Format output -printf function

The format string contains two types of objects ： Ordinary characters and conversion instructions . At output time , Common stock characters are copied to the output stream as is , The transformation specification is not output directly to the output stream , It's used to control printf Conversion and printing of parameters in , Each conversion description consists of a percent character (%) Start , And results in a converted character . In the character % And conversion characters may contain the following components in turn ：

• Minus sign , Used to specify that the converted parameters are output in the form of left alignment .
• Count , Used to specify the minimum field width . The converted parameters will print fields that are not less than the minimum field width .
• decimal point , Used to separate field width from precision .
• Count , Used to specify the precision , That is, specify the maximum number of characters to be printed in the string 、 Floating point number the number of digits after the decimal point 、 The number of integer minimum output digits
• Letter h or l, Letter h The table does not treat integers as short Type print , Letter l Indicates that an integer is taken as long Type print .

printf Basic conversion description of function

character	Parameter type : Output form
d,i	int type : Decimal number
o	int type : Unsigned octal
x,X	int type : Unsigned hexadecimal number
u	int type ： An unsigned decimal number
c	int type ： Single character
s	char * type , Print the characters in the string sequentially , until ’\0’ Or the number of characters specified by precision has been printed
f	double type ： Decimal number
e,E	double type
g,G	double type
p	void * type , The pointer
%	Do not convert parameters ： Print a percent sign %

function sprintf Executed transformations and functions printf identical , But it saves the output to a string

int sprintf(char *string, char *format, arg1, arg2, ...);

sprintf Functions and printf The function is the same , according to format Format format parameter sequence arg1、arg2、…, But it stores the output in string in , Instead of exporting to standard output .

4. Variable length parameter table

In this section, we mainly look at the corresponding printf Function our own implementation , The details are as follows ：

#include <stdio.h>
#include <stdarg.h>

/* minprintf: minimal printf with variable argument list */
void minprintf(char *fmt, ...) {
    
    va_list ap; /* points to each unnamed arg in turn */
    char *p, *sval;
    int ival;
    double dval;
    va_start(ap, fmt); /* make ap point to 1st unnamed arg */
    for (p = fmt; *p; p++) {
    
        if (*p != '%') {
    
            putchar(*p);
            continue;
        }
        switch (*++p) {
    
            case 'd':
                ival = va_arg(ap, int);
                printf("%d", ival);
                break;
            case 'f':
                dval = va_arg(ap, double);
                printf("%f", dval);
                break;
            case 's':
                for (sval = va_arg(ap, char *); *sval; sval++)
                    putchar(*sval);
                break;
            default:
                putchar(*p);
                break;
        }
    }
    va_end(ap); /* clean up when done */
}

among , Ellipsis indicates that the number and type of parameters in the parameter table are variable . Ellipsis can only appear at the end of the parameter table .

va_list Type is used to declare a variable , This variable references each parameter in turn . In function minprintf in , We call this variable ap, Parameter pointer . macro va_start take ap Initialize to a pointer to the first unnamed parameter . In the use of ap Before , The macro must be called once , The parameter table must contain at least one named parameter ,va_start Take the last named parameter as the starting point .

Every time you call va_arg, This function will return a parameter , And will ap Point to the next parameter .va_arg Use a type name to determine the type of object returned 、 Step of pointer movement . Last , Must be called before the function returns va_end, To complete some necessary cleaning work .

5. Format input -scanf function

scanf Function reads a character sequence from standard input , according to format The format description in explains the character sequence , And save the results to the rest of the parameters .

When scanf Function scans its format string , Or when some input cannot match the format control description , The function will terminate , meanwhile , The number of input items successfully matched and assigned will be returned as the function value , therefore , The return value of this function can be used to determine the number of input items that have been matched . If you reach the end of the file , This function will return EOF.

There is also an input function sscanf, It is used from a string ( Instead of standard input ) Read character sequence in ：

int sscanf(char *string, char *format, arg1, arg2, ...)

It follows the format parameters format Scan string in the format specified in string, And save the results to arg1、arg2 Of these parameters , These parameters must be pointers . The specific format is as follows ：

character	Parameter type : Output form
d	int type : Decimal number
i	integer :int * type , It can be octal or hexadecimal
o	int type : Unsigned octal
u	int type : An unsigned decimal number
x	int type ： Unsigned hexadecimal number
c	character :char * type , Enter the next multiple characters ( The default is 1 Characters ) Store in the designated location , This conversion specification usually does not skip whitespace . If you need to read in the next non blank character , have access to %1s
s	character string ( Without quotes )char * type , Point to a string large enough to hold the string ( It also includes trailing characters ’\0’) Array of characters , A terminator will be added to the end of the string ’\0’
e,f,g	Floating point numbers , It can include a sign decimal point Index part float* type
%	character %: No assignment operation

6. File access

The file pointer , It points to a structure that contains file information , This information includes ： Buffer location 、 The position of the current character in the buffer 、 Read or write status of the file 、 Whether there is an error or whether the end of the file has been reached .

Open a file as follows ：

fp = fopen(name, mode);

fopen The first argument to is a string , It contains the file name . The second parameter is the access mode , Also a string , Used to specify how files are used . The allowed modes are ： read (“r”)、 Write (“w”) And add (“a”). Some systems also distinguish between text files and binary files , Access to the latter requires adding characters to the schema string " b";

If you open a nonexistent file for writing or appending , The change file will be created . When opening an existing file in write mode , The original contents of the file will be overwritten . however , If you open a file by appending , The original contents of the modified file will remain unchanged .

Let's look at the following code ：

#include <stdio.h>

/* cat: concatenate files, version 1 */
main(int argc, char *argv[]) {
    
    FILE *fp;
    void filecopy(FILE *, FILE *);
    if (argc == 1) /* no args; copy standard input */
        filecopy(stdin, stdout);
    else
        while (--argc > 0)
            if ((fp = fopen(*++argv, "r")) == NULL) {
    
                printf("cat: can't open %s\n, *argv");
                return 1;
            } else {
    
                filecopy(fp, stdout);
                fclose(fp);
            }
    return 0;
}

/* filecopy: copy file ifp to file ofp */
void filecopy(FILE *ifp, FILE *ofp) {
    
    int c;
    while ((c = getc(ifp)) != EOF)
        putc(c, ofp);
}

The file pointer stdin And stdout All are FILE * Object of type . But they are constants , Not variables . Therefore, they cannot be assigned .

7. Error handling -stderr and exit

cat The error handling function of the program is not perfect . The problem lies in , If one of the files is inaccessible for some reason , The corresponding diagnostic information can only be printed at the end of the output of the connection . When output to the screen , This treatment is acceptable , But if you export to a file or pipeline to another program , It's unacceptable .

So we modified the following program , The details are as follows ：

#include <stdio.h>
#include <stdlib.h>

void exit(int i);

/* cat: concatenate files, version 2 */
main(int argc, char *argv[]) {
    
    FILE *fp;
    void filecopy(FILE *, FILE *);
    char *prog = argv[0];  /* program name for errors */
    if (argc == 1) /* no args; copy standard input */
        filecopy(stdin, stdout);
    else
        while (--argc > 0)
            if ((fp = fopen(*++argv, "r")) == NULL) {
    
                fprintf(stderr, "%s: can't open %s\n", prog, *argv);
                exit(1);
            } else {
    
                filecopy(fp, stdout);
                fclose(fp);
            }
    if (ferror(stdout)) {
    
        fprintf(stderr, "%s: error writing stdout\n", prog);
        exit(2);
    }
    exit(0);
}

The program sends error messages in two ways . First , take fprintf The diagnostic information generated by the function is output to stderr On , Therefore, the diagnostic information will be displayed on the screen , Instead of just exporting to a pipe or output file .

secondly , The program uses standard library functions exit, When a function is called , It will terminate the execution of the calling program .

8. Line input and line output

Let's look directly at the corresponding code , The details are as follows ：

/* fgets: get at most n chars from iop */
char *fgets(char *s, int n, FILE *iop)
{
    
  register int c;
  register char *cs;
  cs = s;
  while (--n > 0 && (c = getc(iop)) != EOF)
    if ((*cs++ = c) == '\n')
      break;
  *cs = '\0';
  return (c == EOF && cs == s) ? NULL : s;
}

/* fputs: put string s on file iop */
int fputs(char *s, FILE *iop)
{
    
  int c;
  while (c = *s++)
    putc(c, iop);
  return ferror(iop) ? EOF : 0;
}

Standard input functions fgets, It and getline Function similar to .

9. Other functions

9.1 String manipulation functions

The following functions are in <string.h> In the definition of

• strcat(s,t) take t The string pointed to is connected to s To the end of the string .
• strncat(s,t,n) take t In the string pointed to n Characters to s To the end of the string .
• strcmp(s,t) according to s The string pointed to is less than (s<t)、 be equal to (s==t) Or greater than (s>t) t Different situations of the string pointed to , Return negative integers respectively 、0 Or a positive integer .
• strcnmp(s,t,n) Same as strcmp identical , But only before n Compare... In characters .
• strcpy(s,t) take t Copy the string to s Point to .
• strncpy(s, t, n) take t In the string pointed to n Characters copied to s Point to .
• strlen(s) return s The length of the string pointed to
• strchr(s,c) stay s Find... In the string pointed to c, If you find , Returns a pointer to the position where it first appears , Otherwise return to NULL
• strrchr(s,c) stay s Find... In the string pointed to c, If you find , Returns a pointer to the position where it last appeared , Otherwise return to NULL

9.2 Character category test and conversion function

The following functions are in <ctype.h> In the definition of

• isapha if c It's the letters , Return a non 0 value , Otherwise return to 0
• isupper if c It's capital letters , Return a non 0 value , Otherwise return to 0
• islower if c It's lowercase , Return a non 0 value , Otherwise return to 0
• isdigit if c It's an array , Return a non 0 value , Otherwise return to 0
• isalnum if isalpha or isdigit, Return a non 0 value , Otherwise return to 0
• isspace if c Is a space 、 Horizontal tabs 、 A newline 、 A carriage return 、 Page feed or vertical tab Return a non 0 value
• toupper return c In capital form
• tolower return c In lowercase

9.3ungetc function

Format of function ：

int ungetc(int c, FILE *fp)

This function converts the character c Write back to the file fp in , If it works , Then return to c, Otherwise return to EOF. Each file can only receive one writeback character .ungetc Function can be used with any input function .

9.4 Command execution function

function system(char* s) The execution is contained in the string s The command . Then continue to execute the current program .

9.5 Storage management functions

function malloc and calloc Used to dynamically allocate storage blocks . The details are as follows

void *malloc(size_t n)

When the allocation is successful , It returns a pointer , Set pointer to n Byte length uninitialized Storage space , Otherwise return to NULL.

void *calloc(size_t n, size_t size)

When the allocation is successful , It returns a pointer , The free space pointed to by this pointer is enough to hold the free space indicated by n An array of objects of specified length . Otherwise return to NULL. The storage space is initialization by 0.

free§ Function to release p Point to the storage space , among ,p It was passed before malloc or calloc Function gets a pointer to .

9.6 Mathematical functions

The following functions are in <math.h> In the definition of

• sin(x) x The sine function of , among x In radians
• cos(x) x Cosine function of , among x In radians
• atan2(y,x) y/x The arctangent function of , among ,x and y In radians
• exp(x) Exponential function ex
• log(x) x The natural logarithm of
• log10(x) x The logarithm of the constant
• pow(x,y) Calculation x Of y The value of the power
• sqrt(x) x The square root of
• fabs(x) x The absolute value of

9.7 Random number generator function

function rand() Build between 0 and RAND_MAX Between the pseudo-random integer sequences . among RAND_MAX Is in header file <stdlib.h> Symbolic constants defined in . The following is a method of generating greater than or equal to 0 But less than 1 Random floating point number method :

#define frand() ((double) rand() / (RAND_MAx + 1.0))

10. At the end

This blog mainly introduces C The standard input and output functions of language , The following chapter mainly introduces UNIX System interface of .