c语言程序设计课件ppt（英文）c program language 之8 file

LOGOChapter Seven FileContentsIntroduction1Open/close file2Read/write file 3Check the state of the opening file4The location of the file pointer5How to get the data while run a program?Where will the result in ?How can input the result of a program to another program ?IntroductionThe procedure of C language Edit LinkCreate the program textCompileCompile it successfullyLink it to the C function library Introductionprogramformat outputcharacter outputstring outputinitializationassignment input from keyboardmemorymemoryformat inputcharacter inputstring input Read data from a file Write data to a fileIntroductionFile is the gather of related information saved in memory device (assistant memory )OS manages system in filesProcessing files means file input/out processing.Procedure 1 open file2 file read/write 3 close file.Interface input/out devices are treated as files in C .File accessing methods:1) Buffered I/O (or stream I/O)2) Unbuffered I/O (or low-level I/O)IntroductionBuffered (temporary storage location)Well discuss Buffered I/O in our lectureIn buffered I/O,-open a communication area in RAMDISkPROGRAMOutput file buffer Input file bufferThere are two ways to deal with file:Binary fileASCII file IntroductionStream is the abstract of file used to deal with file. There are two kinds of streams in C.The character is ASCII code .one ASCII code occupies one byte , new line n is end of line .After read /write a character from /to ASCII file , the fp increases automatically to guarantee the read /write sequences.Text stream Binary streamStoring data to disk in the form the same as in RAM .Example : 10000 In binary file , use 2 bytes In ASCII file , use 5 bytesOpen/close fileHow to magange a file in C ? PointerOperations open file（ load file ） operate file using functions close file （ remove file from memory）BDFACEA file systemcreat a new filedelete an old one read/write access a file by its namemanage the memory file usedlimit the users rightOpen/close fileFile pointer,points to a structure that contains information about the file, such as the location of a buffer , the current character position in the buffer, whether the file is being read or written, and whether errors or end of file have occurred. Users dont need to know the details ,because the definitions obtained from include a structure declaration called FILE. I am a student .2 The function of opening a file :FILE *fp; uppercase letters ,otherwise ,cant be recognized, defined to standard structure , key-word # include is necessaryfopen ( filename ,*mode );stringchar arraychar pointer paths are allowed1 files pointer :opening a file use function fopen ( ) readwriteappendOpen/close fileOpen/close filefp=fopen (“ file.txt ” , “r ” ); open file.txt , read in ASCIIOpening a file means.returns a file pointerspecify the operation to be donedefining a file name firstpoints to the first location of file buffer.r-readfile.txtwhich The returned value is assigned to a file pointer. If process is successful , the returned value is a file pointer. (address ) FLIE *fp; Otherwise, return NULL pointer.if ( ( fp = fopen ( “file.txt” “r ” ) = = NULL ) printf ( “ cant open file n”); exist (1 ) ; Open/close fileSymbol Explanationr readw writea appendt Text streamb Binary stream+ Permit read/write If fopen fails , return value is zero ( NULL pointer) when:1)fopen wants to open a non existed file for reading .2) read a file not permitted read .3) create a file but disk is full .4) disk sector is bad .3 Close a fileClose a file opened ,argument Open/close fileOpened file must be closed before ending a program, otherwise, data may be lost .Writes string into file pointed by streamFile read/writefgets(string,n,stream)Reads a string of up to n-1 characters from stream and places them into stringfputs(string, stream)1 read/write file functionsfgetc (fp), fputc(fp) read/write a char from /to file read : c = fgetc(fp); read a char from named file.write : fputc (c,fp ); write c to named file.Prototype in stdio.h return EOF (-1) if a) at the end of file; b) detecting an error;Using function fgets() and fputs() can realize string input/ouput in files.main ( int argc, char *argv ) int c ; FILE *fpr, *fpd;if ( argc!=3) putchar(007);puts(“ Usage: copy file1, file 2“);exit( ); if ( (fpr=fopen (argv1 ,“r“) = NULL ) printf(“%c“,007);printf(“file %s cant open n“, argv 1);exit( ); if ( fpd = fopen ( argv2 ,“w“ )= NULL) printf ( “ files %s cant openn “ , argv2);exit ( ); while( c=fgetc(fpr)!= EOF)fputc(c, fpd);fclose (fpr);fclose (fpd);Input three strings( file name)Source file name object file name The function of this program is to copy file1 to file2#includemain() FILE *fp; char c1；fp=fopen(“file.dat”,”r”)；c1=fgetc(fp);putchar(c1); fclose(fp);#includemain() FILE *fp; char ch=a;fp=fopen(“file.dat”,”w”);fputc(ch,fp);fclose(fp); Read the character from file.dat write character a to file.dat . File read/writefscanf ( stream, format , arg,.)Reads data from stream using format conversion specifier into address specifierd by arg .fprintf (stream,format, arg,.)Writes arg into stream using format conversion specifier.main( ) FILE *fp; char *c1=“Turbo C”; char c210;fp=fopen(“file2.dat”,”w”);fputs(c1,fp); fclose(fp);fp=fopen(“file2.dat”,”r”);fgets(c2,8,fp);printf(“%sn”,c2); fclose(fp); File read/write read a string from determined file (fp) , have read 7 chars;Returned value: normal the 1st location of the string; error or end of file NULLwrite a string to determined file (fp)Returned value: normal: the number of chars; error : NULLmain() FILE *fp;char *c1=“Turbo C”;char c210;fp=fopen(“file3.dat”,”w”);fprintf(fp,”%s”,c1); fclose(fp);fp=fopen(“file3.dat”,”r”);fscanf(fp,”%s”,c2);printf(“%sn”,c2); fclose(fp); File read/writeIt is similar to printf , add fp file pointerIt is similar to scanf, add fp file pointermain( ) FILE *fp; char str81, filename80;gets(filename);if(fp=fopen(filename,”w”)=NULL) printf(“can not open this filen”); exit(0); while(strlen(gets(str)0 ) fputs(str,fp);fputs(“n”,fp); fclose(fp); Input several strings , then save them to diskCan you use “filename” here ?How many strings should you input ?File read/write fread reads n items of size length from stream into buffer. File read/writefwrite( ) / fread( ) * Two standard library functions are available to facilitate the storage of data arrays in binary forms. These are fwrite( ) and fread( ).fwrite( buffer, size, n, stream); fread( buffer, size, n, stream); buffer :a pointer to the start of the buffer to be written into stream. n: items in the buffer of length size in bytes.when computer are started, 3 standard files are opened automatically, and 3 associated file pointers are provided 。 User neednt write statements to open standard files and after program are executed,standard files automatically closed.2 Interface input/out devices stdin file inputstdout standard output pointerstderr errorexit(1) : return to os . Innormal existexit(0) : - normal exist .3 Exit functions program will stop right away.File read/writeThe files state is necessary when using input/out functions ,especially testing the end of a file. There are one couple of ways to test the end of file1 EOFWhen reading /writing ASCII file, EOF is used to test if the file pointer points to the end of file, if so, the value of EOF is -1 Cant suit for binary file 2 function feof ( ) feof(file pointer name )The return value is nonzero at the end of file ,Otherwise , it is not the end of file .Check the state of the opening file while(feof(fp)=0)c=fgetc(fp);Check the state of the opening file while(c=fgetc(fp)!=EOF) putchar( c ); Example : read characters from a file The location of he file pointer1 rewindrewind( file pointer name ） Sets file pointer to stream to beginning of file2 fseek Locates file pointer for stream at offset bytes from origins fseek(file pointer name ， offsets， origin ） No return value Generally it is Used for binary file0:the begining1:current position 2:the end The number of bytes + move backwards- move forwards return: a zero if the pointer was reset without error.3、 The current position ftell( file pointer name ） Returns current position of file pointed from stream Return 1L means there is something wrong The location o f the file pointerWhere is the current position of the file pointer?0 Beginning of file1 Current position of file pointer2 End of filefseek(file pointer name ， distance， origin ）main ( int argc, char *argv ) char s80; int a;FILE *fp;if ( (fp=fopen (“test“ ,“w“) = NULL ) printf(“cant open file“ );exit( ); fscanf(stdin, “%s%d“, s, /*read from keyboard*/fprintf(fp, “%s %d“, s, a); /* write to file*/fclose(fp);if ( (fp=fopen (“test“ ,“r“) = NULL ) printf(“cant open file“