C-Programming File-handling-C.pptx

C-Programming File-handling-C.pptx

• 1.
  FILE HANDLING INC
• 2.
  CONSOLE ORIENTED INPUT/OUTPUT • Consoleoriented – use terminal (keyboard/screen) scanf(“%d”,&i) – read data from keyboard printf(“%d”,i) – print data to monitor • Suitable for small volumes of data • Data lost when program terminated
• 3.
  REAL-LIFE APPLICATIONS • Largedata volumes • E.g. physical experiments (CERN collider), human genome, population records etc. • Need for flexible approach to store/retrieve data • Concept of files
• 4.
  Files • File –place on disc where group of related data is stored • E.g. your C programs, executables • High-level programming languages support file operations • Naming • Opening • Reading • Writing • Closing
• 5.
  DEFINING AND OPENINGFILE • To store data file in secondary memory (disc) must specify to OS • Filename (e.G. Sort.C, input.Data) • Data structure (e.G. File) • Purpose (e.G. Reading, writing, appending)
• 6.
  FILENAME • String ofcharacters that make up a valid filename for OS. • May contain two parts • Primary • Optional period with extension • Examples: a.out, prog.c, temp, text.out
• 7.
  GENERAL FORMAT FOROPENING FILE • fp • contains all information about file • Communication link between system and program • Mode can be • r open file for reading only • w open file for writing only • a open file for appending (adding) data FILE *fp; /*variable fp is pointer to type FILE*/ fp = fopen(“filename”, “mode”); /*opens file with name filename , assigns identifier to fp */
• 8.
  DIFFERENT MODES • Writingmode • if file already exists then contents are deleted, • else new file with specified name created • Appending mode • if file already exists then file opened with contents safe • else new file created • Reading mode • if file already exists then opened with contents safe • else error occurs. FILE *p1, *p2; p1 = fopen(“data”,”r”); p2= fopen(“results”, w”);
• 9.
  ADDITIONAL MODES • r+open to beginning for both reading/writing • w+ same as w except both for reading and writing • a+ same as ‘a’ except both for reading and writing
• 10.
  CLOSING A FILE •File must be closed as soon as all operations on it completed. • Ensures • All outstanding information associated with file flushed out from buffers • All links to file broken • Accidental misuse of file prevented. • If want to change mode of file, then first close and open again.
• 11.
  CLOSING A FILE •pointer can be reused after closing Syntax: fclose(file_pointer); Example: FILE *p1, *p2; p1 = fopen(“INPUT.txt”, “r”); p2 =fopen(“OUTPUT.txt”, “w”); …….. …….. fclose(p1); fclose(p2);
• 12.
  INPUT/OUTPUT OPERATIONS ON FILES •C provides several different functions for reading/writing 1. getc() – read a character 2. putc() – write a character 3. fprintf() – write set of data values 4. fscanf() – read set of data values 5. getw() – read integer 6. putw() – write integer
• 13.
  GETC() AND PUTC() •Handle one character at a time like getchar() and putchar() • syntax: putc(c,fp1); • c : a character variable • fp1 : pointer to file opened with mode w • syntax: c = getc(fp2); • c : a character variable • fp2 : pointer to file opened with mode r • file pointer moves by one character position after every getc() and putc() • getc() returns end-of-file marker EOF when file end reached
• 14.
  PROGRAM TO READ/WRITEUSING GETC/PUTC main() { FILE *fp1; char c; f1= fopen(“INPUT”, “w”); /* open file for writing */ while((c=getchar()) != EOF) /*get char from keyboard putc(c,f1); /*write a character to INPUT */ fclose(f1); /* close INPUT */ f1=fopen(“INPUT”, “r”); /* reopen file */ while((c=getc(f1))!=EOF) /*read character from file INPUT*/ printf(“%c”, c); /* print character to screen */ fclose(f1); } /*end main */
• 15.
  FSCANF() AND FPRINTF() •Similar to scanf() and printf() • In addition provide file-pointer • Given the following • file-pointer f1 (points to file opened in write mode) • file-pointer f2 (points to file opened in read mode) • integer variable i • float variable f • Example: fprintf(f1, “%d %fn”, i, f); fprintf(stdout, “%f n”, f); /*note: stdout refers to screen */ fscanf(f2, “%d %f”, &i, &f); • fscanf returns EOF when end-of-file reached
• 16.
  GETW() AND PUTW() •Handle one integer at a time • syntax: putw(i,fp1); • i : an integer variable • fp1 : pointer to file ipened with mode w • syntax: i = getw(fp2); • i : an integer variable • fp2 : pointer to file opened with mode r • File pointer moves by one integer position, data stored in binary format native to local system • getw() returns end-of-file marker EOF when file end reached
• 17.
  C PROGRAM USINGGETW, PUTW,FSCANF, FPRINTF main() { int i,sum1=0; FILE *f1; /* open files */ f1 = fopen("int_data.bin","w"); for(i=10;i<15;i++) putw(i,f1); fclose(f1); f1 = fopen("int_data.bin","r"); while((i=getw(f1))!=EOF) { sum1+=i; printf("binary file: i=%dn",i); } /* end while getw */ printf("binary sum=%d,sum1); fclose(f1); }
• 18.
  OUTPUT: $ ./a.out binaryfile: i=10 binary file: i=11 binary file: i=12 binary file: i=13 binary file: i=14 binary sum=60, $ cat int_data.txt 10 11 12 13 14
• 19.
  ERRORS THAT OCCURDURING I/O • Typical errors that occur: 1. trying to read beyond end-of-file 2. trying to use a file that has not been opened 3. perform operation on file not permitted by ‘fopen’ mode 4. open file with invalid filename 5. write to write-protected file
• 20.
  ERROR HANDLING • Givenfile-pointer, check if EOF reached, errors while handling file, problems opening file etc. • Check if EOF reached: feof() • feof() takes file-pointer as input, returns nonzero if all data read and zero otherwise if(feof(fp)) printf(“End of datan”); • ferror() takes file-pointer as input, returns nonzero integer if error detected else returns zero if(ferror(fp) !=0) printf(“An error has occurredn”);
• 21.
  ERROR WHILE OPENINGFILE • if file cannot be opened then fopen returns a NULL pointer • Good practice to check if pointer is NULL before proceeding fp = fopen(“input.dat”, “r”); if (fp == NULL) printf(“File could not be opened n ”);
• 22.
  RANDOM ACCESS TOFILES • fseek (file-pointer, offset, position); • Position: 0 (beginning), 1 (current), 2 (end) • Offset: number of locations to move from position Example: fseek(fp,-m, 1); /* move back by m bytes from current position */ fseek(fp,m,0); /* move to (m+1)th byte in file */ fseek(fp, -10, 2); /* what is this? */ • ftell(fp) returns current byte position in file • rewind(fp) resets position to start of file
• 23.
  COMMAND LINE ARGUMENTS •main ( int argc, char *argv[] ) • argc – gives a count of number of arguments (including program name) • char *argv[] defines an array of pointers to character (or array of strings) • argv[0] – program name • argv[1] to argv[argc -1] give the other arguments as strings