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7.0 files and c input


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7.0 files and c input

  1. 1. Files and C Input/Output
  2. 2. Learning outcomes  At the end of this section you will be able to  Understand the concepts involved in accessing secondary storage by computer programs  Understand how to deal with disk files in C  Write programs those read and write into files in a secondary storage
  3. 3. Introduction  All input and output in C are from and to files.  File is a sequence of bytes. (text files use n)  Open, Process, Close  An open file is identified by a “File Access Control Block”
  4. 4. File Access Control Blocks  struct iobuf { char *_ptr; // Current ch. position int *_cnt; // Remaining bytes char *_base; // Buffer base address char _flag; // Access control flags char _file; } // File number  #define FILE struct _iobuf /* stdio.h */  #define EOF -1  typedef struct _iobuf FILE;
  5. 5. FCB’s cont…  First 3 file access control blocks are automatically assigned to  standard input  standard output and  standard error devices  FILE *stdin, *stdout, *stderr  Standard input and output devices can be redefined for a program at the command line as,  Process <infile >outfile
  6. 6. stdio.h  Many IO libray functions come in pairs (or in triplets) whose operational differs only slightly.  printf(“Hello world”); // prints to stdout  fprintf(fp,“Hello world”) // prints to a file identified by fp  fprintf(stdout,“Hello world”) // same as the first  fp is a pointer to a file access control block and is returned by a successful fopen() operation.
  7. 7. fopen() FILE *fp; /* declares a file pointer or a stream */ char s[]=“c:mydirtemp.txt”; If ((fp=fopen(s,“wt”)) == NULL) printf(“file open error”);  Mode string  “r | w | a [+] [ t | b ]”  r : open for read  w : create for write  a : open for append/create for write  + : permits both read and write  t | b : in text or binary mode
  8. 8. fclose()  To close an opened file,  if (fclose(fp)!=0) printf(“file close error”);  If necessary file’s buffer is first flushed (written back to the file) before it is closed.  fclose() is automatically called for each open file when a program terminates normally.
  9. 9. Character transfers to/from files  getc(fp) and fgetc((fp) reads a ch from the stream fp.  putc(c,fp) and fputc(c,fp) writes c into the stream fp.  ungetc(c,fp) push back the character just read to fp.  All 5 returns the ch. on success and EOF on failure Eg – while ((*ptr++=getc(fp))!=EOF) …; while (putc(*ptr++)) …;  #define getchar() getc(stdin)  #define putchar(c) putc(c,stdout) // in stdio.h
  10. 10. Line IO  Line  ch sequence terminating in a ‘n’  Simplest form of a record in a C file.  fgets(s,n,fp) reads the next line (including ‘n’) up to a maximum of n chs from stream fp and writes into memory buffer starting from s with a terminating ‘0’.  gets(s) reads a line from stdin and placed in memory starting from s, ‘n’ replaced by a ’0’.  gets() and fgets() return s on success and a NULL on failure.
  11. 11.  fputs(s,fp) writes a string beginning at s onto the stream fp without ‘0’.  puts(s) same as fputs(s,stdout) but appends a ‘n’.  puts() fputs() returns last ch or 0 on success and EOF on failure.  These return values permit statements such as,  while(strlen(gets(s)) …;  while (fgets(s,MAX_LINE,fp1) != NULL) fputs(s,fp2);
  12. 12. Formatted IO  The characters –123.45E-7 printed on a display or typed in a keyboard is totally different from the form it is stored in memory.  printf(f), fprintf(fp,f), sprintf(s,f) - Return the no. of bytes output on success and EOF on failure.  Eg- printf(“nAverage = %-8.4f”,avg);  scanf(f), fscanf(fp,f), sscanf(s,f) – On success return the no. of fields scanned and stored. Return EOF on failure.
  13. 13. Binary IO  fread(ptr,i,n,fp)  Transfers n data blocks of each of size i bytes from stream fp into memory buffer at ptr.  Returns the no.of blocks read.  fwrite(ptr,i,n,fp)  Appends n data items in memory at ptr each of size i to the stream fp.  Returns no. of bytes written.
  14. 14. Further File Operations  unlink(“fname”)  removes fname from the file system  rewind(fp)  set the current file position at start of file.  fseek(fp,L,i)  place the file position L bytes from i  i is 0|1|2 for start | current | end  L is a long integer  unlink and fseek return 0 on success and –1 on failure. rewind does not return any value.  ftell(fp)  returns current byte position as a long and -1 on failure  feof() detects end-of-file marker in a file