6.21 Files

6.21 Files

• When to use files
• Content of files
• Array sizes and memory costs
• Access times - Sequential and random
• File buffering
• Files in memory

6.21.1 stdlib I/O functions

• fopen()
  Open a file for any type of access. Kernighan p242 p160. Hanly & Koffman pages 78, 88, 601, 626, and AP5. Harbison p346.
• fclose()
  Close a stream and flush all data. Harbison p346.
• fgets()
  Read a line from a stream. Harbison p356.
• fputs()
  Write a line to a stream. Harbison p366.
• fseek() Sets file position. Kernighan p248. Harbison p352.
• ftell() Returns file position. Kernighan p248. Harbison p352.
• rewind()
  Sets file position to zero. Harbison p352.
• fgetpos()
  Gets file position pointer. Harbison p352.
• fsetpos()
  Positions file pointer. Harbison p352.
• fflush()
  Writes output to file. Harbison p346.
• remove()
  Deletes a file. Harbison p382.
• rename()
  Deletes a file. Harbison p382.

6.21.2 stdlib error functions

• extern int errno; This integer is set when a routine reports an error. It is NEVER cleared by a routine. It should only be checked after an error occurs.
• strerror() "Convert" the error number to a string.
• perror() Print the string associated with an error.
• ferror() File error. Harbison p382.
• feof() End-of-file.
• clearerr() Clear file error.

6.21.3 Unix I/O functions

(Avoid these functions for portability)

• open
• close
• creat
• read
• write
• lseek

6.21.4 I/O function Examples

File access examples. ftp://lt.tucson.az.us/pub/c/file-or-stdin.c

/* Louis Taber, 3/18/2001
 * 
 * --  Use stdin or file name on command line. 
 *
 * copy stdin to stdout if no file name is specified 
 *
 *  else opern file and send to stdout */ 

#include <stdio.h>

void filecopy(FILE *, FILE *);

int main(int argc, char *argv[])
{
FILE *fp;

if( argc == 1 )   /* No arguments -- use std input */
  {
  filecopy(stdin, stdout);
  }
else
  {
  if((fp = fopen( argv[1], "r")) == NULL)
    {
    printf("%s(line %d): can't open: %s\n", 
           __FILE__, __LINE__,  argv[1]);
    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);
}

Program to read the roads for the route lab. ftp://lt.tucson.az.us/pub/c/read-roads.c

/* Louis Taber 3/18/2001
 *
 * --  Read roads 
 *
 * Example program for fscanf()
 *
 *                               */

#include <stdio.h>

int main(void)
{
FILE *fp;
int zip1, zip2, miles;
char comment[200];

if((fp = fopen( "route.dat", "r")) == NULL)
  {
  printf("%s(line %d): can't open: route.dat\n", 
         __FILE__, __LINE__);
  return 1;
  }
else
  {
  do
    {
    if( fscanf( fp, "%d %d %d %s\n", 
                    &zip1, &zip2, &miles, comment) <0 ) break;
    printf("From zip %05d to  zip %05d is %5d miles  (%s) \n", 
           zip1, zip2, miles, comment);
    }  while( zip1 != 0 );
  fclose(fp);
  }
return 0;
}

6.21 Files