Text Files in C

There is a current position in the file's character stream. The current position starts out at the first character in the file, and moves one character over as a result of a character read (or write) to the file; to read the 10th character you need to first read the first 9 characters (or you need to explicitly move the current position in the file to the 10th character).

There are special hidden chars (just like there are in the stdin input stream), '\n', '\t', etc. In a file there is another special hidden char, EOF, marking the end of the file.

Using text files in C

1. DECLARE a FILE * variable

     FILE *infile; 
     FILE *outfile;
   

2. OPEN the file: associate the variable with an actual file using fopen you can open a file in read, "r", write, "w", or append, "a" mode

     infile = fopen("input.txt", "r");       // using relative path name of file
   
     if ( infile == NULL ) {  // error checking with fopen call
       printf("Unable to open file."); 
       exit(1);
     } 
   
     outfile = fopen("/home/newhall/output.txt", "w");  // using absolute path name of file 
   
     if ( outfile == NULL ) { // error checking with fopen 
       printf("Unable to open file."); 
       exit(1);
     }
   

3. USE I/O operations to read, write, or move the current position in the file. This example uses the function fscanf to read from infile and fprintf to write values to outfile (see below for other functions to read and write, including getc, putc):

     int x, y, num;
   
     num = fscanf(infile, "%d%d", &x, &y);  // read 2 int values from infile into x and y
   
     if( num != 2 ) {  // fscanf returns number of values read
       printf("fscanf error");
       exit(1);
     }
   
     num = fprintf(outfile, "%d\n", x);  // write 1 int value (value of x) and newline to outfile
   
     if( num != 1 ) {  // fprintf returns number of values written
       printf("fprint error");
       exit(1);
     }
   
   

4. CLOSE the file: use fclose to close the file after you are done with it

    
     fclose(infile); 
     fclose(outfile);
   

   You can also move the current file position in a file:

      // to reset current position to beginning of file
      void rewind(FILE *f);   
   
      rewind(infile);
   
      // to move to a specific location in the file:
      fseek(FILE *f, long offset, int whence);  
   
      fseek(f, 0, SEEK_SET);    // seek to the beginning of the file
      fseek(f, 3, SEEK_CUR);    // seek to 3 chars from the current position
      fseek(f, -3, SEEK_END);   // seek to 3 chars from the end of the file
   
   

File I/O operations in stdio.h

---------------
Character Based
---------------

int getc(FILE *f): returns the next character in the file stream f 
		    as an integer or EOF 
int fgetc(FILE *f): same as getc

int putc(int c, FILE *f): writes the character c to the file stream f and 
 			  returns the character written
int fputc(int c, FILE *f): same as putc

int ungetc(int c, FILE *f): pushes the character c back onto the file stream f 
    			    returns the chars pushed

   	you can only push back one character and EOF cannot be pushed back
    
    	used when you need to read in a char value and test it, and based on
	test results need to put it back in the file.

int getchar();    read from stdin 

putchar(int c);   write to stdout (we usually use printf to do this instead)

-----------
Line  Based:  WARNING THESE CAN BE A BIT MORE TRICKY TO USE  than getc & putc
------------

char *fgets(char *s, int n, FILE *f);

    reads at most n-1 characters into the array s stopping if a newline is 
    encountered, newline is included in the array which is '\0' terminated 

int fputs(char *s, FILE *f);

    writes the string s which need not contain a newline onto the file stream f 

---------
Formatted:  WARNING THESE CAN BE REALLY TRICKY TO USE 
---------   (USING scanf for reading in input from a user is NOT ROBUST to bad input)

int fscanf(FILE *f, char *format, arg1, arg2, ...);

    designed to be the counterpart to printf, uses a similar control string     
    returns EOF if end of file or an error occurs
    otherwise returns the number of input items converted and assigned

    int scanf(char *format, arg1, arg2, ...);   for stdin

%d integer 		    
%f float 		    
%lf double 		    
%c character 	      	    
%s string, up to first white space

%[...] string, up to first character not in brackets
%[0123456789] would read in digits
%[^...] string, up to first character in brackets
%[^\n] would read everything up to a newline


int fprintf(FILE *f, char *format, addrofarg1, addrofarg2, ...);

    just like standard printf, which assumes a file stream of stdout
    allows you to specify other file streams

Examples:
----------
int x;
double d;
char c, array[MAX];

scanf("%d %c", &x, &c);   // read int & char from stdin, ignore ALL whitespace

fscanf(infile, "%d,%c", &x, &c); // read an int & char from file where int and
				 // char are separated by a comma

fprintf(outfile, "%d:%c\n", x, c); // write int & char char values to file
				 // separated by colon, followd by new line char

fscanf(infile,"%s", array);  // read a string from file into array 
			     // stops at white space

fscanf(infile, "%lf %24s", &d, array);  // read a double and a string upto
					// 24 chars from infile

fscanf(infile, "%20[012345]",array);  // read a string of at most 20 chars
				      // consisting of only chars in set

fscanf(infile, "%[^.,:!;]", array); // read in a string, stop when hit 
				    // punctuation mark

fscanf(infile, "%ld %d%c", &x, &b, &c);  // read in two integer values store
					 // first in long, second in int
					 // read in end of line char into c 

fscanf returns the number of items read (above example would return 3) or
it returns EOF if it reaches EOF while reading