File I/O

• A file is a unit of stored memory, usually on disk
• A file can also be... directories, buffers (standard input/output are files), a socket could be a file, a program is a file
• You can write to and read from a file
• Files may be plaintext or binary (or plaintext but not intended for human consumption: EDI, XML, JSON, base-64 encoding)
• The basic steps to follow are:
  1. Open the file
  2. Process the file
  3. Close the file

File I/O Iin C

• There is a special FILE * pointer built into the standard library
• You can open a file using fopen()
• It takes two arguments (both strings): the path/name of the file to open and a mode: "r" (reading, file input), "w" (writing, file output)

//open a file data.txt in the current working directory (CWD)
FILE *f = fopen("data.txt", "r");

//for writing instead:
FILE *f = fopen("data.txt", "w");

//you can also use relative paths:
FILE *f = fopen("../../mydata/data.txt", "r");

//you can use absolute paths:
FILE *f = fopen("/etc/shadow", "r");

• If fopen fails, it returns NULL
• You may not have permission(s) to read/write a file, the file may not exist, etc.
• If you open a new file for writing that does not already exist, it will be create for you. However, if it did already exist, you will overwrite its data
• Once you are done processing the file, you should always close it
• To close a file use fclose()

FILE *f = fopen("data.txt", "r");
//...
fclose(f);

File Output in C

• There are many ways to do file output, keep it simple:
• Use fprintf(): same usage as printf but takes an additional first argument, the file you want to "print" to

FILE *f = fopen("data.txt", "w");

fprintf(f, "Hello world!\n");
int x = 10;
fprintf(f, "x = %d\n", x);
char name[] = "Chris";
fprintf(f, "hello, %s\n", name);

fclose(f);

File Input in C

• There are many dangerous ways to read from a file
• I'm going to keep it simple and safe: fgetc, fgets
• fgetc gets a single character at a time from the file

FILE *f = fopen("lorem.txt", "r");

int i;
char c = fgetc(f);
while(c != EOF) {
	printf("%c\n", c);
	c = fgetc(f);
}

fclose(f);

• fgetc returns a special EOF (end of file) value when it gets to the end of a file
• It is generally more efficient to read an entire line at a time from a file: fgets
• fgets reads up to an entire line from the file: it stops when it reaches the first endline character \n OR it reaches the limit you gave it
• You provide a limit parameter that limits the number of characters/bytes that fgets reads at a time to prevent buffer overflows
• fgets reads an entire string from a file, *fgets(char *s, int size, FILE *stream);
  • s is the string that the line will be stored in (it is your responsibility to ensure it is "big enough")
  • size limits the number of byte reads, fgets will actually only read at most size-1 bytes; it automatically inserts the null terminator for you!
  • stream is the file you are reading from
  • It returns a char * pointer (which points to the same buffer string you read into) BUT when you reach the end of a file, it returns NULL

FILE *f = fopen("lorem.txt", "r");

char buff[100];
char *s = fgets(buff, 100, f);
while(s != NULL) {
	//you can remove the endline character as follows:                              
	//removing the endline character is known as "chomping"                         
	buff[strlen(buff)-1] = '\0';
	printf("%s\n", buff);
	s = fgets(buff, 100, f);
}

fclose(f);

Binary Data in C

• C provides both fread for reading binary data and fwrite
• size_t fwrite(const void *restrict ptr, size_t size, size_t nitems, FILE *restrict stream);

File IO in Java

• Java provides a File class

File f = new File("file.txt");
File f = new File("/path/to/some/file.txt");
File f = new File("/../../mydata/file.txt");

//input:
Scanner s = new Scanner(f);
int x = s.nextInt();
String line = s.nextLine();

//closing: you close the scanner not the file
s.close();

//reading a file line by line:
File f = new File("data.txt");
Scanner s = new Scanner(f);
while(s.hasNextLine()) {
	String line = s.nextLine();
	System.out.println(line);
}

File Output

• There are at least a dozen ways to do file output
• Simplest: use a PrintWriter to do file output

File f = new File("outputdata.txt");
PrintWriter pw = new PrintWriter(f);
pw.println("hello!");
pw.print("no endline character here, do it manually:\n");
int x = 10;
pw.printf("%d\n", x);
pw.println("x = " + x);