Files

Applications require information to be read from or written to a memory device

• All files are administered by the Operating System(OS) which allocates and deallocates disk blocks.

• The OS also controls the transfer of data between programs and the files they access

The most convenient way to handle such large scales of data is to read/write data from/to files.

Handling Files in C programs

A file has to be opened before data can be read from or written to it.

When the file is opened, the OS associates a stream with the file

A common buffer and a file postion indication are maintained in the memory for a function to know how much of the file has already been read.

The stream is disconnected when the files is closed.

Opening a File

FILE *file_pointer_name;
file_pointer_name = fopen("filename", "mode");

mode can be one of the following:

• r: Open a file for reading. The file must exist.
• w: Create an empty file for writing. If a file with the same name already exists, its content is erased and the file is considered as a new empty file.
• a: Append to a file. Writing operations append data at the end of the file. The file is created if it does not exist.
• r+ or w+: Open a file for update both reading and writing. The file must exist.
• a+: Supports both reading and writing but but writing is only supported if it is appended at the end of the file.
• rb: Open a file for reading in binary mode.
• wb: Create an empty file for writing in binary mode.
• ab: Append to a file in binary mode.
• rb+ or wb+: Open a file for update both reading and writing in binary mode.

Closing a File

Although it is not required to close a file. However, it is a good practice to do so for various reasons:

Operating systems have a limit on the number of files that a program can open.

Multiple programs might be trying to access the same file, and they cannot do so until the current program closes the file stream.

fclose(file_pointer_name);

Reading and Writing to a File

The standard library offers a number of functions for performing read and write on the files

1. character oriented functions (fgetc and fputc)
2. line oriented functions (fgets and fputs`)
3. formatted functions (fscanf and fprintf)

• fgetc:
  • It fetches the next character that has not yet been read from the file.
  • fgetc(FILE *stream);
  • It could be used as follows: int c = fgetc(fp);
  • If there are no more characters to be read from the file, fgetc returns EOF.

while((c = fgetc(fp)) != EOF) {
    // Process the current character in c here.
}

• fputc:

  • It outputs a character to the current file.
  • fputc(int c, FILE *stream);
  • Note that although the syntax specifies int, the output is a character, so we need to enter the ASCII code. For example, fputc(70, fp); outputs ‘F’, because the ASCII code of ‘F’ is 70.

• fgets:

  • fgets(char *s, int size, FILE *stream);
  • This is a line-oriented function. fgets reads at most size - 1 characters from the file stream and saves the characters in s. After that, \0, the string-terminating character, is automatically appended to s.
  • Note that if a newline is encountered before size - 1 characters, then fgets automatically stops reading. So, fgets will read size - 1 characters or one entire line, whichever is smaller. Example usage: char buf[10]; fgets(buf, 10, fp);

• fputs:

  • fputs(const char* s, FILE *stream);
  • The fputs function writes the string of characters in s to the file stream.

• fscanf and fprintf:

  • These function exactly like scanf and printf, except we need to specify the file stream as the first argument.

  • fscanf(FILE *stream, ….) (the rest is the same as normal scanf/printf).

    fscanf(fp1, “%s%d%d”, name, &marks1, &marks1);
    fprintf(fp2, “%s %d %d %d”, name, marks1, marks2, marks1 + marks2);
    

Example

#include <stdio.h>

int main() {
    FILE* fp = open("file.txt", "w+");
    char buf[] = "A1234 abcd";
    fputs(buf, fp); // Writes "A1234 abcd" to fp.
    rewind(fp); // Go back to position 0 of fp.
    int c = fgetc(fp); // c = 'A';
    fputc(c, stdout); // Prints A to standard output (explained later).
    int x;
    fscanf(fp, "%d", &x); // x = 1234
    fprintf(stdout, " %d", x); // Prints 1234
    fclose(fp);
}

Special Files

• stdin: Standard input file. It is a file that is automatically opened when a program starts. It is used to read input from the keyboard.
• stdout: Standard output file. It is a file that is automatically opened when a program starts. It is used to write output to the screen. It is associated with the terminal.
• stderr: Standard error file. It is mainly used for debugging purposes. The default output of stderr is also in the terminal, the same as stdout.

fscanf() and fprintf() can achieve all of the functionality of scanf() and printf(), simply by using the special files, stdin and stdout.

fscanf() can read from file streams, but scanf() cannot.

Error Handling

The functions that open files return NULL if they fail to open the file.

FILE *fp = fopen("file.txt", "r");
if (fp == NULL) {
    printf("Error opening file\n");
    return 1;
}