Files and Streams

Question 1

What does the scanf() function do?

Answer 1

It’s the reverse of printf() - used for formatted input from the console.

Question 2

What does the return value of scanf() represent?

Answer 2

The number of data items successfully assigned a value.

Question 3

What’s the correct way to read an integer using scanf()?

Answer 3

scanf(“%d”, &k) - The ampersand before the variable name is required to pass by reference.

Question 4

What is a stream in C?

Answer 4

An abstraction of a file that provides a consistent interface to the programmer, regardless of the actual device.

Question 5

What are two key characteristics of streams?

Answer 5

Streams are buffered, and may be text or binary.

Question 6

What’s the difference between text and binary streams?

Answer 6

Text streams are sequences of characters with possible character translation (e.g., newline → CR+LF), while binary streams are sequences of bytes with no character translation.

Question 7

What header file is required to use stream functions?

Answer 7

<stdio.h>
</stdio.h>

Question 8

What is the purpose of a file pointer?

Answer 8

A file pointer (FILE *) is used to identify and manage a stream for file operations.

Question 9

How do you open a file in C?

Answer 9

Using fopen(): FILE *fp = fopen(“filename”, “mode”);

Question 10

Why should you always check if a file was opened successfully?

Answer 10

Because fopen() returns NULL if it fails to open the file.

Question 11

What is the correct pattern to check if a file opened successfully?

Answer 11

if ((fp = fopen(“filename”, “r”)) == NULL) {
/* error handling */
}

Question 12

How do you close a file in C?

Answer 12

Using fclose(fp);

Question 13

What does the “r” mode do when opening a file?

Answer 13

Opens a text file for reading from

Question 14

Q: What does the “w” mode do when opening a file?

Answer 14

Creates a text file for writing to (overwrites if file exists).

Question 15

What does the “a” mode do when opening a file?

Answer 15

Opens a text file for appending to (writing to the end of it).

Question 16

What’s the difference between “r+” and “w+”?

Answer 16

“r+” opens an existing file for both reading and writing, while “w+” creates a new file (or truncates existing) for both reading and writing.

Question 17

How do you specify a binary file mode?

Answer 17

By adding “b” to the mode, e.g., “rb”, “wb”, “ab”.

Question 18

What is errno in C?

Answer 18

A global variable defined in errno.h that is updated by system functions when an error occurs.

Question 19

How can you get the text description of an error based on errno?

Answer 19

Using strerror(errno) from <string.h>

Question 20

What does the ferror() function do?

Answer 20

Determines whether the most recent operation on a file produced an error.

Question 21

When is the value of ferror() reset?

Answer 21

It’s reset by each file operation.

Question 22

What function writes a single character to a stream?

Answer 22

fputc(ch, fp)

Question 23

What function writes a string to a stream?

Answer 23

fputs(str, fp)

Question 24

Does fputs() output the null terminator?

Answer 24

No, the null terminator is not output.

Question 25

What function allows formatted output to a stream?

Answer 25

fprintf(fp, "format", arguments...)

Question 26

What function reads a single character from a stream?

Answer 26

fgetc(fp)

Question 27

What function reads a string from a stream?

Answer 27

fgets(str, count, fp)

Question 28

How does `fgets()` determine when to stop reading?

Answer 28

It reads up to a newline character, or a maximum of `count-1` characters.

Question 29

What function allows formatted input from a stream?

Answer 29

fscanf(fp, "format", &variables...)

Question 30

What does `fflush(fp)` do?

Answer 30

Forces the output buffer to be written to the file.

Question 31

What does `remove("filename")` do?

Answer 31

Deletes the specified file

Question 32

What does `rewind(fp)` do?

Answer 32

Resets the file position indicator back to the beginning of the file

Question 33

What does `fseek(fp, offset, whence)` do?

Answer 33

Moves the file position indicator to `offset` bytes from `whence` (beginning, end, or current position)

Question 34

What does `sprintf()` do?

Answer 34

Like `fprintf()`, but outputs to a string buffer instead of a file.

Question 35

What does `sscanf()` do?

Answer 35

Like `fscanf()`, but reads input from a string buffer instead of a file.

Question 36

What header file is needed for `sprintf()` and `sscanf()`?

Question 37

What are the three standard streams available in C programs?

Answer 37

`stdin` (input from terminal), `stdout` (normal output to terminal), and `stderr` (error messages to terminal).

Question 38

What is `scanf()` equivalent to?

Answer 38

`fscanf(stdin, ...)`

Question 39

What is `printf()` equivalent to?

Answer 39

`fprintf(stdout, ...)`

Question 40

Why separate `stdout` and `stderr`?

Answer 40

To allow normal output and error messages to be redirected separately

Question 41

How do you redirect `stdout` to a file in Linux?

Answer 41

Using the `>` operator: `program > output_file`

Question 42

How do you redirect `stderr` to a file in Linux?

Answer 42

Using the `2>` operator: `program 2> error_file`

Question 43

How do you redirect both `stdout` and `stderr` to the same file?

Answer 43

Using: `program > out.file 2>&1`

Question 44

What does it mean that streams are buffered?

Answer 44

Data is not necessarily written to the device when the write command is issued.

Question 45

When are streams automatically flushed?

Answer 45

At the end of each line when output to a terminal, when the buffer is full, or when the program ends.

Question 46

What is the typical buffer size for streams?

Answer 46

Implementation dependent, but often 8192 bytes.

Question 47

How can you ensure output is immediately written?

Answer 47

By using `fflush(stdout)` to manually flush the buffer.