week 3 c programming

Question 1

what are void pointers

Answer 1

pointers for when you do not know what type to point to

Question 2

what must you make sure you do with void pointers

Answer 2

You must cast it to the correct type before using
the underlying values
Pointer arithmetic on void* is illegal in C
You must cast it to the correct pointer type before
doing ptr arithmetic.

Question 3

what is a function pointer

Answer 3

a pointer that points to a function

Question 4

explain this : int (*func)(float, int);

Answer 4

This example means: func is a pointer to a
function that returns an int and takes a float and
an int as parameters

Question 5

what are examples of standard input/output functions in C

Answer 5

scanf()
printf()
fprintf()
fscanf()
getchar()
put char()
getline()

Question 6

what is getchar()

Answer 6

gets a character from standard input

Question 7

what is putchar()

Answer 7

writes a character to standard output

Question 8

can you write code that uses getchar() and putchar()

Answer 8

int main () {
char c;
printf(“Enter a character: “);
c = getchar();
printf(“Character entered: “);
putchar(c);
return(0);
}

Question 9

what is get line()

Answer 9

reads an entire line

Question 10

can you write code using getline()

Answer 10

int main(){
char *str = NULL;
size_t n;
int res;
printf(“Enter a string: “);
res = getline (&str, &n, stdin);
if (res != -1) {
printf (“%s”, str);
}
free(str);
return 0;
}

Question 11

hwta do we use files for in C

Answer 11

Large data volumes

Question 12

what do we to open a file in C

Answer 12

fopen(“filename”,”mode”)

Question 13

write code for fopen()

Answer 13

FILE fp; /variable fp is pointer to type FILE*/
fp = fopen(“filename”, “mode”);

Question 14

what happens if file does not exists for fopen()

Answer 14

returns NULL if it is unable to open the specified file

Question 15

File *fp is a poinet -> what is it pointing to?

Answer 15

File pointer fp points to the ‘file’ resource
* contains all information about file
* Communication link between system and program

Question 16

what are the different modes for file opening

Answer 16

r - reading mode
w - writing mode
a - appending mode
r+
w+
a+

Question 17

what is reading mode (r) in C

Answer 17

• if the file already exists then it is opened as read-only
• sets up a pointer which points to the first character in it.
• else error occurs

Question 18

what is writing mode (w) in C

Answer 18

if the file already exists then it is overwritten by a new file
* else a new file with specified name created

Question 19

what is appending mode (a) in C

Answer 19

if the file already exists then it is opened
* else new file created
* sets up a pointer that points to the last character in it
* any new content is appended after existing content

Question 20

whatare additional modes for file opening in C

Answer 20

r+
w+
a+

Question 21

what is r+

Answer 21

r+ opens file for both reading/writing an existing file
* doesn’t delete the content of if the existing file

Question 22

what is w+

Answer 22

w+ opens file for reading and writing a new file
* Overwrites if the specified file already exists

Question 23

what is a+

Answer 23

a+ open file for reading and writing from the last character in
the file

Question 24

what can you use to read/write frm a file
e>g the standard input/output

Answer 24

getc()
putc()
get line()
fprintf()
fscanf()

Question 25

what is getc()

Answer 25

read a char from a file

Question 26

what is putc()

Answer 26

write a char to a file

Question 27

write some code using getc() and putc()

Answer 27

int main(){
char ch;
FILE *fp0, *fp1;
fp0 = fopen(“infile.txt”, “r”);
fp1 = fopen(“outfile.txt”, “w”);
if (fp0==NULL || fp1==NULL){
printf(“Cannot open files\n”);
exit(-1);
}
ch = getc(fp0); // reads one char from first file
while (ch != EOF){ // EOF is ‘end of file’
printf(“%c”, ch); // Displays on screen
putc(ch, fp1); // writes to second file
ch = getc(fp0); // reads another char from first file
}
fclose(fp0);
fclose(fp1);
return 0;
}

Question 28

what is the difference between fprintf() and fscanf() compared to printf() and scanf()

Answer 28

in addition the file pointer is provide as an input

Question 29

To read one int from a file with file pointer fp0

Answer 29

int i;
fp0=fopen(“some_file”, “r”);
fscanf(fp0, “%d”, &i);

Question 30

To write one int to a file with file pointer fp1

Answer 30

int i=4;
fp1=fopen(“some_file”, “w”);
fprintf(f1, “%d”, i);

Question 31

what are some file errors that could occur

Answer 31

Typically, errors happen when a program
* tries to read beyond end-of-file (EOF)
* tries to use a file that has not been opened
* performs operation on file not permitted by ‘fopen’ mode
opens file with invalid filename
* writes to write-protected file
Example: files with read-only permission

Question 32

how to handle file errors that could occur in C

Answer 32

feof(fp) returns a non-zero value when End-of-File is reached,
else it returns zero
ferror(fp) returns nonzero value if error detected else returns
zero

Question 33

write code for foef(fp)

Answer 33

fp = fopen(“somefile”, “somemode”);
. . .
if(feof(fp)){
printf(“End of file\n”);
}

Question 34

write code for ferror(fp)

Answer 34

fp = fopen(“somefile”, “somemode”);
. . .
if(ferror(fp) !=0)
printf(“An error has occurred\n”);

Question 35

what is fseek()

Answer 35

Data in a file is basically a collection of bytes.
We can directly jump to a target byte-number in a file without
reading previous data using fseek()

Question 36

what is the syntax of fseek()

Answer 36

Syntax: fseek(file-pointer, offset, position);
* position: 0 (beginning), 1 (current), 2 (end)
* offset: number of locations to move from specified position

Question 37

what does this do?
fseek(fp, -m, 1);

Answer 37

// move back by m bytes from current

Question 38

what does this do?
fseek(fp, m, 0);

Answer 38

// move to (m+1)th byte in file

Question 39

what does ftell(fp) do

Answer 39

returns current byte position in file

Question 40

what does rewind(fp) do?

Answer 40

resets position to start of file

Question 41

what is the syntax for a command line argument

Answer 41

int main(int argc, char *argv[]){
…
}

Question 42

what is argc(argument count)

Answer 42

int and automatically stores the number of
command-line arguments passed by the user including name of program

Question 43

what type is arc

Answer 43

it is int

Question 44

what is argc (argument vector)

Answer 44

is array of char pointers listing all the arguments

Question 45

what is the minimum numer of argv can there be

Answer 45

1 as argv[0] is the name of the program
argv[1] is the first command-line argument etc

Question 46

how to deal with larger programs

Answer 46

use an automated build system

Question 47

what is a makefile

Answer 47

an automated build system

Question 48

descibe object file

Answer 48

Object files contain the compiled machine code.
Do not commit them to version control.
They are in the final step linked together for the
full executable

Question 49

what is in an object file

Answer 49

c Sources
.h Headers
.o Object files (compiled C)

Question 50

what does this mean in make file:

<target>: <file0> <file1> ...
<command></command>
</file1></file0></target>

Answer 50

Means: if target does not exist or any of the files
has changed, execute the command

Question 51

what is a preprocessor

Answer 51

Runs before
compilation and does text substitution

Question 52

syntax of preprocessor

Answer 52

include “xyz.h”

Question 53

what does this mean?
#include “xyz.h”

Answer 53

This will take xyz.h and literally paste it at the
location of the #include statement