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Excerpts that are difficult to read in C

Last edited: 2024-11-26

Based on "So you think you know C?", from https://wordsandbuttons.online/so_you_think_you_know_c.html

return

  • The following examples use the operating system exit code to show program output.
  • In the C language, the main() function returns an integer, but the operating system uses the byte type and therefore values above 255 are out of range.
  • Values over 255 get wrapped around.
int main(void) {
  return 260;
}

[C kernel] Executable exited with code 4

sizeof(structure)

  • When the sizeof() of elements are different in a structure, depending on the compiler and implementation, padding is added by the compiler to adapt to the hardware characteristics.
  • Ref.: https://www.geeksforgeeks.org/is-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member/
struct S {
  short i;
} s;

int main(void) {
  return sizeof(s);
}

[C kernel] Executable exited with code 2

struct S {
  int i;
} s;

int main(void) {
  return sizeof(s);
}

[C kernel] Executable exited with code 4

struct S {
  double i;
} s;

int main(void) {
  return sizeof(s);
}

[C kernel] Executable exited with code 8

struct S {
  char c;
} s;

int main(void) {
  return sizeof(s);
}

[C kernel] Executable exited with code 1

int main(void) {
  char c;
  return sizeof(c);
}

[C kernel] Executable exited with code 1

int main(void) {
  int i;
  return sizeof(i);
}

[C kernel] Executable exited with code 4

struct S {
  int i;
  char c;
} s;

int main(void) {
  return sizeof(s);
}

[C kernel] Executable exited with code 8

struct S {
  int i;
  char c;
} s;

int main(void) {
  return sizeof( * ( &s ));
}

[C kernel] Executable exited with code 8

struct S {
  double i;
  char c;
} s;

int main(void) {
  return sizeof( * ( &s ));
}

[C kernel] Executable exited with code 16

sizeof( two variables )

int main(void) {
  short int b = 0;
  return sizeof( b );
}

[C kernel] Executable exited with code 2

int main(void) {
  char a = 0;
  short int b = 0;
  return sizeof( a + b );
}

[C kernel] Executable exited with code 4

char as byte

int main(void) {
  char a = ' ' * 2;
  return a;
}

[C kernel] Executable exited with code 64

shift

#include <stdio.h>
int main(void) {
  int i = 16;
  printf("%d\n", 1 << i );
  printf("%d\n", (1 << i) >> i );
  printf("%d\n", ((1 << i) >> i) <= i );
}

65536
1
1

i++ + ++i

  • The first i++ returns zero, and then the variable is incremented and the variable's value becomes 1.
  • At this point, the intermediate result becomes (0) + (++i).
  • ++i first increments the value of the variable, changing it to 2, and then returns its value.
  • The intermediate result becomes (0) + (2).
#include <stdio.h>
int main(void) {
    int i =0, j = 0, k = 0, l = 0;
    j = i++;
    printf("%d\n", j );
    k = ++i;
    printf("%d\n", k );
    l = j + k;
    printf("%d\n", l );
    i = 0;
    j = (i++) + (++i);
    printf("%d\n", j );
}

/tmp/tmpmn8nioy4.c: In function ‘main’:
/tmp/tmpmn8nioy4.c:11:11: warning: operation on ‘i’ may be undefined [-Wsequence-point]
   11 |     j = (i++) + (++i);
      |         ~~^~~


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