strings in C

Complete

Summary

Strings

  • array of chars that ends with a ‘\0’(null) char
c
// valid strings
char *a = "hello"; // " is used to wrap strings, ' for chars
char b[] = { 'h', 'e', 'l', 'l', 'o', '\0' };
char c[6] = { 'h', 'e', 'l', 'l', 'o' }; // last char is initialized as 0 -> '\0'

// invalid strings
char d[5] = { 'h', 'e', 'l', 'l', 'o' };

Assignment

  • same as arrays
  • cannot assign after declaration

String functions

  • string.h
c
strlen(s); // counts upto but not including '\0'

strcmp(s1, s2); // 0 if equal, -ve if s1 < s2, +ve if s1 > s2
strncmp(s1, s2, n); // compare only the first n characters

strcpy(s1, s2); // copy the string at s2 into s1
strncpy(s1, s2, n); // copy only the first n characters

Concept

I/O functions

  • reading from stdin
c
char *str;

scanf("%s", str); // reads until whitespace
fgets(str, size, stdin); // reads size-1 chars or until newline
  • printing to stdout
c
printf("%s\n", str);
puts(str); // terminates implicitly with a newline

will trace through the string until the '\0'

Application

String terminator

c
char d[5] = { 'h', 'e', 'l', 'l', 'o' };
puts(d); // hello??????... we don't know whats stored in memory after the array
d'h''e''l''l''o'???maynotbeterminator

this can leak information about what’s stored in memory

Overflow

c
// strcpy and most fns that don't take into account string length are unsafe
char name[10];
strcpy(name, "Bob Ross"); // 1

strcpy(name, "Ovuvuevuevue Enyetuenwuevue Ugbemugbem Osas"); // 2
name'B''o''b''''R''o''s''s''n0'?name'O''v''u''v''u''e''v''u''e''v''u''e''''E''n''y''e''t''u''e''n''w''u''e''v''u''e''''U''g''b''e''m''u''g''b''e''m''''O''s''a''s''n0'

a stack canary will protect overflowing in most cases, compile with -fno-stack-protector to compile without a stack canary

Removing the newline char

c
fgets(str, size, stdin);
	len = strlen(str);
	if (str[len – 1] == '\n')
		str[len – 1] = '\0';

Jank

c
long x = 470020878965; // 0x00 00 00 6D 6F 6D 72 75
puts((void *)&x); // urmom