C Strings

Treat text as a character array that ends at \0, keep capacity visible, and never use gets().

A C string is a character array terminated by \0

C has no built-in string object. char city[16] = "Rabat"; creates a writable array. The visible letters occupy positions 0 through 4; C places a terminating null character \0 at position 5. String-aware functions use that marker to know where text ends.

Capacity must include both the visible characters and the terminator. char city[6] = "Rabat"; fits exactly. char city[5] = "Rabat"; does not leave room for \0 and is invalid. Array indexes and index < count loops are the same rules as C Arrays; the extra rule is the terminator.

#include <stdio.h>

int main(void) {
  char city[16] = "Rabat";

  printf("City: %s\n", city);
  printf("First character: %c\n", city[0]);
  return 0;
}

Edit the program, predict its output, then run it.

Read text with a capacity-aware policy

gets() is removed from modern C and was never safe: it writes until a newline with no limit. Do not use it. scanf("%s", city) is also unsafe for a fixed buffer because it has no width. For a char city[16] that reads one whitespace-delimited word, use scanf("%15s", city);. The width leaves one slot for \0. It stops at whitespace, so it is not suitable for a name with spaces.

For a line of text, prefer fgets(city, sizeof city, stdin). It receives the buffer and its capacity, and it can retain spaces. Check the return value. It may keep a trailing newline when there is room; real programs often remove that newline before comparison or display.

string.h counts and compares characters, not capacity

#include <stdio.h>
#include <string.h>

int main(void) {
  char city[16] = "Rabat";

  printf("Length: %zu\n", strlen(city));
  if (strcmp(city, "Rabat") == 0) {
    puts("Match");
  }
  return 0;
}

Do not write through a pointer to a string literal

char city[] = "Rabat"; creates an array you can modify. char *city = "Rabat"; points at a string literal. Treat that literal as read-only: city[0] = 'r'; has undefined behavior. Use a character array whenever the program must change the characters. Pointer details continue in C Pointers.

Independent lab: a labeled report name

Store a report title in a char array large enough for the text plus \0, print it with %s, and print the first character with %c. Compile locally to add strlen and a strcmp check. Do not call gets().

Lesson review

• A C string is a char array that ends at \0.
• Capacity must include the terminator.
• %s prints the whole terminated string; %c prints one character.
• strlen and strcmp need a terminator; they do not know the buffer size.
• Never use gets(). Bound scanf or prefer fgets.