Strings and the standard library

In plain English

A C string is a sequence of characters stored in a char array. The null terminator is a special character with the value zero that marks where the string ends, and every string function relies on it. The standard library is the set of ready-made functions that ship with C; strlen returns the length of a string, strcpy copies one string into another, and snprintf writes formatted text into a buffer without overflowing it. The old function gets reads input with no size limit, so a long line can write past the end of the array and corrupt memory, which is why it is unsafe and should never be used.

Step by step

1. 1Declare a char array and initialise it with a string literal.
2. 2Print the string and its length using strlen.
3. 3Copy it into a second array with strcpy.
4. 4Build a longer message with snprintf and a size.
5. 5Look up why gets has no size argument and is therefore unsafe.

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

int main(void) {
    char title[32] = "C Programming";
    printf("length = %zu\n", strlen(title));

    char copy[32];
    strcpy(copy, title);
    printf("copy = %s\n", copy);

    char buffer[64];
    snprintf(buffer, sizeof buffer, "%s has %d lessons", title, 5);
    printf("%s\n", buffer);

    return 0;
}

Common mistakes

• Forgetting to leave room for the null terminator makes a buffer one byte too small for its string.
• Using strcpy into a destination that is too small overwrites whatever happens to sit next to it in memory.
• Calling strlen on a char array that has no null terminator reads past the end of the array.
• Using gets at all is a mistake, because it cannot stop a long line from overflowing the buffer.