Lesson 10

ArtyArty
4 min read

String Manipulation <string.h>

  • Provides useful functions:

    • copy strings

    • concatenating strings

    • comparing strings

    • tokenizing strings

    • determining the length of strings

    • etc

Copying and Concatenation

FunctionDescription
strcpy(dest, src)Copies src into dest (unsafe, may cause buffer overflow).
strncpy(dest, src, n)Copies up to n characters from src to dest.
strcat(dest, src)Appends src to dest (unsafe, may cause buffer overflow).
strncat(dest, src, n)Appends up to n characters from src to dest.

  • The first argument is always the one that gets updated.

  • Every function except for strncopy appends the null (\0) character to its result.

  • For concatenation, the NULL character (\0) in the first string gets replaced with the first character of the second string.

  • Make sure the first array is long enough.

Comparison

FunctionDescription
int strcmp(const char *str1, const char *str2);Compares str1 and str2 lexicographically.
int strncmp(const char *str1, const char *str2, size_t n);Compares up to n characters of str1 and str2.

  • Returns

    • 0 if str1 equals str2.

    • Positive number (0 >) if str1 is greater than str2.

    • Negative number (< 0) if str1 is less than str2

  • The first argument is the one that determines if a one string is greater than the other. Eg. consider char str1 = “a”; char str2 = “b”;

    • -1 = strcmp(str1, str2);

    • 1 = strcmp(str2, str1);

  • It goes through each character and compares its ASCII value. Eg. consider char str1 = "aa"; char str2 = "aA";

    • 32 = strcmp(str1, str2); This is because the first and only character ("a") in str1 is 97 and the first and only character ("A") in str2 is 65.
FunctionDescription
char *strchr(const char *str, int c);Returns a pointer to the first occurrence of c in str.
char *strrchr(const char *str, int c);Returns a pointer to the last occurrence of c in str.
char *strstr(const char *haystack, const char *needle);Finds the first occurrence of needle in haystack.
char *strtok(char *str, const char *delim);Tokenizes str using delimiters from delim.
size_t strspn(const char *str1, const char *str2);Returns the length of the initial segment of str1 containing only characters from str2.
size_t strcspn(const char *str1, const char *str2);Returns the length of the initial segment of str1 containing no characters from str2.
char *strpbrk(const char *str1, const char *str2);Finds the first occurrence in str1 of any character in str2.

  • We can get what the function does based on the suffix

    • chr does a search of a character

    • rchr does a reverse search of a character

    • str does a search for a string

    • tok searches and tokenizes a string

    • spn does a search for a span (a segment) match on both strings (abcdef123 and abcde would match abcde).

    • strspn does a complement search for a span (a segment) mismatch on both strings (abcdef123 and abcde would match 123).

Memory

FunctionDescription
void *memcpy(void *dest, const void *src, size_t n);Copies n bytes from src to dest.
void *memmove(void *dest, const void *src, size_t n);Copies n bytes from src to dest, allowing overlap.
void *memset(void *str, int c, size_t n);Fills the first n bytes of str with c.
int memcmp(const void *str1, const void *str2, size_t n);Compares the first n bytes of str1 and str2.
void memchr(const void str, int c, size_t n);Searches for the first occurrence of a specified byte (c) in the first n bytes of a memory block.

  • Difference between memcpy/memcmp and strcpy/strcmp:

    • Deals with the memory, not a string.

    • Uses a number of memory blocks instead of stopping when the NULL character (\0) is found.

    • Does not add the NULL character (\0) after copying which means that can be used to copy other types of data (like a struct).

    • memcpy/memcmp is faster

    • strcpy/strcmp is safer because of its reliability on the NULL character (\0)

Other

Function PrototypeDescription
size_t strlen(const char *str);Returns the length of str (excluding the null terminator).
char *strerror(int errnum);Returns a pointer to a human-readable string that describes the error corresponding to the error number errnum.
 
#include <stdio.h>
#include <string.h>
#include <errno.h>

int main() {
    // Example of an error (trying to open a non-existing file)
    FILE *file = fopen("nonexistentfile.txt", "r");
    if (file == NULL) {
        printf("errno: %d\n", errno);
        printf("Error opening file: %s\n", strerror(errno));  // strerror gives the description of the error
    }
    return 0;
}

// output:
// errno: 2
// Error opening file: No such file or directory
0
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