String Functions in C (string.h)

Because C strings are just arrays, you can't compare them with == or join them with +. The standard header <string.h> provides functions for every common job – length, copy, join, compare, search, split. Knowing them, their pitfalls and how to implement a few yourself is core C knowledge.

Length, copy and join

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

int main(void) {
    char first[20] = "Asha";
    char last[] = "Verma";
    char full[40];

    printf("strlen(\"%s\") = %zu, sizeof = %zu\n", first, strlen(first), sizeof first);

    strcpy(full, first);          // copy: full = "Asha"
    strcat(full, " ");            // append: "Asha "
    strcat(full, last);           // "Asha Verma"
    printf("full = %s (%zu chars)\n", full, strlen(full));

    char safe[8];
    snprintf(safe, sizeof safe, "%s %s", first, last);   // never writes past the buffer
    printf("truncated safely: \"%s\"\n", safe);
    return 0;
}

strlen("Asha") = 4, sizeof = 20
full = Asha Verma (10 chars)
truncated safely: "Asha Ve"

Comparing strings

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

int main(void) {
    char a[] = "apple", b[] = "apple", c[] = "banana";
    const char *pa = a, *pb = b;
    printf("pa == pb ? %s\n", pa == pb ? "same" : "different addresses");   // == compares addresses!
    printf("strcmp(a, b) = %d\n", strcmp(a, b));
    printf("strcmp(a, c) %s 0\n", strcmp(a, c) < 0 ? "<" : ">");
    printf("strcmp(\"Zoo\", \"apple\") %s 0\n", strcmp("Zoo", "apple") < 0 ? "<" : ">");   // 'Z' (90) < 'a' (97)
    printf("strncmp(\"interview\", \"internet\", 5) = %d\n", strncmp("interview", "internet", 5));
    return 0;
}

pa == pb ? different addresses
strcmp(a, b) = 0
strcmp(a, c) < 0
strcmp("Zoo", "apple") < 0
strncmp("interview", "internet", 5) = 0

strcmp returns 0 when the strings are equal, a negative value when the first comes earlier, and a positive value when it comes later – comparing character codes, so upper-case letters sort before lower-case. Never use == to compare strings: it compares addresses.

Searching and splitting

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

int main(void) {
    const char *email = "asha.verma@learnfrenzy.com";
    const char *at = strchr(email, '@');                 // first '@'
    if (at != NULL) {
        printf("user = %.*s, domain = %s\n", (int)(at - email), email, at + 1);
    }
    printf("contains \"frenzy\"? %s\n", strstr(email, "frenzy") ? "yes" : "no");

    char csv[] = "Asha,21,Pune,CSE";                     // strtok modifies its input: use an array
    int field = 1;
    for (char *tok = strtok(csv, ","); tok != NULL; tok = strtok(NULL, ",")) {
        printf("field %d: %s\n", field++, tok);
    }
    return 0;
}

user = asha.verma, domain = learnfrenzy.com
contains "frenzy"? yes
field 1: Asha
field 2: 21
field 3: Pune
field 4: CSE

Write your own strlen and strcpy

A favourite interview task. Both simply walk the array until '\0':

#include <stdio.h>

size_t my_strlen(const char *s) {
    size_t n = 0;
    while (s[n] != '\0') n++;
    return n;
}

char *my_strcpy(char *dest, const char *src) {
    char *start = dest;
    while ((*dest++ = *src++) != '\0') { }   // copies each char, including the final '\0'
    return start;
}

int my_strcmp(const char *a, const char *b) {
    while (*a && *a == *b) { a++; b++; }
    return (unsigned char)*a - (unsigned char)*b;
}

int main(void) {
    char buf[32];
    my_strcpy(buf, "LearnFrenzy");
    printf("%s has %zu characters\n", buf, my_strlen(buf));
    printf("compare: %d %d\n", my_strcmp("cat", "cat"), my_strcmp("cat", "car") > 0);
    return 0;
}

LearnFrenzy has 11 characters
compare: 0 1

Practice

Summary