String functions from <string.h>

The C standard library provides a set of string functions in <string.h>. These functions are the building blocks for working with text in C. They are also a source of notorious bugs — particularly buffer overflows — because they require you to manage destination buffer sizes yourself.

strcpy — copy a string

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

int main(void) {
    char src[] = "Hello";
    char dst[10]; /* must be large enough */

    strcpy(dst, src);
    printf("%s\n", dst); /* Hello */
    return 0;
}

strcpy(dst, src) copies bytes from src to dst including the null terminator. dst must have enough space — strcpy does not check. If dst is too small, you write past its end: a buffer overflow.

Use strncpy for safer copying with a length limit:

char dst[10];
strncpy(dst, src, sizeof(dst) - 1); /* copy at most 9 chars */
dst[sizeof(dst) - 1] = '\0';        /* always null-terminate manually */

strncpy does not guarantee null-termination if the source is longer than the limit, so you must add '\0' manually.

strlen — string length

size_t len = strlen("Hello"); /* 5 -- does not count '\0' */

Remember: strlen runs in O(n) time because it scans to the null terminator. Do not call it inside a loop to check the length repeatedly — store it in a variable.

strcat — concatenate strings

char result[20] = "Hello";
strcat(result, ", World!"); /* appends -- result must have enough space */
printf("%s\n", result); /* Hello, World! */

strcat appends src to the end of dst, overwriting the null terminator of dst and adding a new one at the end. Again, dst must be large enough for the combined result.

Safer version:

strncat(result, extra, sizeof(result) - strlen(result) - 1);

strcmp — compare strings

You cannot use == to compare strings in C — that compares pointers (addresses), not content. Use strcmp:

int cmp = strcmp("apple", "banana");
/* cmp < 0: "apple" comes before "banana" alphabetically */
/* cmp == 0: the strings are equal */
/* cmp > 0: "apple" comes after the other string */

#include <string.h>

int main(void) {
    char input[] = "quit";

    if (strcmp(input, "quit") == 0) {
        printf("Exiting\n");
    } else if (strcmp(input, "help") == 0) {
        printf("Showing help\n");
    }

    return 0;
}

strncmp(a, b, n) compares at most n characters. Use it when comparing a fixed-length prefix.

strchr and strrchr — searching for a character

char str[] = "hello world";
char *pos = strchr(str, 'o');  /* pointer to first 'o' */
char *last = strrchr(str, 'o'); /* pointer to last 'o' */

if (pos != NULL) {
    printf("Found 'o' at index %td\n", pos - str); /* 4 */
}

strchr returns a pointer to the first occurrence of the character, or NULL if not found. strrchr finds the last occurrence. The index is computed as pointer arithmetic (pos - str).

strstr — searching for a substring

char haystack[] = "the quick brown fox";
char *found = strstr(haystack, "quick");

if (found != NULL) {
    printf("Found at index %td\n", found - haystack); /* 4 */
}

sprintf — building a string with printf formatting

#include <stdio.h>

int main(void) {
    char buffer[64];
    int score = 95;
    sprintf(buffer, "Your score is %d%%", score);
    printf("%s\n", buffer); /* Your score is 95% */
    return 0;
}

sprintf works like printf but writes to a buffer instead of stdout. Always ensure the buffer is large enough. snprintf(buffer, size, format, ...) is safer — it limits output to size - 1 characters and always null-terminates.

Summary of key functions

Where to go next

Next: common string pitfalls — the bugs that arise most often when working with C strings, including off-by-one errors, missing null terminators, and using == to compare strings.