strstr() / strchr() / strrchr()

// Searches for needle within haystack.
// Returns: a pointer to the first occurrence, or NULL if not found.
char *strstr(const char *haystack, const char *needle);

// Searches for character c in string s from the beginning.
// Returns: a pointer to the first occurrence, or NULL if not found.
char *strchr(const char *s, int c);

// Searches for character c in string s from the end (finds the last occurrence).
char *strrchr(const char *s, int c);

// Returns a pointer to the first character in s that matches any character in accept.
char *strpbrk(const char *s, const char *accept);

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

int main(void) {
    const char *path = "/home/user/documents/report.txt";

    // Use strstr to check whether the path contains "documents".
    if (strstr(path, "documents")) {
        printf("The file is inside the documents folder.\n");
    }

    // Use strchr to find the position of the first '/'.
    char *first_slash = strchr(path, '/');
    printf("From the first '/': %s\n", first_slash); // "/home/user/documents/report.txt"

    // Use strrchr to extract the file extension from the path.
    char *ext = strrchr(path, '.');
    if (ext) {
        printf("Extension: %s\n", ext); // ".txt"
    }

    // Use strrchr to extract the filename from the path.
    char *filename = strrchr(path, '/');
    if (filename) {
        printf("Filename: %s\n", filename + 1); // "report.txt"
    }

    // Use strpbrk to find the position of a delimiter (comma or space).
    const char *csv = "apple,banana orange";
    char *sep = strpbrk(csv, ", ");
    if (sep) {
        printf("First delimiter: '%c'\n", *sep); // ','
    }

    return 0;
}

These functions return a pointer to the found position, which lets you continue processing from the middle of the original string. Always check whether the return value is NULL before using it. Dereferencing a NULL pointer causes undefined behavior.

You can calculate a zero-based index by subtracting the start address of the string from the found position (e.g., pos - haystack).

For string copying and concatenation, see strlen() / strcpy() and strcat().