strstr – cppreference.com

Original:

Finds the first occurrence of the byte string substr in the byte string pointed to by str.

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Original:

Pointer to the first character of the found substring in str, or NULL if no such substring is found. If substr points to an empty string, str is returned.

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

void find_str(char const* str, char const* substr) 
{
    char* pos = strstr(str, substr);
    if(pos) {
        printf("found the string '%s' in '%s' at position: %d\n", substr, str, pos - str);
    } else {
        printf("the string '%s' was not found in '%s'\n", substr, str);
    }
}

int main(int argc, char* argv[]) 
{
    char* str = "one two three";
    find_str(str, "two");
    find_str(str, "");
    find_str(str, "nine");
    find_str(str, "n");

    return 0;
}

found the string 'two' in 'one two three' at position: 4
found the string '' in 'one two three' at position: 0
the string 'nine' was not found in 'one two three'
found the string 'n' in 'one two three' at position: 1