# strstr

From cppreference.com
< c‎ | string‎ | byte

Defined in header ``<string.h>`` |  |
char* strstr( const char* str, const char* substr ); | (1) |
/*QChar*/* strstr( /*QChar*/* str, const char* substr ); | (2) | (since C23)

1) Finds the first occurrence of the null-terminated byte string pointed to by substr in the null-terminated byte string pointed to by str. The terminating null characters are not compared.
2) Type-generic function equivalent to (1). Let `T` be an unqualified character object type.

  - If `str` is of type const T*, the return type is const char*.
  - Otherwise, if `str` is of type T*, the return type is char*.
  - Otherwise, the behavior is undefined.

If a macro definition of each of these generic functions is suppressed to access an actual function (e.g. if (strstr) or a function pointer is used), the actual function declaration (1) becomes visible.

The behavior is undefined if either str or substr is not a pointer to a null-terminated byte string.

### Parameters

str | - | pointer to the null-terminated byte string to examine
substr | - | pointer to the null-terminated byte string to search for

### Return value

Pointer to the first character of the found substring in str, or a null pointer if such substring is not found. If substr points to an empty string, str is returned.

### Example

Run this code

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

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

    int main(void)
    {
        char const* str = "one two three";
        find_str(str, "two");
        find_str(str, "");
        find_str(str, "nine");
        find_str(str, "n");

        return 0;
    }

Output:

    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

### References

- C23 standard (ISO/IEC 9899:2024):

  - 7.24.5.7 The strstr function (p: TBD)

- C17 standard (ISO/IEC 9899:2018):

  - 7.24.5.7 The strstr function (p: 269)

- C11 standard (ISO/IEC 9899:2011):

  - 7.24.5.7 The strstr function (p: 369)

- C99 standard (ISO/IEC 9899:1999):

  - 7.21.5.7 The strstr function (p: 332)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.11.5.7 The strstr function

### See also

strchr | finds the first occurrence of a character (function)
strrchr | finds the last occurrence of a character (function)
C++ documentation for strstr

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