strstr

 

C

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Technical Specifications

Symbol index

 

Strings library

Byte strings

Multibyte strings

Wide strings

 

Null-terminated byte strings

Functions
Character manipulation
isalnum isalpha islower isupper isdigit isxdigit isblank (C99)	iscntrl isgraph isspace isprint ispunct tolower toupper
Conversions to and from numeric formats
atoiatolatoll (C99) atof strtolstrtoll (C99) strtoulstrtoull (C99)	strtoimaxstrtoumax (C99)(C99) strtofstrtodstrtold (C99)(C99) strfromfstrfromdstrfroml (C23)(C23)(C23)
String manipulation
strcpystrcpy_s (C11) strncpystrncpy_s (C11) strcatstrcat_s (C11)	strncatstrncat_s (C11) strxfrm strdup (C23) strndup (C23)
String examination
strlenstrnlen_s (C11) strcmp strncmp strcoll strchr strrchr	strspn strcspn strpbrk strstr strtokstrtok_s (C11)
Memory manipulation
memchr memcmp memsetmemset_explicitmemset_s (C23)(C11)	memcpymemcpy_s (C11) memmovememmove_s (C11) memccpy (C23)
Miscellaneous
strerrorstrerror_sstrerrorlen_s (C11)(C11)

 

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