The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


strdup, strndup - duplicate a specific number of bytes from a string


[CX] [Option Start] #include <string.h>

char *strdup(const char *
char *strndup(const char *
s, size_t size); [Option End]


The strdup() function shall return a pointer to a new string, which is a duplicate of the string pointed to by s. The returned pointer can be passed to free(). A null pointer is returned if the new string cannot be created.

The strndup() function shall be equivalent to the strdup() function, duplicating the provided s in a new block of memory allocated as if by using malloc(), with the exception being that strndup() copies at most size plus one bytes into the newly allocated memory, terminating the new string with a NUL character. If the length of s is larger than size, only size bytes shall be duplicated. If size is larger than the length of s, all bytes in s shall be copied into the new memory buffer, including the terminating NUL character. The newly created string shall always be properly terminated.


The strdup() function shall return a pointer to a new string on success. Otherwise, it shall return a null pointer and set errno to indicate the error.

Upon successful completion, the strndup() function shall return a pointer to the newly allocated memory containing the duplicated string. Otherwise, it shall return a null pointer and set errno to indicate the error.


These functions shall fail if:

Storage space available is insufficient.

The following sections are informative.




For functions that allocate memory as if by malloc(), the application should release such memory when it is no longer required by a call to free(). For strdup() and strndup(), this is the return value.

Implementations are free to malloc() a buffer containing either (size + 1) bytes or (strnlen(s, size) + 1) bytes. Applications should not assume that strndup() will allocate ( size + 1) bytes when strlen(s) is smaller than size.






free, wcsdup

XBD <string.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

Issue 7

Austin Group Interpretation 1003.1-2001 #044 is applied, changing the ``may fail'' [ENOMEM] error to become a ``shall fail'' error.

The strdup() function is moved from the XSI option to the Base.

The strndup() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 1.

The APPLICATION USAGE section is updated to clarify that memory is allocated as if by malloc().

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0338 [738] is applied.

End of informative text.


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