STRDUP(3P) POSIX Programmer's Manual STRDUP(3P)

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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

#include <string.h>
char *strdup(const char *s);
char *strndup(const char *s, size_t size);

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()

The Base Definitions volume of POSIX.1‐2017, <string.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see .

2017 IEEE/The Open Group