strdup / strndup / strdupa / strndupa (3) - glibc

Split from the chapter glibc: strings, ctype and stdlib arithmetic into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <string.h>

char *strdup(const char *s);
char *strndup(const char *s, size_t n);

#define _GNU_SOURCE          /* strdupa, strndupa: macros, need GCC */
#include <string.h>
char *strdupa(const char *s);
char *strndupa(const char *s, size_t n);

Header gates (/usr/include/string.h): strdup under __USE_XOPEN_EXTENDED || __USE_XOPEN2K8 || __GLIBC_USE(LIB_EXT2/ISOC23) (lines 184-189); strndup under __USE_XOPEN2K8 || LIB_EXT2 || ISOC23 (lines 194-197); strdupa/strndupa are #define macros under __USE_GNU && __GNUC__ (lines 199-220). Feature-macro history per strdup(3): _XOPEN_SOURCE >= 500 or _POSIX_C_SOURCE >= 200809L for strdup; _POSIX_C_SOURCE >= 200809L since glibc 2.10 for strndup.

Semantics

strdup returns a malloc(3)-allocated duplicate of s, freeable with free(3); on failure it returns NULL with errno set to ENOMEM (strdup(3)). strndup copies at most n bytes and always adds a terminating null, so the result is a valid string even when s is longer (strdup(3); manual Truncating Strings node stresses this differs from strncpy). strdupa/strndupa are the same operations on alloca-ed stack storage (strdup(3)).

Gotchas and portability

• The a-suffixed pair are macros, not functions: they evaluate s more than once, must not be used where a function call is required (e.g. inside another call’s argument list), and allocate on the stack, so they inherit every alloca(3) limitation: no error return, and dead on large strings or deep recursion (manual Truncating Strings, strndupa entry; strdup(3)).
• The macro implementation in string.h is strlen(old) + 1 bytes via __builtin_alloca plus memcpy; because the size expression is not constant, the allocation cannot move out of loops.
• strndup is the bounded form that terminates; pairing it with strncpy-style truncation is the usual fix for unterminated fixed buffers.
• Failure handling: only the malloc’ing pair can fail (ENOMEM); strdup(3) documents exactly that error.
• musl 1.2.5 provides strdup/strndup as real functions and strdupa as a macro strcpy(alloca(strlen(x)+1), x) (musl include/string.h line 92, src/string/strdup.c, src/string/strndup.c).

References

• glibc manual: string.texi, nodes Copying Strings and Arrays (strdup), Truncating Strings (strndup, strndupa)
• man3: strdup(3) (covers all four)
• header: /usr/include/string.h
• symbols: nm -D --with-symbol-versions libc.so.6