implicit declaration error even after including required header - c

Question

I get the following error when I run my code:
error: implicit declaration of function ‘mkdtemp’ [-Werror=implicit-function-declaration]
This occurs even after including the correct header files for mkdtemp():
#include <stdlib.h>
Any ideas why this might be occurring?

Answer 1

The <stdlib.h> header is mandated by the C standard. The C standard makes no reference to a mkdtemp() function. If you're using gcc -std=c11 or some similar option, only the definitions provided by the C standard are exposed. If you compile using gcc -std=gnu11, then you'll get an indeterminate set of extension features enabled (and mkdtemp() would be one of them).
Since mkdtemp() is a POSIX function, you can explicitly request it by defining the appropriate enabling macro before including any standard header. A command-line option -D_XOPEN_SOURCE=700 would (probably) do the job, for example; there's also the option of using -D_POSIX_C_SOURCE=200809 but remembering the correct number is harder (it is the date of the POSIX 2008 standard as year and month).
Or you can place the appropriate #define at the top of the file:
#ifndef _XOPEN_SOURCE
#define _XOPEN_SOURCE 700
#endif
or:
#ifndef _POSIX_C_SOURCE
#define _POSIX_C_SOURCE 200809L
#endif
These stanzas allow you to override the POSIX version on the command line. Simply writing the #define without the conditional around it would generate a warning (or error) for a non-benign redefinition of the macro.
There used to be major differences between the POSIX and X/Open functionality — X/Open included some things that POSIX doesn't. This distinction is smaller these days, and generally, you'll not get into trouble using the X/Open macro.
There are other enabling macros for other platforms, but one of these two will enable the declaration of mkdtemp(). On Linux (RHEL 7.x), /usr/include/features.h) documents these enabling macros:
/* These are defined by the user (or the compiler)
to specify the desired environment:
__STRICT_ANSI__ ISO Standard C.
_ISOC99_SOURCE Extensions to ISO C89 from ISO C99.
_ISOC11_SOURCE Extensions to ISO C99 from ISO C11.
_POSIX_SOURCE IEEE Std 1003.1.
_POSIX_C_SOURCE If ==1, like _POSIX_SOURCE; if >=2 add IEEE Std 1003.2;
if >=199309L, add IEEE Std 1003.1b-1993;
if >=199506L, add IEEE Std 1003.1c-1995;
if >=200112L, all of IEEE 1003.1-2004
if >=200809L, all of IEEE 1003.1-2008
_XOPEN_SOURCE Includes POSIX and XPG things. Set to 500 if
Single Unix conformance is wanted, to 600 for the
sixth revision, to 700 for the seventh revision.
_XOPEN_SOURCE_EXTENDED XPG things and X/Open Unix extensions.
_LARGEFILE_SOURCE Some more functions for correct standard I/O.
_LARGEFILE64_SOURCE Additional functionality from LFS for large files.
_FILE_OFFSET_BITS=N Select default filesystem interface.
_BSD_SOURCE ISO C, POSIX, and 4.3BSD things.
_SVID_SOURCE ISO C, POSIX, and SVID things.
_ATFILE_SOURCE Additional *at interfaces.
_GNU_SOURCE All of the above, plus GNU extensions.
_REENTRANT Select additionally reentrant object.
_THREAD_SAFE Same as _REENTRANT, often used by other systems.
_FORTIFY_SOURCE If set to numeric value > 0 additional security
measures are defined, according to level.
Note, too, that the manual page for mkdtemp() shows what is needed:
NAME
mkdtemp - create a unique temporary directory
SYNOPSIS
#include <stdlib.h>
char *mkdtemp(char *template);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
mkdtemp():
_BSD_SOURCE
|| /* Since glibc 2.10: */
(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
What I called 'enabling macros' are also known as 'Feature Test' macros.
See also POSIX System Interfaces: General Information: The Compilation Environment.