5.6.2 Particular Header Checks

These macros check for particular system header files—whether they exist, and in some cases whether they declare certain symbols.

Macro: AC_HEADER_ASSERT

Check whether to enable assertions in the style of ‘assert.h’. Assertions are enabled by default, but the user can override this by invoking configure with the ‘--disable-assert’ option.

Macro: AC_HEADER_DIRENT

Check for the following header files. For the first one that is found and defines ‘DIR’, define the listed C preprocessor macro:

‘dirent.h’	HAVE_DIRENT_H
‘sys/ndir.h’	HAVE_SYS_NDIR_H
‘sys/dir.h’	HAVE_SYS_DIR_H
‘ndir.h’	HAVE_NDIR_H

The directory-library declarations in your source code should look something like the following:

#include <sys/types.h> #ifdef HAVE_DIRENT_H # include <dirent.h> # define NAMLEN(dirent) strlen ((dirent)->d_name) #else # define dirent direct # define NAMLEN(dirent) ((dirent)->d_namlen) # ifdef HAVE_SYS_NDIR_H # include <sys/ndir.h> # endif # ifdef HAVE_SYS_DIR_H # include <sys/dir.h> # endif # ifdef HAVE_NDIR_H # include <ndir.h> # endif #endif

Using the above declarations, the program would declare variables to be of type struct dirent, not struct direct, and would access the length of a directory entry name by passing a pointer to a struct dirent to the NAMLEN macro.

This macro also checks for the SCO Xenix ‘dir’ and ‘x’ libraries.

This macro is obsolescent, as all current systems with directory libraries have <dirent.h>. New programs need not use this macro.

Also see AC_STRUCT_DIRENT_D_INO and AC_STRUCT_DIRENT_D_TYPE (see section Particular Structure Checks).

Macro: AC_HEADER_MAJOR

If ‘sys/types.h’ does not define major, minor, and makedev, but ‘sys/mkdev.h’ does, define MAJOR_IN_MKDEV; otherwise, if ‘sys/sysmacros.h’ does, define MAJOR_IN_SYSMACROS.

Macro: AC_HEADER_RESOLV

Checks for header ‘resolv.h’, checking for prerequisites first. To properly use ‘resolv.h’, your code should contain something like the following:

#ifdef HAVE_SYS_TYPES_H
#  include <sys/types.h>
#endif
#ifdef HAVE_NETINET_IN_H
#  include <netinet/in.h>   /* inet_ functions / structs */
#endif
#ifdef HAVE_ARPA_NAMESER_H
#  include <arpa/nameser.h> /* DNS HEADER struct */
#endif
#ifdef HAVE_NETDB_H
#  include <netdb.h>
#endif
#include <resolv.h>

Macro: AC_HEADER_STAT

If the macros S_ISDIR, S_ISREG, etc. defined in ‘sys/stat.h’ do not work properly (returning false positives), define STAT_MACROS_BROKEN. This is the case on Tektronix UTekV, Amdahl UTS and Motorola System V/88.

This macro is obsolescent, as no current systems have the bug. New programs need not use this macro.

Macro: AC_HEADER_STDBOOL

If ‘stdbool.h’ exists and conforms to C99, define HAVE_STDBOOL_H to 1; if the type _Bool is defined, define HAVE__BOOL to 1. To fulfill the C99 requirements, your ‘system.h’ could contain the following code:

#ifdef HAVE_STDBOOL_H
# include <stdbool.h>
#else
# ifndef HAVE__BOOL
#  ifdef __cplusplus
typedef bool _Bool;
#  else
#   define _Bool signed char
#  endif
# endif
# define bool _Bool
# define false 0
# define true 1
# define __bool_true_false_are_defined 1
#endif

Alternatively you can use the ‘stdbool’ package of Gnulib (see section Gnulib); it packages the above code into a replacement header and contains a few other bells and whistles.

Macro: AC_HEADER_STDC

Define STDC_HEADERS if the system has C header files conforming to ANSI C89 (ISO C90). Specifically, this macro checks for ‘stdlib.h’, ‘stdarg.h’, ‘string.h’, and ‘float.h’; if the system has those, it probably has the rest of the C89 header files. This macro also checks whether ‘string.h’ declares memchr (and thus presumably the other mem functions), whether ‘stdlib.h’ declare free (and thus presumably malloc and other related functions), and whether the ‘ctype.h’ macros work on characters with the high bit set, as the C standard requires.

If you use this macro, your code can refer to STDC_HEADERS to determine whether the system has conforming header files (and probably C library functions).

This macro is obsolescent, as current systems have conforming header files. New programs need not use this macro.

Nowadays ‘string.h’ is part of the C standard and declares functions like strcpy, and ‘strings.h’ is standardized by Posix and declares BSD functions like bcopy; but historically, string functions were a major sticking point in this area. If you still want to worry about portability to ancient systems without standard headers, there is so much variation that it is probably easier to declare the functions you use than to figure out exactly what the system header files declare. Some ancient systems contained a mix of functions from the C standard and from BSD; some were mostly standard but lacked ‘memmove’; some defined the BSD functions as macros in ‘string.h’ or ‘strings.h’; some had only the BSD functions but ‘string.h’; some declared the memory functions in ‘memory.h’, some in ‘string.h’; etc. It is probably sufficient to check for one string function and one memory function; if the library had the standard versions of those then it probably had most of the others. If you put the following in ‘configure.ac’:

# This example is obsolescent. # Nowadays you can omit these macro calls. AC_HEADER_STDC AC_CHECK_FUNCS([strchr memcpy])

then, in your code, you can use declarations like this:

/* This example is obsolescent. Nowadays you can just #include <string.h>. */ #ifdef STDC_HEADERS # include <string.h> #else # ifndef HAVE_STRCHR # define strchr index # define strrchr rindex # endif char *strchr (), *strrchr (); # ifndef HAVE_MEMCPY # define memcpy(d, s, n) bcopy ((s), (d), (n)) # define memmove(d, s, n) bcopy ((s), (d), (n)) # endif #endif

If you use a function like memchr, memset, strtok, or strspn, which have no BSD equivalent, then macros don't suffice to port to ancient hosts; you must provide an implementation of each function. An easy way to incorporate your implementations only when needed (since the ones in system C libraries may be hand optimized) is to, taking memchr for example, put it in ‘memchr.c’ and use ‘AC_REPLACE_FUNCS([memchr])’.

Macro: AC_HEADER_SYS_WAIT

If ‘sys/wait.h’ exists and is compatible with Posix, define HAVE_SYS_WAIT_H. Incompatibility can occur if ‘sys/wait.h’ does not exist, or if it uses the old BSD union wait instead of int to store a status value. If ‘sys/wait.h’ is not Posix compatible, then instead of including it, define the Posix macros with their usual interpretations. Here is an example:

#include <sys/types.h> #ifdef HAVE_SYS_WAIT_H # include <sys/wait.h> #endif #ifndef WEXITSTATUS # define WEXITSTATUS(stat_val) ((unsigned int) (stat_val) >> 8) #endif #ifndef WIFEXITED # define WIFEXITED(stat_val) (((stat_val) & 255) == 0) #endif

This macro is obsolescent, as current systems are compatible with Posix. New programs need not use this macro.

_POSIX_VERSION is defined when ‘unistd.h’ is included on Posix systems. If there is no ‘unistd.h’, it is definitely not a Posix system. However, some non-Posix systems do have ‘unistd.h’.

The way to check whether the system supports Posix is:

#ifdef HAVE_UNISTD_H
# include <sys/types.h>
# include <unistd.h>
#endif

#ifdef _POSIX_VERSION
/* Code for Posix systems.  */
#endif

Macro: AC_HEADER_TIME

If a program may include both ‘time.h’ and ‘sys/time.h’, define TIME_WITH_SYS_TIME. On some ancient systems, ‘sys/time.h’ included ‘time.h’, but ‘time.h’ was not protected against multiple inclusion, so programs could not explicitly include both files. This macro is useful in programs that use, for example, struct timeval as well as struct tm. It is best used in conjunction with HAVE_SYS_TIME_H, which can be checked for using AC_CHECK_HEADERS([sys/time.h]).

#ifdef TIME_WITH_SYS_TIME # include <sys/time.h> # include <time.h> #else # ifdef HAVE_SYS_TIME_H # include <sys/time.h> # else # include <time.h> # endif #endif

This macro is obsolescent, as current systems can include both files when they exist. New programs need not use this macro.

Macro: AC_HEADER_TIOCGWINSZ

If the use of TIOCGWINSZ requires ‘<sys/ioctl.h>’, then define GWINSZ_IN_SYS_IOCTL. Otherwise TIOCGWINSZ can be found in ‘<termios.h>’.

Use:

#ifdef HAVE_TERMIOS_H # include <termios.h> #endif #ifdef GWINSZ_IN_SYS_IOCTL # include <sys/ioctl.h> #endif