Have a time date string I'd like to convert to a tm object. Google tells me the POSIX (but not the C) standard includes a function called strptime() that will do the job.
The man page says it's in <time.h> and that I need to include #define _XOPEN_SOURCE before I include the <time.h> file. Easy enough.
But I still get an implicit declaration warning from the compiler. I opened /usr/include/time.h and found the function declaration:
# ifdef __USE_XOPEN
/* Parse S according to FORMAT and store binary time information in TP.
The return value is a pointer to the first unparsed character in S. */
extern char *strptime (const char *__restrict __s,
const char *__restrict __fmt, struct tm *__tp)
__THROW;
#endif
So it looks like I need #define _USE_XOPEN instead.
Except that doesn't work either. The compiler still isn't seeing the declaration.
Any ideas. I'm using a relatively recent version of Linux (Mint) with gcc 5.4.0.
Converting comments into an answer.
To fix this, your options include using -std=gnu11 instead of -std=c11 on the GCC command line, or using #define _XOPEN_SOURCE 700 or equivalent (e.g. -D_XOPEN_SOURCE=700 on the command line). The 700 identifies POSIX 2008; 600 or 500 identify earlier versions of POSIX or X/Open.
In theory, you could also use _POSIX_C_SOURCE 200809L (see POSIX Compilation environment), but that doesn't expose everything that _XOPEN_SOURCE 700 exposes so it is usually better to use the latter.
Note that the POSIX specification of strptime() is annotated as an XSI extension, which means you must set _XOPEN_SOURCE; setting _POSIX_C_SOURCE alone is not sufficient.
Test code
This test code prints the address of the strptime function; it won't compile if strptime() is not declared.
#define _XOPEN_SOURCE 700
#include <time.h>
#include <stdio.h>
int main(void)
{
printf("%p\n", (void *)strptime);
return 0;
}
That should compile for you with gcc -std=c11 -Wall -c test-strptime.c. If you add -ansi to the options, you reset the standard back to C90. GCC 5.4.0 should default to C11 (effectively -std=gnu11) unless someone did something horrible in the build of GCC that you're using (which is unlikely).
Note that the compiler unsets and then sets __USE_XOPEN based on settings like _XOPEN_SOURCE and trying to set it manually doesn't work reliably.
Position matters
You must specify the #define _XOPEN_SOURCE 700 before the first system header is included (whether included directly or indirectly). If you include a system header before trying to set _XOPEN_SOURCE, the settings have been determined and your subsequent operations are effectively ignored. POSIX says (at the 'compilation environment' link already given):
In the compilation of an application that #defines a feature test macro specified by POSIX.1-2008, no header defined by POSIX.1-2008 shall be included prior to the definition of the feature test macro. This restriction also applies to any implementation-provided header in which these feature test macros are used. If the definition of the macro does not precede the #include, the result is undefined.
One common undefined result is that your attempt to set/change the POSIX version is completely ignored.
Related
This question already has answers here:
GCC with -std=c99 complains about not knowing struct timespec
(3 answers)
Closed 2 years ago.
I am currently experiencing the error
Unknown type name 'ssize_t'
I had a look at Where is ssize_t defined in Linux?
but the error remained.
I added the
#include <sys/types.h>
My function is
ssize_t ss_size(sparse_set_ptr sparse_set)
{
return false;
}
What could be the cause of the problem and can it be fixed?
Also I am running C Executable Language Standard: C99
From `man ssize_t:
ssize_t
Include: <sys/types.h>. Alternatively, <aio.h>, <monetary.h>,
<mqueue.h>, <stdio.h>, <sys/msg.h>, <sys/socket.h>, <sys/uio.h>,
or <unistd.h>.
Try all these one by one, with one of them it should work.
Note that even if you include <sys/types.h>, the type might not be defined if you specify a strict C standard with GCC (-std=c99) rather than the GNU variant (-std=gnu99). In fact, you don't need to explicitly include <sys/types.h> with POSIX 2008 (2018) or the prior version, POSIX 2004; it is effectively included automatically.
If you're including <sys/types.h> and still see the problem, then you need to enable the POSIX extensions, probably with #define _XOPEN_SOURCE 700 — the number's easier to remember than #define _POSIX_C_SOURCE 200809L which is a valid alternative. Both these select POSIX 2008; there are other values for other versions of POSIX. There are a few subtle differences between X/Open and POSIX (X/Open might till specify a few things that POSIX does not), but they're minimal and seldom relevant. The #define must appear before any system header is included. It could be specified on the command line as -D_XOPEN_SOURCE=700.
You have to decide which standard you want to use: C99 or Posix.
If you want to use the type ssize_t, then you have to define (activate) that standard in your sources, see man feature_test_macros.
But beware, if you activate any standard, which enables the desired Posix standard, you're not pure C99 anymore.
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?
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.
I'm looking for a way to insert an #undef to the lex generated source code that will appear before the built in lines lex generates.
When compiling a file.l with lex, I generate a lex.yy.c file. In my file.l I have written :
#include "y.tab.h"
#undef __STRICT_ANSI__
#include <string.h>
The #undef helps me compile the code under the flag -std=c99 So it needs to be done before including string.h. But the generated file includes string.h before copying my undef.
Without the #undef I am getting a lot of warnings due to the use of strdup. I have seen the normal fixes using flags, but like I said I can't access the makefile.
Adding 'manually' the line
#undef __STRICT_ANSI__
into lex.yy.c before fixes everything. But i prefer not to touch any of the generated code and have it done by lex.
I have read this,
strdup(): Confused about warnings ('implicit declaration', 'makes pointer...without a cast', memory leak)
And like i said it does solve it.
But only if I can somehow force the generated file to run the undef first.
To start with, #undef __STRICT_ASCII__ is not the correct way to enable the declaration of Posix functions like strdup.
Posix extensions which are declared in standard C library header files are made conditional on "feature test macros". You can read a summary in man feature_test_macros but in any case, the documentation for any function which requires a feature test macro includes a description of which macros are required. In the case of strdup, we can read in man strdup:
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
strdup():
_XOPEN_SOURCE >= 500
(Followed by more possibilities.)
Personally, I always use
#define _XOPEN_SOURCE 700
which requests declarations for all functions in the latest version of Posix.
One way to insert the feature test macro before any include of a standard library function is to do so on the compile command line:
-D_XOPEN_SOURCE=700
I like doing it this way, because I can add it to my Makefile and then it applies to every compilation (which is basically what I want). Usually, makefiles include a feature which allows you to add this option to your compiler flags without modifying the file. For example, the following will often work:
make file CPPFLAGS="-D_XOPEN_SOURCE=700"
(CPPFLAGS is a common makefile variable used to set preprocessor flags.)
But if you want to put it into your flex file, you can use a %top block:
%top {
#define _XOPEN_SOURCE 700
}
%top is like %{ but it puts the inserted code right at the beginning of the generated code.
If nothing else works, you can always just insert the declaration for strdup, (also taken from man strdup) into your flex prologue.
%{
char *strdup(const char *s);
#include "y.tab.h"
%}
Both the C standard and the Posix standard allow explicit declaration of library functions (but not macros) as an alternative to including relevant headers.
I am trying to use the open_memstream function in my C code. However I cannot seem to compile it. Minimal working example as follows:
#include <stdio.h>
int main(void) {
char *buf;
size_t sz;
FILE *stream = open_memstream(&buf, &sz);
putc('A', stream);
fclose(stream);
}
And I also use the following command to compile it:
gcc -std=c99 -o test test.c
After some research, I found that I need to define a macro before I include stdio.h. However the following example code was to no avail.
#define __USE_POSIX
#define __USE_XOPEN
#include <stdio.h>
The following compiler warnings are thrown; I assume the second warning is because of the first one.
test.c:7:17: warning: implicit declaration of function ‘open_memstream’ [-Wimplicit-function-declaration]
FILE *stream = open_memstream(&buf, &sz);
^
test.c:7:17: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
The __USE_* macros are internal to glibc's headers, and defining them yourself does not work. You should instead do one of the following:
Compile your program with -std=gnu11 instead of -std=c99 and don't define any special macros. This is the easiest change. Conveniently, -std=gnu11 is the default with newer versions of GCC.
If you have some concrete reason to want to select an old, strict conformance mode, but also you want POSIX extensions to C, then you can use the documented POSIX feature selection macros:
#define _XOPEN_SOURCE 700
or
#define _POSIX_C_SOURCE 200809L
These must be defined before including any standard headers. The difference is that _XOPEN_SOURCE requests an additional set of features (the "XSI" functions). See the Feature Test macros section of the glibc manual for more detail.
Note that if you need to request strict conformance mode from the library, using a -std=cXX option, then you almost certainly also want to use the -Wall and -Wpedantic options to enable strict conformance checking for the language. (You should use at least -Wall even if you don't need strict conformance checking.)
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
...
lstat(name, &st);
...
I am using CodeBlocks to write a C program. All the other includes work fine. I checked online and lstat requires the three includes listed at the top of the code, but I still get the error message warning: implicit declaration of function 'lstat' when I try to compile. I do not know what is wrong. If I need to include any extra information to get help, please say.
According to lstat(2):
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
lstat():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.10: */ _POSIX_C_SOURCE >= 200112L
This means you need to define one of these feature test macros to use lstat(2).
So choose one of those feature test macros that makes sense to your code, such as _BSD_SOURCE, and define it in the very beginning (before you include any header file) of your source file, or you could define it on the compiler command line, such as -D_BSD_SOURCE for gcc.
lstat() is not compliant with strict ANSI standard.You should use your compiler flags from
-std=c99 to -std=gnu99 . This would include all the Unix based system.