当编译为C99时CRTSCTS未定义

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问题描述

我正在树莓派上编写一些串行代码，并切换到C99.当我完成操作时，我开始收到错误错误:未声明'CRTSCTS'(此功能的首次使用)"

$ c99 -M serial01.c | grep termios.h
 /usr/include/termios.h /usr/include/arm-linux-gnueabihf/bits/termios.h \
$ gcc -M serial01.c | grep termios.h
 /usr/include/termios.h /usr/include/arm-linux-gnueabihf/bits/termios.h \

使用-M显示2个termios.h标头.前者不包含CRTSCTS的定义，而后者则包含.

我认为标准的c89使用的不是好的，而c99使用的不是，但是我不确定，因为-M调用的结果是相同的.谁能向我解释为什么我切换到C99时会发生这种情况以及如何解决?

解决方案

-std=gnu99是解决方案.在下面更详细.

gcc/c99 -M调用的结果相同，但是没有任何意义！对此行为的解释是预处理器宏(特别是 GNU扩展，如Mat所述).

详细信息:

cat -n main.c
     1  #include <termios.h>
     2
     3  int printf(const char *, ...);
     4
     5  int main(void)
     6  {
     7      printf("%d\n", CRTSCTS);
     8      return 0;
     9  }
gcc main.c -o main --std=c89 -> 'CRTSCTS' undeclared compilation error
gcc main.c -o main --std=cgnu89 -> successfully compiled

对于c99和gnu99，此行为相同！

就像卡梅伦所说，-M输出是相同的:

gcc -M --std=c89 main.c | grep termios.h | nl
     1  main.o: main.c /usr/include/termios.h /usr/include/features.h \
     2   /usr/include/arm-linux-gnueabihf/bits/termios.h
gcc -M --std=gnu89 main.c | grep termios.h | nl
1  main.o: main.c /usr/include/termios.h /usr/include/features.h \
     2   /usr/include/arm-linux-gnueabihf/bits/termios.h \

在定义__USE_MISC时，在bits/termios.h中定义了CRTSCTS

     1  #ifdef __USE_MISC
     2  # define CIBAUD   002003600000          /* input baud rate (not used) */
     3  # define CMSPAR   010000000000          /* mark or space (stick) parity */
     4  # define CRTSCTS  020000000000          /* flow control */

让我们看一下__USE_MISC.

gcc -M /usr/include/termios.h | nl
     1  termios.o: /usr/include/termios.h /usr/include/features.h \
     2   /usr/include/arm-linux-gnueabihf/bits/predefs.h \
     3   /usr/include/arm-linux-gnueabihf/sys/cdefs.h \
     4   /usr/include/arm-linux-gnueabihf/bits/wordsize.h \
     5   /usr/include/arm-linux-gnueabihf/gnu/stubs.h \
     6   /usr/include/arm-linux-gnueabihf/bits/types.h \
     7   /usr/include/arm-linux-gnueabihf/bits/typesizes.h \
     8   /usr/include/arm-linux-gnueabihf/bits/termios.h \
     9   /usr/include/arm-linux-gnueabihf/sys/ttydefaults.h

第一个，features.h，包含__USE_MISC的定义，但仅当定义了_BSD_SOURCE或_SVID_SOURCE时

grep 'define __USE_MISC' /usr/include/features.h -B 1 | nl
     1  #if defined _BSD_SOURCE || defined _SVID_SOURCE
     2  # define __USE_MISC     1

并且在定义_GNU_SOURCE或'nothing'时定义了_BSD_SOURCE和_SVID_SOURCE

     1  #ifdef _GNU_SOURCE
     2  # undef  _ISOC95_SOURCE
     3  # define _ISOC95_SOURCE 1
     4  # undef  _ISOC99_SOURCE
     5  # define _ISOC99_SOURCE 1
     6  # undef  _POSIX_SOURCE
     7  # define _POSIX_SOURCE  1
     8  # undef  _POSIX_C_SOURCE
     9  # define _POSIX_C_SOURCE        200809L
    10  # undef  _XOPEN_SOURCE
    11  # define _XOPEN_SOURCE  700
    12  # undef  _XOPEN_SOURCE_EXTENDED
    13  # define _XOPEN_SOURCE_EXTENDED 1
    14  # undef  _LARGEFILE64_SOURCE
    15  # define _LARGEFILE64_SOURCE    1
    16  # undef  _BSD_SOURCE
    17  # define _BSD_SOURCE    1
    18  # undef  _SVID_SOURCE
    19  # define _SVID_SOURCE   1
    20  # undef  _ATFILE_SOURCE
    21  # define _ATFILE_SOURCE 1
    22  #endif

    23  /* If nothing (other than _GNU_SOURCE) is defined,
    24     define _BSD_SOURCE and _SVID_SOURCE.  */
    25  #if (!defined __STRICT_ANSI__ && !defined _ISOC99_SOURCE && \
    26       !defined _POSIX_SOURCE && !defined _POSIX_C_SOURCE && \
    27       !defined _XOPEN_SOURCE && !defined _BSD_SOURCE && !defined _SVID_SOURCE)
    28  # define _BSD_SOURCE    1
    29  # define _SVID_SOURCE   1
    30  #endif

I'm writing some serial code on a raspberry pi and switched to C99. When I did I started getting the error "error: ‘CRTSCTS’ undeclared (first use in this function)"

$ c99 -M serial01.c | grep termios.h
 /usr/include/termios.h /usr/include/arm-linux-gnueabihf/bits/termios.h \
$ gcc -M serial01.c | grep termios.h
 /usr/include/termios.h /usr/include/arm-linux-gnueabihf/bits/termios.h \

using -M reveals 2 termios.h headers. the former does not contain a definition for CRTSCTS and the latter does.

I assume the standard c89 is using the good one and c99 not but I'm not sure since the result of the -M call is identical. Can anyone explain to me why this is happening when I switch to C99 and how to fix it?

解决方案

-std=gnu99 is the solution. More detailed below.

Result of the gcc/c99 -M call is identical but it does not mean anything! The explanation of this behavior are preprocessor macros (specifically GNU extensions, like said Mat).

In detail:

cat -n main.c
     1  #include <termios.h>
     2
     3  int printf(const char *, ...);
     4
     5  int main(void)
     6  {
     7      printf("%d\n", CRTSCTS);
     8      return 0;
     9  }
gcc main.c -o main --std=c89 -> 'CRTSCTS' undeclared compilation error
gcc main.c -o main --std=cgnu89 -> successfully compiled

This behavior is the same for c99 and gnu99!

Like said Cameron, -M output is identical:

gcc -M --std=c89 main.c | grep termios.h | nl
     1  main.o: main.c /usr/include/termios.h /usr/include/features.h \
     2   /usr/include/arm-linux-gnueabihf/bits/termios.h
gcc -M --std=gnu89 main.c | grep termios.h | nl
1  main.o: main.c /usr/include/termios.h /usr/include/features.h \
     2   /usr/include/arm-linux-gnueabihf/bits/termios.h \

CRTSCTS is defined in the bits/termios.h when __USE_MISC is defined

     1  #ifdef __USE_MISC
     2  # define CIBAUD   002003600000          /* input baud rate (not used) */
     3  # define CMSPAR   010000000000          /* mark or space (stick) parity */
     4  # define CRTSCTS  020000000000          /* flow control */

Let's look at __USE_MISC.

gcc -M /usr/include/termios.h | nl
     1  termios.o: /usr/include/termios.h /usr/include/features.h \
     2   /usr/include/arm-linux-gnueabihf/bits/predefs.h \
     3   /usr/include/arm-linux-gnueabihf/sys/cdefs.h \
     4   /usr/include/arm-linux-gnueabihf/bits/wordsize.h \
     5   /usr/include/arm-linux-gnueabihf/gnu/stubs.h \
     6   /usr/include/arm-linux-gnueabihf/bits/types.h \
     7   /usr/include/arm-linux-gnueabihf/bits/typesizes.h \
     8   /usr/include/arm-linux-gnueabihf/bits/termios.h \
     9   /usr/include/arm-linux-gnueabihf/sys/ttydefaults.h

First one, features.h, contains definition of __USE_MISC, but only when _BSD_SOURCE or _SVID_SOURCE is defined

grep 'define __USE_MISC' /usr/include/features.h -B 1 | nl
     1  #if defined _BSD_SOURCE || defined _SVID_SOURCE
     2  # define __USE_MISC     1

and _BSD_SOURCE and _SVID_SOURCE is defined when _GNU_SOURCE or 'nothing' is defined

     1  #ifdef _GNU_SOURCE
     2  # undef  _ISOC95_SOURCE
     3  # define _ISOC95_SOURCE 1
     4  # undef  _ISOC99_SOURCE
     5  # define _ISOC99_SOURCE 1
     6  # undef  _POSIX_SOURCE
     7  # define _POSIX_SOURCE  1
     8  # undef  _POSIX_C_SOURCE
     9  # define _POSIX_C_SOURCE        200809L
    10  # undef  _XOPEN_SOURCE
    11  # define _XOPEN_SOURCE  700
    12  # undef  _XOPEN_SOURCE_EXTENDED
    13  # define _XOPEN_SOURCE_EXTENDED 1
    14  # undef  _LARGEFILE64_SOURCE
    15  # define _LARGEFILE64_SOURCE    1
    16  # undef  _BSD_SOURCE
    17  # define _BSD_SOURCE    1
    18  # undef  _SVID_SOURCE
    19  # define _SVID_SOURCE   1
    20  # undef  _ATFILE_SOURCE
    21  # define _ATFILE_SOURCE 1
    22  #endif

    23  /* If nothing (other than _GNU_SOURCE) is defined,
    24     define _BSD_SOURCE and _SVID_SOURCE.  */
    25  #if (!defined __STRICT_ANSI__ && !defined _ISOC99_SOURCE && \
    26       !defined _POSIX_SOURCE && !defined _POSIX_C_SOURCE && \
    27       !defined _XOPEN_SOURCE && !defined _BSD_SOURCE && !defined _SVID_SOURCE)
    28  # define _BSD_SOURCE    1
    29  # define _SVID_SOURCE   1
    30  #endif

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