C函数定义和外部关键字

Question

我一直在尝试了解关于_sbrk函数遇到的链接错误，并偶然发现了库中的该函数定义。

extern caddr_t _sbrk(int incr);

// ... some other definitions ...

extern caddr_t _sbrk(int incr)
{
static unsigned char *heap = NULL;
unsigned char *prev_heap;

if (heap == NULL) {
    heap = (unsigned char *)&_end;
}
prev_heap = heap;

heap += incr;

return (caddr_t) prev_heap;
}

现在，我知道extern对函数声明有什么作用，但是我不知道在函数定义中使用extern的含义...

有人知道这样使用extern有什么意思吗？

该文件位于Atmel软件框架（ASF）的asf/sam/utils/syscalls/gcc/syscalls.c目录中。

它在嵌入式环境中，我遇到了大量与缺少_exit ， _kill ， _sbrk定义有关的linkrt错误。

生成存根很有意义，但是我希望至少给定的_sbrk定义行得通吗？

更新：

好的，因此似乎可以添加一些有关如何将所有这些链接在一起的信息。

我有一个可执行项目（将用GCC生成），并且有一个静态库项目被链接到生成的可执行文件中（也用GCC生成）。

他们都没有启用优化功能（这使得在嵌入式目标上进行调试更容易，而不会像在药物上那样跳动）。

静态库包含上述ASF代码。 ASF由Atmel Studio 6.0中包含的某些向导自动生成。

静态库中的某些代码包含<stdio.h> ，这对我来说不是必需的，但我真的不想更改自动生成的代码（它肯定会撤消我的更改）。

来自链接器的错误如下：

Invoking: ARM/GNU Linker : (crosstool-NG 1.15.3 - Atmel build: 59) 4.7.0
    "C:\Program Files (x86)\Atmel\Atmel Studio 
6.0\extensions\Atmel\ARMGCC\3.3.1.128\ARMGCCToolchain\bin\arm-none-eabi-gcc.exe" -o 
Bootloader_Stage1.elf  cmsis/src/startup_sam3n.o cmsis/src/system_sam3n.o 
Bootloader_Stage1.o   -Wl,-Map="Bootloader_Stage1.map" -Wl,--start-group -lm -
lBootloaderShared  -Wl,--end-group -L"../cmsis/linkerScripts" -
L"../../BootloaderShared/Debug"  -Wl,--gc-sections -Tsam3n4b_flash.ld  -mcpu=cortex-m3 
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-abort.o): In function 
`abort':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/stdlib/abort.c(63,1): undefined reference to `_exit'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-sbrkr.o): In function 
`_sbrk_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/sbrkr.c(60,1): undefined reference to `_sbrk'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-signalr.o): In function 
`_kill_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/signalr.c(61,1): undefined reference to `_kill'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-signalr.o): In function 
`_getpid_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/signalr.c(96,1): undefined reference to `_getpid'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-writer.o): In function 
`_write_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/writer.c(58,1): undefined reference to `_write'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-closer.o): In function 
`_close_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/closer.c(53,1): undefined reference to `_close'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-fstatr.o): In function 
`_fstat_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/fstatr.c(62,1): undefined reference to `_fstat'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-isattyr.o): In function 
`_isatty_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/isattyr.c(58,1): undefined reference to `_isatty'
    Invoking: ARM/GNU Linker : (crosstool-NG 1.15.3 - Atmel build: 59) 4.7.0
    "C:\Program Files (x86)\Atmel\Atmel Studio 
6.0\extensions\Atmel\ARMGCC\3.3.1.128\ARMGCCToolchain\bin\arm-none-eabi-gcc.exe" -o 
Bootloader_Stage1.elf  cmsis/src/startup_sam3n.o cmsis/src/system_sam3n.o 
Bootloader_Stage1.o   -Wl,-Map="Bootloader_Stage1.map" -Wl,--start-group -lm -
lBootloaderShared  -Wl,--end-group -L"../cmsis/linkerScripts" -
L"../../BootloaderShared/Debug"  -Wl,--gc-sections -Tsam3n4b_flash.ld  -mcpu=cortex-m3 
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-abort.o): In function 
`abort':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/stdlib/abort.c(63,1): undefined reference to `_exit'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-sbrkr.o): In function 
`_sbrk_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/sbrkr.c(60,1): undefined reference to `_sbrk'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-signalr.o): In function  
`_kill_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/signalr.c(61,1): undefined reference to `_kill'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-signalr.o): In function 
`_getpid_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/signalr.c(96,1): undefined reference to `_getpid'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-writer.o): In function 
`_write_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/writer.c(58,1): undefined reference to `_write'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-closer.o): In function 
`_close_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/closer.c(53,1): undefined reference to `_close'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-fstatr.o): In function 
`_fstat_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/fstatr.c(62,1): undefined reference to `_fstat'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-isattyr.o): In function 
`_isatty_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/isattyr.c(58,1): undefined reference to `_isatty'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-lseekr.o): In function 
`_lseek_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/lseekr.c(58,1): undefined reference to `_lseek'
    c:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-readr.o): In function 
`_read_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/readr.c(58,1): undefined reference to `_read'
collect2.exe(0,0): ld returned 1 exit statusc:/program files (x86)/atmel/atmel studio 
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-lseekr.o): In function 
`_lseek_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/lseekr.c(58,1): undefined reference to `_lseek'
    c:/program files (x86)/atmel/atmel studio  
6.0/extensions/atmel/armgcc/3.3.1.128/armgcctoolchain/bin/../lib/gcc/arm-none-
eabi/4.7.0/../../../../arm-none-eabi/lib/thumb\libc.a(lib_a-readr.o): In function 
`_read_r':
/usr/local/avr32studio/hudson/workspace/arm-gnu-toolchain/.build/src/newlib-
1.19.0/newlib/libc/reent/readr.c(58,1): undefined reference to `_read'
collect2.exe(0,0): ld returned 1 exit status

更新2：

我已经解决了我的链接问题。 看来Atmel软件框架包括#include <assert.h> ，它是调用所有其他功能的那个。 在该标头中， assert()宏的定义如下

#ifdef NDEBUG           /* required by ANSI standard */
# define assert(__e) ((void)0)
#else
# define assert(__e) ((__e) ? (void)0 : __assert_func (__FILE__, __LINE__, \
                           __ASSERT_FUNC, #__e))

定义NDEBUG后，链接问题就消失了。 据我所知， NDEBUG在其他任何地方都没有使用（不删除我依赖的任何其他代码），因此我可以保留该符号的定义。

正如我之前提到的，我已经接受了回答extern问题的答案，并投票赞成有助于解决链接问题的答案。

Answer 1

在函数声明中指定extern没问题。 从C99 6.9.1 / 4“功能定义”开始：

句法

 function-definition: declaration-specifiers declarator declaration-list[opt] compound-statement 

...

声明说明符中的存储类说明符（如有）应为extern或static 。

但是，关于函数定义的extern并不是特别有用。 从C99 6.2.2 / 4“标识符的链接”：

对于在可见该标识符的先前声明的范围内用存储类说明符extern声明的标识符，如果该先前声明指定了内部或外部链接，则该标识符在后面的声明中的链接与该链接相同在事先声明中指定。 如果没有在先声明可见，或者在先声明没有指定链接，则标识符具有外部链接。

如果函数标识符的声明没有存储类说明符，则将其链接确定为与使用存储类说明符extern声明的完全相同。

因此，在函数的第一个声明之后，即使该第一个声明为static ，后续函数声明上的所有extern将被忽略。

Answer 2

函数定义前面的extern仅表示该函数具有外部链接(which it has anyway by default) 。

只要在声明中包含了“ extern”，并且在定义的编译中已经出现过，就不必在函数定义中使用它。 记住definitions are declarations also 。

Answer 3

在部分答案到连接问题，接头抱怨它都没有一起联的事情，例如，对象文件（以及可能的预编译库），包括_sbrk（）函数，它是由代码 ，在被称为那里。 其他几个低层库函数也是如此。

extern不会影响您的问题。