C语言不要重复包含.h头文件和.c文件

1.不要重复包含头文件

--以上出自《C语言程序设计：现代方法(第2版)》

f3.h

//#ifndef AE_OK#define AE_OK 0typedef int        ngx_int_t;//#endif

f2.h

#include "f3.h"

f1.h

#include "f3.h"

test.c

#include <stdio.h>#include "f1.h"#include "f2.h"int main(){ngx_int_t a1=1;printf("%d",AE_OK);printf("%d",a1);return 0;}

编译不过去：

导出预编译文件：

...以上省略# 2 "test.c" 2# 1 "f1.h" 1# 1 "f3.h" 1typedef int ngx_int_t;# 1 "f1.h" 2# 4 "test.c" 2# 1 "f2.h" 1# 1 "f3.h" 1typedef int ngx_int_t;# 1 "f2.h" 2# 5 "test.c" 2int main(){ ngx_int_t a1=1; printf("%d",0); printf("%d",a1); return 0;}

如果我们在f3.h中增加#ifndef就不会出问题了，直接导出预编译文件：

# 2 "test.c" 2# 1 "f1.h" 1# 1 "f3.h" 1typedef int ngx_int_t;# 1 "f1.h" 2# 4 "test.c" 2# 1 "f2.h" 1# 5 "test.c" 2int main(){ ngx_int_t a1=1; printf("%d",0); printf("%d",a1); return 0;}

2. .c文件编译注意不要重复引入

这个是我从redis源码中抽取其事件库的编译

在redis源码的ae.c文件：

#include <stdio.h>#include <sys/time.h>#include <sys/types.h>#include <unistd.h>#include <stdlib.h>#include <poll.h>#include <string.h>#include <time.h>#include <errno.h>#include "ae.h"#include "zmalloc.h"#include "config.h"/* Include the best multiplexing layer supported by this system. * The following should be ordered by performances, descending. */#ifdef HAVE_EVPORT#include "ae_evport.c"#else    #ifdef HAVE_EPOLL    #include "ae_epoll.c"    #else        #ifdef HAVE_KQUEUE        #include "ae_kqueue.c"        #else        #include "ae_select.c"        #endif    #endif#endifaeEventLoop *aeCreateEventLoop(int setsize) {    aeEventLoop *eventLoop;    int i;    if ((eventLoop = zmalloc(sizeof(*eventLoop))) == NULL) goto err;    eventLoop->events = zmalloc(sizeof(aeFileEvent)*setsize);    eventLoop->fired = zmalloc(sizeof(aeFiredEvent)*setsize);    if (eventLoop->events == NULL || eventLoop->fired == NULL) goto err;    eventLoop->setsize = setsize;    eventLoop->lastTime = time(NULL);    eventLoop->timeEventHead = NULL;    eventLoop->timeEventNextId = 0;    eventLoop->stop = 0;    eventLoop->maxfd = -1;    eventLoop->beforesleep = NULL;    if (aeApiCreate(eventLoop) == -1) goto err;    /* Events with mask == AE_NONE are not set. So let's initialize the     * vector with it. */    for (i = 0; i < setsize; i++)        eventLoop->events[i].mask = AE_NONE;    return eventLoop;err:    if (eventLoop) {        zfree(eventLoop->events);        zfree(eventLoop->fired);        zfree(eventLoop);    }    return NULL;}

HAVE_EPOLL是前面定义的：

/* Test for polling API */#ifdef __linux__#define HAVE_EPOLL 1#endif

在ae_epoll.c中的aeApiCreate函数

#include <sys/epoll.h>typedef struct aeApiState {    int epfd;    struct epoll_event *events;} aeApiState;static int aeApiCreate(aeEventLoop *eventLoop) {    aeApiState *state = zmalloc(sizeof(aeApiState));    if (!state) return -1;    state->events = zmalloc(sizeof(struct epoll_event)*eventLoop->setsize);    if (!state->events) {        zfree(state);        return -1;    }    state->epfd = epoll_create(1024); /* 1024 is just an hint for the kernel */    if (state->epfd == -1) {        zfree(state->events);        zfree(state);        return -1;    }    eventLoop->apidata = state;    return 0;}

我想抽取出redis的事件库

一开始不知道include的.c文件编译的时候不需要重复引入，不然编译报错：

ae_epoll.c不用被引入，因为在ae.c已经引入了。

成功编译：

生成了redis文件。

2. .h文件编译注意不要重复引入

报错anet.h:47: 错误：expected declaration specifiers or ‘...’ before ‘size_t’

原因是anet.h头文件重复引用，去掉anet.h，重新编译就可以了：

原因分析：

b.h中#include "../../a.h" 而a.h中的函数声明中用到了b.h中的结构体或者typedef，那么就会出现在包含a.h的时候b.h中的结构体或者typedef还没有声明，从而陷入错误。