c - gcc 是如何编译的

Question

我有两个文件 sendfdsock.c, accessories.c .在 accessories.c 中，我定义了各种常用函数(如 logp - 用于打印日志)并包含了各种常用库(如 sys/socket.h、string.h、stdio.h ....)。现在而不是包括 accessories.c在sendfdsock.c或先制作 acceessories.h然后将其包含在 sendfdsock.c 中, 我的计划是先制作 sendfdsock.c and accessories.c 的目标文件然后在发送时将它们链接在一起。执行此操作的命令(我没有制作任何 accessories.h 文件，我直接链接 accessories.c ):

gcc -c sendfdsock.c
gcc -c accessories.c
gcc -o send sendfdsock.o accessories.o

但是在第一个命令之后 gcc -c sendfdsock.c我收到一些错误和警告。这些错误的原因是 sendfdsock.c需要在那些 .h 中定义的一些函数和变量我包含在 accessories.h 中的文件.这意味着 gcc 在创建目标文件时检查各种函数和变量的定义。 但是奇怪的是函数logp()没有错误或警告我在 accessories.c 中定义的并用在main() of sendfdsock.c的第一行.
也就是说，用户定义的函数没有错误/警告，但标准库则相反。 为什么？
这个编译是如何在 gcc 中完成的？有没有什么好的资源可以简要地告诉我关于这个编译和链接的东西。
我也分享了两个文件的代码(如果有人想看的话):

accessories.c :

#include <malloc.h>
#include <time.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <pthread.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <error.h>
#include <signal.h>

#define PORT "4444" //port we are listening on

int sendall(int fd, char *buf, int *len)
{
    int total = 0;        // how many bytes we've sent
    int bytesleft = *len; // how many we have left to send
    int n;

    while(total < *len) {
        n = send(fd, buf+total, bytesleft, 0);
        if (n == -1) { break; }
        total += n;
        bytesleft -= n;
    }

    *len = total; // return number actually sent here

    return n==-1?-1:0; // return -1 on failure, 0 on success
}

int recvall(int fd, char *buf, int *len)
{
    int total = 0;        // how many bytes we've sent
    int bytesleft = *len; // how many we have left to send
    int n;

    while(total < *len) {
        n = recv(fd, buf+total, bytesleft, 0);
        if (n == -1) { break; }
        total += n;
        bytesleft -= n;
    }

    *len = total; // return number actually sent here

    return n==-1?-1:0; // return -1 on failure, 0 on success
} 

void logp(int typ, char* msg) // typ --> type(category) of message [1-Normal Log, 2-Warning(any unexpected thing happened), 3-Error, 4-Debugging Log ]
{
    int fd;
    time_t now;
    ssize_t wlength=0;
    char * dat;
    char * str;
    int size = 45+strlen(msg);//14+24+5+sizeof msg+1

    str= (char *) malloc(size);

    time(&now);//system time in seconds
    dat = ctime(&now); // converting seconds to date-time format
    dat = strtok(dat,"\n");

    //Appending type of log
    switch(typ)
    {
    case 1:
        strcpy(str,"__LOG__    |  ");
        strcat(str,dat);
        break;
    case 2:
        strcpy(str,"__WARN__   |  ");
        strcat(str,dat);
        break;
    case 3:
        strcpy(str,"__ERR__    |  ");
        strcat(str,dat);
        break;
    case 4:
        strcpy(str,"__DEBUG__  |  ");
        strcat(str,dat);
        break;
    default:
        strcpy(str,"__UNDEF__  |  ");
        strcat(str,dat);
        break;
    }


    strcat(str,"  |  ");
    strcat(str,msg);//appending message
    strcat(str,"\n");

    fd = open("log", O_WRONLY | O_CREAT | O_APPEND, 0644); // should be opened somewhere else
    if (fd == -1)
        printf("Could not open log - %s\n",strerror(errno));
    else
    {//need to add lock to the file and printing error message
        while ( wlength < strlen(str) )
        {
            wlength = write(fd, str,strlen(str));
            if (wlength == -1)
            {
                printf("Error : writing log\n");
                break;
            }
        }


    }
}
void errorp(char *where, int boolean, int errn,char *what)
{
    char errmsg[21+strlen(where)];
    strcpy(errmsg,"Where - ");
    strcat(errmsg,where);
    strcat(errmsg,"  |  Error - ");

    if(boolean == 1)//we got error number
    {
        strcat(errmsg,strerror(errn));
        //fprintf(stderr,"ERROR - In %s and error is %s\n",where ,strerror(errn));
        logp(3,errmsg);
    }
    else if(boolean == 0)//we got a message
    {
        strcat(errmsg,what);
        //fprintf(stderr,"ERROR - In %s and error is %s\n",where ,what);
        logp(3,errmsg);
    }
    else//we got nothing
    {
        strcat(errmsg,"No Message");
        //fprintf(stderr,"ERROR - In %s\n",where);
        logp(3,errmsg); 
    }
}

sendfdsock.c :

#include <stropts.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/un.h>




#define CONTROLLEN  CMSG_LEN(sizeof(int))
static struct cmsghdr   *cmptr = NULL;  /* malloc'ed first time */ 

int send_err(int fd, int errcode, const char *msg);
int send_fd(int fd, int fd_to_send);

int main(int argc, char const *argv[])
{   
    logp(1,"started"); 
    int fd_to_send;
    if((fd_to_send = open("vi",O_RDONLY)) < 0)
        printf("vi open failed");

    struct sockaddr_un address;
    int  socket_fd, nbytes;
    char buffer[256];

    socket_fd = socket(PF_UNIX, SOCK_STREAM, 0);
    if(socket_fd < 0)
    {
        printf("socket() failed\n");
        return 1;
    }

    /* start with a clean address structure */
    memset(&address, 0, sizeof(struct sockaddr_un));

    address.sun_family = AF_UNIX;
    snprintf(address.sun_path, sizeof(address.sun_path)-1, "./demo_socket");

    if(connect(socket_fd, (struct sockaddr *) &address, sizeof(struct sockaddr_un)) != 0)
    {
        printf("connect() failed\n");
        return 1;
    }

    nbytes = snprintf(buffer, 256, "hello from a client");
//    write(socket_fd, buffer, nbytes);

  //  nbytes = read(socket_fd, buffer, 256);
    buffer[nbytes] = 0;

    //printf("MESSAGE FROM SERVER: %s\n", buffer);

    //sending the file descriptor    
    printf("From send_fd %d \n",send_fd(socket_fd,fd_to_send));
    printf("Main end");
    close(socket_fd);

    exit(0);

}

int send_err(int fd, int errcode, const char *msg)
{
    int     n;

    if ((n = strlen(msg)) > 0)
        if (write(fd, msg, n) != n)    /* send the error message */
            return(-1);

    if (errcode >= 0)
        errcode = -1;   /* must be negative */

    if (send_fd(fd, errcode) < 0)
        return(-1);

    return(0);
}

int send_fd(int fd, int fd_to_send)
{

    ssize_t temp;
    struct iovec    iov[1];
    struct msghdr   msg;
    char            buf[2]; /* send_fd()/recv_fd() 2-byte protocol */

    iov[0].iov_base = buf;
    iov[0].iov_len  = 2;
    msg.msg_iov     = iov;
    msg.msg_iovlen  = 1;
    msg.msg_name    = NULL;
    msg.msg_namelen = 0;
    if (fd_to_send < 0) {
        msg.msg_control    = NULL;
        msg.msg_controllen = 0;
        buf[1] = -fd_to_send;   /* nonzero status means error */
        if (buf[1] == 0)
            buf[1] = 1; /* -256, etc. would screw up protocol */
    } else {
        if (cmptr == NULL && (cmptr = malloc(CONTROLLEN)) == NULL)
            return(-1);
        cmptr->cmsg_level  = SOL_SOCKET;
        cmptr->cmsg_type   = SCM_RIGHTS;
        cmptr->cmsg_len    = CONTROLLEN;
        msg.msg_control    = cmptr;
        msg.msg_controllen = CONTROLLEN;
        *(int *)CMSG_DATA(cmptr) = fd_to_send;     /* the fd to pass */
        buf[1] = 0;          /* z ero status means OK */
    }
    buf[0] = 0;              /* null byte flag to recv_fd() */
    printf("before sendmsg \n");
    temp = sendmsg(fd, &msg, 0);
    if (temp != 2)
    {
        printf("inside sendmsg condition %d\n",temp);
        return(-1);
    }
    printf("after sendmsg %d\n",temp);
    return(0);

}

Answer 1

使用 -Wall -pedantic 编译并仔细检查:您可能会收到警告，告诉您您隐式定义了 int logp 如果 gcc 没有定义，这就是它所做的'找不到声明。

附言。如果您不确定声明和定义的区别，请开始阅读相关内容 ;-)