如何使用管道发送动态分配的字符串

我有一个功课，我需要使用管道在进程之间发送一串字符串 .

当我使用管道从子进程发送 string which is created compile time 到主进程时，它被成功发送 . 这是我在主要和子进程中所做的 .

儿童过程：

char* str ="from child";
    int lengthOfString=strlen(str)+1;//+1 for null terminator

  write(pipes[0][1],&lengthOfString,sizeof(int));//send the length of string beforehand
  write(pipes[0][1],&str,lengthOfString); //send the actual string

主要流程：

int numberOfChar;
    char* buffer;
    buffer=(char*)malloc(sizeof(char)*15);
        read(pipes[0][0],&numberOfChar,sizeof(int));//get length of upcoming string
        read(pipes[0][0],&buffer,numberOfChar);//read the actual sting
        printf("received from pipe :%s\n",buffer);

但是，如果我使用 string which is dynamically allocated 尝试相同的操作，我在主进程中只接收NULL作为字符串 . 这就是我做的 .

儿童过程：

char* dynamicallyAllocatedString=(char*)malloc(sizeof(char)*strlen(str));
    strcpy(dynamicallyAllocatedString,str);
    int lengthOfString=strlen(dynamicallyAllocatedString)+1;//+1 for null terminator

  write(pipes[0][1],&lengthOfString,sizeof(int));//send the length of string beforehand
  write(pipes[0][1],&dynamicallyAllocatedString,lengthOfString); //send the actual string

主要流程：

read(pipes[0][0],&numberOfChar,sizeof(int));//get length of upcoming string
        buffer=(char*)malloc(sizeof(char)*numberOfChar);
        read(pipes[0][0],&buffer,numberOfChar);//read the actual sting
        printf("received from pipe :%s\n",buffer);

这是完整的代码：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc,char * argv[])
{
pid_t pid;
int numberOfPipe=1;
int ret;
int c;
int pipeNumber=-1;
int** pipes= (int**)malloc(numberOfPipe*sizeof(int));
 for(c=0;c<numberOfPipe;c++)//create pipe for each process
{
    pipes[c]= (int*)malloc(sizeof(int)*2);
    ret = pipe(pipes[c]);
    if(ret==-1)
        {
            perror("pipe error");
            exit(1);
        }
}


for(c=0;c<numberOfPipe;c++)//create child process
{
pid = fork();
pipeNumber++;
if(0==pid)
{
    break;
}
}
if(0==pid)
{
    // Child process
    int i=0;
    char* str ="from child";
    char* dynamicallyAllocatedString=(char*)malloc(sizeof(char)*strlen(str));
    strcpy(dynamicallyAllocatedString,str);
    int lengthOfString=strlen(dynamicallyAllocatedString)+1;//+1 for null terminator
    close(pipes[pipeNumber][0]); //close read side

  write(pipes[pipeNumber][1],&lengthOfString,sizeof(int));//send the length of string beforehand
  write(pipes[pipeNumber][1],&dynamicallyAllocatedString,lengthOfString); //send the actual string

    close(pipes[pipeNumber][1]); //close write side
    exit(0);
}
else
{
    //main process
    printf("main process\n");

    int numberOfChar;
    char* buffer;


    for(c=0;c<numberOfPipe;c++)//close write side of each pipe
    {
        close(pipes[c][1]);
    }

    for(c=0;c<numberOfPipe;c++)//iterate each pipe
    {
        read(pipes[c][0],&numberOfChar,sizeof(int));//get length of upcoming string
        buffer=(char*)malloc(sizeof(char)*numberOfChar);
        read(pipes[c][0],&buffer,numberOfChar);//read the actual sting
        printf("received from pipe :%s\n",buffer);
    }

    for(c=0;c<numberOfPipe;c++)//close read side of each pipe
    {
        close(pipes[c][0]);
    }
}

printf("end");
return 0;

}

我该怎么做才能获得实际的字符串而不是NULL？谢谢 .