由Linux中管道的buffer，浅谈C语言中char类型字符串拷贝使用strcpy()和=赋值符号的区别

今天在写父子进程用两个单向管道通信时，出现了错误：

Segmentation fault (core dumped)

打开core文件发现：

附上源码：

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
#include <errno.h>
#include <string.h>

void erreur(const char *msg)
{
    perror(msg);
}

#define NBCAR 256

int main(void)
{
    pid_t pid;
    int tube[2];
    int tube2[2];
    int ret_out, ret_in;
    char *buffer,*buffer2;

    if (pipe(tube) == -1) {//from parent to son
    erreur("Erreur de creation du pipe");
    exit(EXIT_FAILURE);
    }
if (pipe(tube2) == -1) {//from son to parent
    erreur("Erreur de creation du pipe");
    exit(EXIT_FAILURE);
    }
    buffer = (char *) malloc(NBCAR * sizeof(char));
    buffer2 =  (char *) malloc(NBCAR * sizeof(char));
    switch (pid = fork()) {
         case (pid_t) -1:
            erreur("fork");
        case (pid_t) 0:    
            close(tube[1]);
            close(tube2[0]);
            if (printf("Je suis le fils de PID %d
 write dans le tube : %s
 Nbr Caracteres lus: %d
",getpid(), buffer, ret_in =write(tube2[1], buffer="tube1 s to p", NBCAR - 1)) == -1) {
            erreur(" Pb Lecture ");
            exit(EXIT_FAILURE);
                }
            if (printf("Je suis le fils de PID %d
 Lecture dans le tube : %s
 Nbr Caracteres lus: %d
",getpid(), buffer2, ret_in =read(tube[0], buffer2, NBCAR - 1)) == -1) {
            erreur(" Pb Lecture ");
            exit(EXIT_FAILURE);
                }

            exit(0);
            break;
          default:    
        close(tube[0]);
        close(tube2[1]);
        if (printf("Je suis le parent de PID %d
 Lecture dans le tube : %s
 Nbr Caracteres lus: %d
",getpid(), buffer, ret_in =read(tube2[0], buffer, NBCAR - 1)) == -1) {
            erreur(" Pb Lecture ");
            exit(EXIT_FAILURE);}
        if (printf("Je suis le parent de PID %d
 write dans le tube : %s
 Nbr Caracteres lus: %d
",getpid(), buffer2, ret_in =write(tube[1], buffer2="tube2 p to s", NBCAR - 1)) == -1) {
            erreur(" Pb Lecture ");
            exit(EXIT_FAILURE);}

    wait(NULL);
    break;

    }
    free(buffer);
    free(buffer2);
    return EXIT_SUCCESS;
}

关于SIGSEGV错误

SIGSEGV --- Segment Fault. The possible cases of your encountering this error are:

1.buffer overflow --- usually caused by a pointer reference out of range.

2.stack overflow --- please keep in mind that the default stack size is 8192K.

3.illegal file access --- file operations are forbidden on our judge system.

后来通过邮件问老师，发现是buffer出现内存泄漏。

先梳理一下C语言中char和字符型、字符串型、单引号、双引号、字符串截止符号的概念：

　　首先C中没有专门的字符串变量（没有C++中的String类），单个的char就代表一个字符，赋值时应该是：char c = 'a'; 故而单引号表示单个字符。如果用char表示字符串，需要定义char的数组，并有两种主要赋值方法：

char greeting[6] = {'H', 'e', 'l', 'l', 'o', ''};
char greeting[] = "Hello";
//第二例子
char c[10]={'I', ' ', 'a', 'm', ' ', 'h', 'a', 'p', 'p', 'y'}; 
char c[]="I am happy";

第一种方法：若初值个数小于数组长度,则只将这些字符赋绐数组中前面的元素,其余元素自动定为空字符(即'')。

第二种方法：编译器会在初始化数组时，自动把 '' 放在字符串的末尾。

回过头看代码中对buffer的操作：

对buffer和buffer2都分配了256个字节的空间（注意他们是指针，指向256个字节连续空间的首地址），但在赋值时直接对buffer使用等号=，把常量字符串赋给一个指针本身，这显然是不对的····原来分配的两块256字节的内存现在没有指针指向了，最后free的时候并不能把他们释放掉，因此造成内存泄漏

但write()和read()中间参数又必须是指针，这时就需要strcpy(）函数，改成：

buffer=strcpy(buffer,"tube1 s to p")
buffer2=strcpy(buffer2,"tube2 p to s")

完美运行，不会报错

 借此机会再复习一下char字符串用等号=赋值和用strcpy()赋值的区别：

等号赋值，两个char指向同一个空间

函数赋值，两个char各有一个相同的字符串拷贝

本文完

参考:

https://blog.csdn.net/yal179/article/details/13019817

https://www.runoob.com/cprogramming/c-strings.html

http://www.voidcn.com/article/p-wlesexyc-ben.html

https://zhidao.baidu.com/question/53110127.html

https://www.runoob.com/cprogramming/c-function-strcpy.html

SIGSEGV错误

https://blog.csdn.net/brace/article/details/1102422

关于core dumped

https://blog.51cto.com/xjsunjie/1954870

https://blog.csdn.net/peiyao456/article/details/70045195