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C:返回值為指針的函數解讀

編輯：關於C語言

一、先看看下面的小程序：

[cpp] #include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
    char *pName; /*姓名*/
    int iId;     /*ID*/
}stStaffInfo;

/*根據ID獲取姓名*/
char* GetStaffNameById(int iId)
{
    int i;

    stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           return stTmpInfo[i].pName;
        }
    }

    return "No match Name";

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

system("PAUSE");

return 0;
}
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
char *pName; /*姓名*/
int iId; /*ID*/
}stStaffInfo;

/*根據ID獲取姓名*/
char* GetStaffNameById(int iId)
{
    int i;

    stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           return stTmpInfo[i].pName;
        }
    }

    return "No match Name";

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

system("PAUSE");

return 0;
}

運行結果：

[plain]
Staff Name is : dyx1024(id = 2)
請按任意鍵繼續. . .
Staff Name is : dyx1024(id = 2)
請按任意鍵繼續. . .

運行結果與預期相符，但此程序卻是一個有問題的程序，stTmpInfo是一個定義在函數GetStaffNameById中的臨時變量，調用完此程序，它即被銷毀，所以main函數中，pName所指對象的值是不確定的，在沒被覆蓋前，正確( 像此程序)，但若被其他的函數調用覆蓋，誰也不知道其值是什麼。

二、如何改進？

方法有以下四種，推薦使用方法三。

1、將stTmpInfo定義為全局變量，如下

[cpp]
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
    char *pName; /*姓名*/
    int iId;     /*ID*/
}stStaffInfo;

static stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

/*根據ID獲取姓名*/
char* GetStaffNameById(int iId)
{
    int i;

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           return stTmpInfo[i].pName;
        }
    }

    return "No match Name";

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

system("PAUSE");<span style="white-space:pre"> </span>

return 0;
}
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
char *pName; /*姓名*/
int iId; /*ID*/
}stStaffInfo;

static stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

/*根據ID獲取姓名*/
char* GetStaffNameById(int iId)
{
    int i;

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           return stTmpInfo[i].pName;
        }
    }

    return "No match Name";

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

system("PAUSE");<span style="white-space:pre"> </span>

return 0;
}

2、程序內部將stTmpInfo定義為static，如下：

[cpp]
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
    char *pName;
    int iId;     /*ID*/
}stStaffInfo;

char* GetStaffNameById(int iId)
{
    int i;

    static stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };


    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           return stTmpInfo[i].pName;
        }
    }

    return "No match Name";

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

system("PAUSE");

return 0;
}
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
char *pName;
int iId; /*ID*/
}stStaffInfo;

char* GetStaffNameById(int iId)
{
    int i;

    static stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           return stTmpInfo[i].pName;
        }
    }

    return "No match Name";

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

system("PAUSE");

return 0;
}

3、修改接口，通過出參取回。

[cpp]
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
    char *pName;
    int iId;
}stStaffInfo;

void GetStaffNameById(int iId, int iNameLen, char *pOutName)
{
    int i;

    stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

    if (NULL == pOutName)
    {
       return;
    }

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           strncpy(pOutName, stTmpInfo[i].pName, iNameLen);
           return;
        }
    }

    strncpy(pOutName, "No match Name", iNameLen);

    return;

}

int main(int argc, char *argv[])
{
int iId = 20;
char szName[21] = "\0";

GetStaffNameById(iId, 20, szName);

printf("Staff Name is : %s(id = %d)\n", szName, iId);

system("PAUSE");

return 0;
}
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
char *pName;
int iId;
}stStaffInfo;

void GetStaffNameById(int iId, int iNameLen, char *pOutName)
{
    int i;

    stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

    if (NULL == pOutName)
    {
       return;
    }

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           strncpy(pOutName, stTmpInfo[i].pName, iNameLen);
           return;
        }
    }

    strncpy(pOutName, "No match Name", iNameLen);

    return;

}

int main(int argc, char *argv[])
{
int iId = 20;
char szName[21] = "\0";

GetStaffNameById(iId, 20, szName);

printf("Staff Name is : %s(id = %d)\n", szName, iId);

system("PAUSE");

return 0;
}

4、通過子程序分配內存，調用者釋放解決。

[cpp]
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
    char *pName; /*ÐÕÃû*/
    int iId;     /*ID*/
}stStaffInfo;

/*¸ù¾ÝID»ñÈ¡ÐÕÃû*/
char* GetStaffNameById(int iId)
{
    int i;
    int iMaxNameLen = 20;

    stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

    char *pRetName = (char *)malloc(iMaxNameLen);
    if (NULL == pRetName)
    {
        exit(1);
    }

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           strncpy(pRetName, stTmpInfo[i].pName, iMaxNameLen);
           return pRetName;
        }
    }

    strncpy(pRetName, "No match Name", iMaxNameLen);

    return pRetName;

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

free(pName);
pName = NULL;

system("PAUSE");

return 0;
}
#include <stdio.h>
#include <stdlib.h>

typedef struct _STSTAFFINFO
{
char *pName; /*ÐÕÃû*/
int iId; /*ID*/
}stStaffInfo;

/*¸ù¾ÝID»ñÈ¡ÐÕÃû*/
char* GetStaffNameById(int iId)
{
    int i;
    int iMaxNameLen = 20;

    stStaffInfo stTmpInfo[] =
    {
        {"Socrates", 1},
        {"dyx1024", 2},
        {"Kevin", 3},
        {"Jim", 4}
    };

    char *pRetName = (char *)malloc(iMaxNameLen);
    if (NULL == pRetName)
    {
        exit(1);
    }

    for (i = 0; i < sizeof(stTmpInfo) / sizeof(stTmpInfo[0]); i++)
    {
        if (stTmpInfo[i].iId == iId)
        {
           strncpy(pRetName, stTmpInfo[i].pName, iMaxNameLen);
           return pRetName;
        }
    }

    strncpy(pRetName, "No match Name", iMaxNameLen);

    return pRetName;

}

int main(int argc, char *argv[])
{
int iId = 2;

char *pName = GetStaffNameById(iId);

printf("Staff Name is : %s(id = %d)\n", pName, iId);

free(pName);
pName = NULL;

system("PAUSE");

return 0;
}