有fread()和fwrite()引起的一些思考

在做cgi编程的时候，遇到用fread()函数读取数据。于是查阅了Linux c函数库。遇到些问题，一并总结。

1.fread()

首先，fread()原型：

/*fread():read datas from file-stream.--------------------------------------------------------
size_t fread(void *ptr, size_t size, sizt_t nmenb, FILE *stream);
stream:the open-file pointer.
ptr:point to the space that want to store the read-date.
size:the type size,like int, char or struct.
nmemb*size:the number char that read.
Note:fread() returns the actual number that read,nmemb.While IF the actual number is smaller than the parameter 'nmemb', maybe it means it reads to the file-end or error happens.In this case, we must use feof() or ferror() to see what happened.
-----------------------------------------fread()----------------------------------------------

关于fread()函数例子：

#include <stdio.h>
#include <stdlib.h>
#define nmem 3

struct test
{
    char name[20];
    int size;
}s[nmem];

int main()
{
    FILE *stream;
    stream = fopen("/test.c", "r");
    fread(s, sizeof(struct test), nmem, stream);
    fclose(stream);
    for(int i=0; i<nmem; i++)
        printf("name[%d]=%-20s: size[%d]=%d\n", i, s[i].name, i, s[i].size);
    return 0;
}

在VC6.0中编译，产生错误：

D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(18) : error C2143: syntax error : missing ';' before 'type'
D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(18) : error C2143: syntax error : missing ';' before 'type'
D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(18) : error C2143: syntax error : missing ')' before 'type'
D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(18) : error C2143: syntax error : missing ';' before 'type'
D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(18) : error C2065: 'i' : undeclared identifier
D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(18) : warning C4552: '<' : operator has no effect; expected operator with side-effect
D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(18) : error C2059: syntax error : ')'
D:\Visual C++ 6.0 Enterprise Setup\MSDev98\MyProjects\fread.c(19) : error C2146: syntax error : missing ';' before identifier 'printf'
Error executing cl.exe.

fread.obj - 7 error(s), 1 warning(s)

但从句法分析，虽说是语法错误，但貌似没有啊 ：（

后来在Linux下编译，擦擦，错误信息很明了啊，如下：

fread.c: In function ‘main’:
fread.c:17: error: ‘for’ loop initial declaration used outside C99 mode
原来问题出在C99语法与C89语法的问题。顺便在网上搜了下，看这里。

所以，将i在main函数开始定义即可。也可以这样：gcc -std=c99 fread.c -o fread，使支持c99标准。

貌似这种定义以前一直用，今天换了下产生这个问题。还是编译器的原因吧。

 

附：

左对齐："-"   "%-20s"
右对齐："+"   "%+20s"

顺便搜集一下常用到的输入输出格式，在另外一篇日记里（网上检索的）。

2.fwrite():

size_t fwrite(const void *ptr, size_t size, size_tnmemb, FILE *stream);

同read类似，只不过是“写”。

 

例子：

#include
#incldue

#define set_s (x, y) {strcpy(s[x].name, y); s[x].size = strlen(y);}

struct test
{
    char name[20];
    int size;
}s[nmem];
main()
{
    FILE *stream;
    set_s(0, "Linux");
    set_s(1, "Windows");
    set_s(2, "Macintosh");
    stream = fopen("./test.c", "w");
    fwrite(s, sizeof(struct test), nmem, stream);
    fclose(stream);
}