为什么 fread() 没有得到预期的字节数？

Question

我正在尝试使用 C 以二进制模式读取文件，但它仅将前 43 个字符存储在缓冲区中。我想以 245 字节为一组读取文件。它包含多字符字节和空字符。这是文件的十六进制内容：

323031353037303735393036333130343739332032373231333732534e30323033323545533036303130340000000008557c0000000000693c0000000000000c0000000008557c0000000000693c0000000000000c0000000008557c0000000000693c0000000000000c00001c00001c00001c00000c00000c00000c00001c4d4e202020204942202020204f393920202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202020202039444b524d4144

这是我的代码：

char* to_hex(const char* strin) {
    char * strout = malloc(2 * strlen(strin) + 1);
    int x;

    for (x = 0; x < strlen(strin);x++){
        sprintf(&strout[x+x],"%02x", (int)(*(unsigned char*)(&strin[x])) );
    }

    strout[2 * strlen(strin)]='[=11=]'
    return strout;
}

int main(int argc, char *argv[]) {

    FILE * pfinput = fopen("stack.bin", "rb");
    int lrec = 245;
    char* sbuff = (char *)malloc((lrec + 1) * sizeof(char));

    if (pfinput != NULL) {
        while (fread (sbuff, 1, lrec, pfinput) > 0){
            sbuff[lrec] = '[=11=]';

            printf("len=%d hex=%s\n\n", strlen(sbuff), to_hex(sbuff) );

        }
    }

    return 0;
}

它returns以下内容：

len=43 hex=323031353037303735393036333130343739332032373231333732534e3032303332354553303630313034

Why it only reads 43 characters instead of 245?
Do you have any alternative to do it?

Answer 1

当您的字符串中嵌入了空字符时，您无法使用 strlen 来可靠地计算字符数。需要抓取fread读取的字符数，使用

int nread = 0;
while (( nread = fread (sbuff, 1, lrec, pfinput)) > 0)

而不是

printf("len=%d hex=%s\n\n", strlen(sbuff), to_hex(sbuff) );

您需要使用：

printf("len=%d hex=%s\n\n", nread, to_hex(sbuff) );

您还需要将 nread 传递给 to_hex，以便您能够在该函数中适当地处理嵌入的空字符。

char* to_hex(const char* strin, int nread) {
    char * strout = malloc(2 * nread + 1);
    int x;

    for (x = 0; x < nread; x++){
        sprintf(&strout[x+x],"%02x", (int)(*(unsigned char*)(&strin[x])) );
    }

    strout[2 * nread]='[=13=]';
    return strout;
}

之后，printf行需要是：

printf("len=%d hex=%s\n\n", nread, to_hex(sbuff, nread) );

PS 注意这里是内存泄漏。由 to_hex 分配的内存用于调用 printf 但之后它不会被释放。您可能希望在变量中捕获该内存并释放它。

char* hexstring = to_hex(sbuff, nread);
printf("len=%d hex=%s\n\n", nread, hexstring);
free(hexstring);

此外，在从 main 返回之前释放 sbuff。

free(sbuff);

PS 2 我会简化行

        sprintf(&strout[x+x],"%02x", (int)(*(unsigned char*)(&strin[x])) );

至

        int c = strin[x];
        sprintf(&strout[x+x],"%02x", c );

Answer 2

'Upon successful completion, fread() shall return the number of elements successfully read',

fread() 返回的值是确定已将多少字节读入缓冲区的唯一方法。在文件末尾，可能会读取少于 'lrec' 个字符。

为什么 fread() 没有得到预期的字节数？

Why fread() is not getting the expected bytes?

c

null

file

fread