读取/malloc的问题

Question

FILE *infp, *outfp;
infp = fopen(argv[2], "r"); 
int len;
char *text;
fseek(infp, 0, SEEK_END); 
len = ftell(infp);
printf("%dn", len);
if ((text = (char *) malloc(500000000)) == NULL)
{
        fprintf(stderr, "Error allocating memoryn");
        exit(1);
}
fread(text, len, 1, infp);
text[len] = '';
fclose(infp);
printf("Text = %s,  Address = %un", text, text);

返回

138
Text = ,  Address = 3794927632

我不知道为什么文本不打印任何东西。我用错词了吗?

Answer 1

您需要像这样重置rewind()或fseek(3)的文件位置

FILE *infp;
FILE *outfp;
int length;
char *text;
if ((infp = fopen(argv[2], "r")) == NULL)
{
     fprintf(stderr, "Error openning `%s'n", argv[2]);
     return -1;
}
fseek(infp, 0L, SEEK_END); 
len = ftell(infp);
/* reset position */
fseek(infp, 0L, SEEK_SET); /* essentially rewind(infp); */
printf("%dn", length);
if ((text = malloc(length + 1)) == NULL)
{
    fprintf(stderr, "Error allocating memoryn");
    return -1;
}
if (fread(text, 1, length, infp) == length)
{
    text[length] = '';
    printf("Text = %s,  Address = %un", text, text);
    free(text); /* never forget to `free' */
}
else
{
    free(text);
    text = NULL:
}
fclose(infp);

你也应该

1. 检查fopen()的返回值，您从未检查文件是否实际打开，我认为这是主要问题。
2. 只分配必要的空间
3. 确保fread()不失效

4. 交换fread(3)的size参数，首先是元素的大小，然后是元素的个数

   size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
   

   和返回值应该等于nmemb，阅读上面链接的手册页

Answer 2

After

fseek(infp, 0, SEEK_END); 

infp指向文件的末尾。你需要倒带文件。

rewind(infp);