为什么 fread 总是返回 0？

Question

I used this code to read file. But fread function always return 0. What is my mistake?

FILE *file = fopen(pathToSourceFile, "rb");
if(file!=NULL) 
{
    char aByte[50000];
    int ret = fread(aByte, sizeof(aByte), 1, file);
    if(ret != 0)
    {
        not jump into there;
        fseek(file, 0, SEEK_SET);
        fwrite(aByte, ret, 1, file);
    }
} 
fclose(file); 

Answer 1

are you sure that your file has a size greater than 50000 ? otherwise you could try:

 fread(aByte,1, sizeof(aByte),  file);

Answer 2

ferror() will tell when something is wrong.

You can print the actual error message using perror() .

Answer 3

You can't fwrite to a file open in rb mode.

Your statement that ret is always zero is false. If you had properly instrumented your code, you'd not be making false claims:

#include <stdio.h>
#include <stdlib.h>

int main() {
    FILE *file = fopen("junk.dat", "rb");
    if(file!=NULL)
    {
        char aByte[50000];
        int ret = fread(aByte, sizeof(aByte), 1, file);
        fprintf(stderr, "fread returned %d\n", ret);

        if(ret != 0)
        {
            int fs = fseek(file, 0, SEEK_SET);
            if(fs == -1) {
                perror("fseek");
                exit(1);
            }
            fs = fwrite(aByte, ret, 1, file);
            if(fs != ret) {
                perror("fwrite");
                exit(1);
            }
        }
    }
    fclose(file);
    return 0;
}

Yields:

fread returned 1
fwrite: Bad file descriptor

when run.

Answer 4

In case someone else runs into this. I just ran into a similar issue. It is because the 2nd argument to fread should be the size of each element in the buffer . In OP's code it is the size of the pointer to the buffer.

This should work provided buff has at least 1 element:

int ret = fread(aByte, sizeof(aByte[0]), 1, file);

Answer 5

In my case I wanted to read a file of size 6553600 bytes (an mp3), and it was returning 0 bytes read. It drove me crazy, till, I tried to manually hardcode 30 bytes, and it did read 30 bytes.

I started playing with it and see how much can it read, and it turns out that it can read exactly 262144 (2^18) bytes, if you ask it to read 262145 bytes it reads 0.

Conclusion: at least with this function you can't load the whole file in one go.

Answer 6

Please check man fread

man fread(3)

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

RETURN VALUE

On success, fread() and fwrite() return the number of items read or written. This number equals the number of bytes transferred only when size is 1 . If an error occurs, or the end of the file is reached, the return value is a short item count (or zero).

As your file is smaller than 50000Bytes aka. size of a item, the read item count is 0.

Answer 7

In my case,

    fseek(rFile, 0, SEEK_END);
    iTotalSize = ftell(rFile);
    fseek(rFile, 0, SEEK_SET);   // <-- I wrote SEEK_END, not SEEK_SET 

so it read 0 byte(anything)

Answer 8

Did you:

#include <unistd.h>

If not, and if you compile without -Wall, the C compiler can incorrectly assume that the second argument to fread() is an int rather than an off_t, which can mess up the function call. Your code snippet doesn't show any #include statements, so please make sure you're including everything that you're using.