C语言中fread函数工作原理及相关细节疑问

Hey there, let's unpack your questions step by step—this is a common point of confusion when working with C file I/O, so you're not alone!

First, let's recap the fread() signature you shared:

size_t fread(void *buffer, size_t size, size_t qty, FILE *inptr);

Your first question: Does inptr store the memory address of each character in the file? #

No, that's not how the FILE* pointer works. The FILE struct (which inptr points to) doesn't hold addresses for every character in the file. Instead, it maintains state information about the open file, like:

• The current position in the file (where the next read/write will happen)
• A small internal buffer to optimize I/O operations (so it doesn't hit the disk every single time you read a byte)
• Flags indicating if the file is open for reading/writing, if an error occurred, if we've reached the end of the file, etc.

Think of inptr as a "handle" that tells the C standard library which file you're working with and how to interact with it—not a map of the file's contents.

Your second question: Is the memory address from inptr copied to *buffer? #

Absolutely not. The buffer parameter is a pointer to a block of memory you've allocated (either on the stack or heap) where fread() will copy the actual content of the file into. The inptr is just used to identify which file to read from; its own data (the FILE struct) isn't copied into your buffer at all.

Your third question: When calling fread(), do we copy the file content itself or the memory addresses of that content? #

You're copying the actual file content bytes, not memory addresses. The size * qty calculation tells fread() how many total bytes to read from the file (starting at the current position tracked by inptr) and write into the memory location pointed to by buffer.

Let's use a simple example to make this concrete:

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

int main() {
    FILE *inptr = fopen("example.txt", "r");
    if (inptr == NULL) {
        perror("Failed to open file");
        return 1;
    }

    // Allocate a buffer to hold up to 100 bytes of the file
    char *buffer = malloc(100 * sizeof(char));
    if (buffer == NULL) {
        perror("Failed to allocate buffer");
        fclose(inptr);
        return 1;
    }

    // Read 100 bytes (1 byte per element, 100 elements)
    size_t bytes_read = fread(buffer, sizeof(char), 100, inptr);

    // Print what we read
    printf("Read %zu bytes: %.*s\n", bytes_read, (int)bytes_read, buffer);

    // Cleanup
    free(buffer);
    fclose(inptr);
    return 0;
}

In this code, fread() takes the bytes from example.txt and copies them directly into the buffer we allocated. No addresses are copied—just the raw character data from the file.

内容的提问来源于stack exchange，提问作者Prav