是否可以在不将文件加载到内存的情况下读取文件?

问题描述：

我想读取一个文件，但是它太大了，无法完全将其加载到内存中.

I want to read a file but it is too big to load it completely into memory.

有没有一种方法可以读取它而不将其加载到内存中?还是有更好的解决方案?

Is there a way to read it without loading it into memory? Or there is a better solution?

答

我需要内容来执行校验和，所以我需要完整的消息

许多校验和库都支持对校验和的增量更新.例如，GLib具有 g_checksum_update() .因此，您可以使用 fread 一次读取一个文件块并更新读取时校验和.

Many checksum libraries support incremental updates to the checksum. For example, the GLib has g_checksum_update(). So you can read the file a block at a time with fread and update the checksum as you read.

#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdlib.h>
#include <glib.h>

int main(void) {
    char filename[] = "test.txt";

    // Create a SHA256 checksum
    GChecksum *sum = g_checksum_new(G_CHECKSUM_SHA256);
    if( sum == NULL ) {
        fprintf(stderr, "Could not create checksum.\n");
        exit(1);
    }

    // Open the file we'll be checksuming.
    FILE *fp = fopen( filename, "rb" );
    if( fp == NULL ) {
        fprintf(stderr, "Could not open %s: %s.\n", filename, strerror(errno));
        exit(1);
    }

    // Read one buffer full at a time (BUFSIZ is from stdio.h)
    // and update the checksum.    
    unsigned char buf[BUFSIZ];
    size_t size_read = 0;
    while( (size_read = fread(buf, 1, sizeof(buf), fp)) != 0 ) {
        // Update the checksum
        g_checksum_update(sum, buf, (gssize)size_read);
    }

    // Print the checksum.
    printf("%s %s\n", g_checksum_get_string(sum), filename);
}

我们可以通过将结果与sha256sum进行比较来检查其是否有效.

And we can check it works by comparing the result with sha256sum.

$ ./test
0c46af5bce717d706cc44e8c60dde57dbc13ad8106a8e056122a39175e2caef8 test.txt
$ sha256sum test.txt 
0c46af5bce717d706cc44e8c60dde57dbc13ad8106a8e056122a39175e2caef8  test.txt

是否可以在不将文件加载到内存的情况下读取文件?

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