将两个文件合并为C中的第三个文件（fread和fwrite）

Question

I'm trying to make a function that takes a series of files and writes the contents of each one into a new one.

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

    void file_copy(FILE *f1, FILE *f2) {
    char buffer[BUFFER_SIZE];
    size_t sz;
    sz = fread(buffer, sizeof(buffer),1,f1);
    while (sz == 1) {
        fwrite(buffer, sz,1,f2);
        sz = fread(buffer, sizeof(buffer),1,f1);
    }
}

int main(int argc, char const *argv[])
{
    FILE *f_in, *f_out;
    int i;
    if (argc < 3)
    {
        puts("Not enough arguments.");
        exit(1);
    }

    if ((f_out = fopen(argv[argc-1], "w")) == NULL)
    {
        printf("Can't open %s for writing.\n", argv[argc-1]);
        exit(1);
    }

    for (i = 0; i < argc-1; ++i)
    {
        if ((f_in = fopen(*++argv, "r")) == NULL)
        {
            printf("Can't open %s for reading\n", *argv);
            exit(1);
        }
        else
            file_copy(f_in, f_out);
        close(f_in);
        close(f_out);
    }


    return 0;
}

and I dont' get any output in my output file. It seems to be closing them just right.

Answer 1

There are Several fails:

1. buffer must be allocated somehow
2. write have to write number of bytes read, not full buffer

So it should be like this:

#define BUF_SIZE 1024

void file_copy(FILE *f1, FILE *f2) {
    char buffer[BUF_SIZE];

    while (!feof(f1)) {
        size_t r = fread(buffer, 1, sizeof(buffer), f1);
        fwrite(buffer, 1, r, f2);
    }
}

Answer 2

Did you plan to save the text from the list of files in the last file? It isn't really clear. There are multiple problems with the code aside from unreadable syntax. You should be using fclose instead of close . And reading a number of one byte elements instead of one element of a giant array.

Here is the fixed code:

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

void file_copy(FILE *f1, FILE *f2) {
    char buffer[1024];
    ssize_t sz;

    while (!feof(f1)) {
        sz = fread(buffer, 1, sizeof(buffer), f1);

        fwrite(buffer, 1, sz, f2);
    }
}

int main(int argc, char const *argv[])
{
    FILE *f_in, *f_out;
    int i;
    if (argc < 3)
    {
        puts("Not enough arguments.");
        exit(1);
    }

    if ((f_out = fopen(argv[argc-1], "w")) == NULL)
    {
        printf("Can't open %s for writing.\n", argv[argc-1]);
        exit(1);
    }

    for (i = 1; i < argc-1; i++)
    {
        if ((f_in = fopen(argv[i], "r")) == NULL)
        {
            printf("Can't open %s for reading\n", *argv);
            exit(1);
        }
        else
        {
            file_copy(f_in, f_out);
            fclose(f_in);
        }
    }
    fclose(f_out);

    return 0;
}

Answer 3

You should definitely know how much to write, because you end up writing 1024 bytes no matter what, as is now.

Depending on what file type you are dealing with, this is an option for txt-files at least:

void file_copy(FILE *f1, FILE *f2) {

    fseek(f1, 0, SEEK_END);
    int buf_size = ftell(f1);
    char buf[buf_size];

    //Reset file position
    fseek(f1, 0, SEEK_SET);
    sz = fread(buf, sizeof(buf), 1, f1);
    fwrite(buf, sz, 1, f2);
}