c - ftruncate 无法与 msys2 一起正常工作

Question

我正在尝试修改文件的大小。我正在使用 msys2。我的代码如下所示:

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

void dummy_file(){
    FILE *f = fopen("file.txt", "w");
    if (f == NULL)
    {
        printf("Error opening file!\n");
        exit(1);
    }

    /* print some text */
    const char *text = "Write this to the file";
    fprintf(f, "Some text: %s\n", text);

    /* print integers and floats */
    int i = 1;
    float py = 3.1415927;
    fprintf(f, "Integer: %d, float: %f\n", i, py);

    /* printing single chatacters */
    char c = 'A';
    fprintf(f, "A character: %c\n", c);
    fclose(f);
}

void print_size(){
    FILE *f = fopen("file.txt", "r");
    long sz = 0;
    if (f == NULL)
    {
        printf("Error opening file!\n");
        exit(1);
    }
    fseek(f, 0L, SEEK_END);
    sz = ftell(f);
    printf("%ld\n",sz);
    fclose(f);
}

void truncate_file(){
    FILE *f = fopen("file.txt", "w");
    if (f == NULL)
    {
        printf("Error opening file!\n");
        exit(1);
    }
    ftruncate(fileno(f), 40);
    fclose(f);
}

int main()
{
    printf("Begin\n");
    dummy_file();
    print_size();
    // truncate_file();
    print_size();
    printf("End\n");
    return 0;
}

输出看起来像这样:

开始 80 40 结束

文件已更改为 40 字节，但内容为大量空值。

我做错了什么？有没有其他方法可以在保留文件内容的同时截断文件？

Answer 1

当您在 w 模式下使用 fopen 打开文件时，您会将文件截断为零长度。之后，当您运行 ftruncate 时，它将用 \0 填充文件以达到您指定的大小。

来自 fopen 手册页，

w Truncate file to zero length or create text file for writing. The stream is positioned at the beginning of the file.

您可以改为使用 r+ 模式打开它，该模式不会截断文件(如果存在)并允许写入。

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

void dummy_file(){
    FILE *f = fopen("file.txt", "w");
    if (f == NULL)
    {
        printf("Error opening file!\n");
        exit(1);
    }

    /* print some text */
    const char *text = "Write this to the file";
    fprintf(f, "Some text: %s\n", text);

    /* print integers and floats */
    int i = 1;
    float py = 3.1415927;
    fprintf(f, "Integer: %d, float: %f\n", i, py);

    /* printing single chatacters */
    char c = 'A';
    fprintf(f, "A character: %c\n", c);
    fclose(f);
}

void print_size(){
    FILE *f = fopen("file.txt", "r");
    long sz = 0;
    if (f == NULL)
    {
        printf("Error opening file!\n");
        exit(1);
    }
    fseek(f, 0L, SEEK_END);
    sz = ftell(f);
    printf("%ld\n",sz);
    fclose(f);
}

void truncate_file(){
    FILE *f = fopen("file.txt", "r+");
    if (f == NULL)
    {
        printf("Error opening file!\n");
        exit(1);
    }
    ftruncate(fileno(f), 40);
    fclose(f);
}

int main()
{
    printf("Begin\n");
    dummy_file();
    print_size();
    truncate_file();
    print_size();
    printf("End\n");
    return 0;
}

---

只是在扩展代码时要考虑的旁注:如果不遵循某些步骤，混合文件描述符和 STD I/O 流可能会导致未定义的行为(有关更多信息，请参阅 this answer，它指向 this link) .