c - 必须使用 `%.4s` 的精度修饰符才能显示 `char[4]` ，为什么简单的 `%s` 不能正常工作？

Question

我有以下代码(仅发布必要的行)。 这里我复制了 WAVE 文件的前 44 个字节(PCM，即线性量化)。 问题在于显示 struct 的 char[4] 记录。

无法弄清楚，虽然试图了解原因但没有结果。

问题:为什么我必须使用精度修饰符 %.*s 来仅显示 char[N] 的前 4 个符号仅使用 %s 打印 char[4] 变量？

BUT 如果我将修饰符写入 %s --> %.4s 的 printf() 函数正确打印所有内容:“RIFF”、“WAVE”、“fmt”、“data”。 它仅显示每个 char[4] 记录的前 4 个符号。但是未显示的符号又如何呢？

我通过 %s 得到以下信息:

Enter input and output filenames (with no extension):
europe_-_final_countdown
europe_-_final_countdown_out
Opening file: "europe_-_final_countdown.wav" has been successfully opened.

*********************************
       ChunkID: RIFF4?AWAVEfmt     // instead of "RIFF"
     ChunkSize: 54629940
        Format: WAVEfmt            // instead of "WAVE"
   SubChunk1ID: fmt                // here is "fmt " -> ok
 SubChunk1Size: 16
   AudioFormat: 1
   NumChannels: 2
    SampleRate: 44100
      ByteRate: 176400
    BlockAlign: 4
 BitsPerSample: 16
   SubChunk2ID: data?A             // instead of "data"
 SubChunk2Size: 54629904
*********************************
Process returned 0 (0x0)   execution time : 34.554 s
Press ENTER to continue.

---

所以这是代码:

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

#define MAX_NAME_LENGTH 255

typedef unsigned short byte2;
typedef unsigned int   byte4;

struct header
{
    // totally 44 bytes;        endian (all the rest are little)
    // RIFF
    char    ChunkID[4];         // big
    byte4   ChunkSize;
    char    Format[4];          // big
    // fmt
    char    SubChunk1ID[4];     // big
    byte4   SubChunk1Size;
    byte2   AudioFormat;
    byte2   NumChannels;
    byte4   SampleRate;
    byte4   ByteRate;
    byte2   BlockAlign;
    byte2   BitsPerSample;
    // data
    char    SubChunk2ID[4];     // big
    byte4   SubChunk2Size;
};

struct header hdr;



int main()
{
    char nameInput[MAX_NAME_LENGTH];
    char nameOutput[MAX_NAME_LENGTH];
    printf("Enter input and output filenames (with no extension):\n");

    scanf("%s", nameInput);
    strcat(nameInput, ".wav");
    //printf("nameInput: %s", nameInput);

    scanf ("%s", nameOutput);
    strcat(nameOutput, ".wav");
    //printf("nameInput: %s", nameOutput);

    /// Opening the input file.
    FILE *input = fopen(nameInput, "rb");
    if (input == NULL)
    {
        printf ("Opening file: Couldn’t open file %s; %s.\n",
                 nameInput, strerror (errno));
        exit (EXIT_FAILURE);
    }
    else
        printf("Opening file: \"%s\" has been successfully opened.\n",
                nameInput);


    /// copying header (first 44 bytes) to `hdr`
    char buf[44];
    fread(&hdr, 1, sizeof(buf), input);


    /// hdr
    printf("\n*********************************\n");
    printf("       ChunkID: %s\n",     hdr.ChunkID   );         // Contains the letters "RIFF" in ASCII.
    printf("     ChunkSize: %d\n",     hdr.ChunkSize );         // 36 + SubChunk2Size (which we don't mention)
    printf("        Format: %s\n",     hdr.Format    );         // Contains the letters "WAVE"
    printf("   SubChunk1ID: %s\n",     hdr.SubChunk1ID   );  // Contains the letters "fmt "
    printf(" SubChunk1Size: %d\n",     hdr.SubChunk1Size );  // 16 for PCM.  This is the size of the rest of the Subchunk which follows this number.
    printf("   AudioFormat: %d\n",     hdr.AudioFormat   );  // PCM = 1 (i.e. Linear quantization). Values other than 1 indicate some form of compression.
    printf("   NumChannels: %d\n",     hdr.NumChannels   );  // Mono = 1, Stereo = 2, etc.
    printf("    SampleRate: %d\n",     hdr.SampleRate    );  // 8000, 44100, etc.
    printf("      ByteRate: %d\n",     hdr.ByteRate      );  // == SampleRate * NumChannels * BitsPerSample/8
    printf("    BlockAlign: %d\n",     hdr.BlockAlign    );  // == NumChannels * BitsPerSample/8. The number of bytes for one sample including all channels.
    printf(" BitsPerSample: %d\n",     hdr.BitsPerSample );  // 8 bits = 8, 16 bits = 16, etc.
    printf("   SubChunk2ID: %s\n",     hdr.SubChunk2ID);
    printf(" SubChunk2Size: %d\n",     hdr.SubChunk2Size);
    printf("\n*********************************\n");

    /// ...
    return 0;
}

Answer 1

%s 格式说明符导致 printf 打印以 null 结尾的字符串。您的字符串不是以 null 结尾的。

当 printf 尝试打印以 null 结尾的字符串时，它只会打印字符，直到遇到零字节为止。在您的情况下，这会导致字符串预期结尾之后的数据被解释为字符并打印，直到遇到零字节。