c语言 - 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(只张贴必要的行)。这里我复制了WAVE文件的前44个字节(PCM，即线性量化)。问题是在显示一个结构的char[4]记录。

不明白，虽然我试图了解原因，但没有结果。

问题:为什么我必须使用精度修饰符%.*s只显示char[N]的前4个符号，而不是使用%s只打印char[4]变量?

BUT如果我给%s -> %.4s写修饰符，printf()函数正确打印所有内容:"RIFF"，"WAVE"，"fmt"，"data"。它只显示每个char[4]记录的前4个符号。但是那些没有显示出来的符号呢?

我得到以下%s:

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.

---

代码如下:

#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: %sn",     hdr.ChunkID   );         // Contains the letters "RIFF" in ASCII.
    printf("     ChunkSize: %dn",     hdr.ChunkSize );         // 36 + SubChunk2Size (which we don't mention)
    printf("        Format: %sn",     hdr.Format    );         // Contains the letters "WAVE"
    printf("   SubChunk1ID: %sn",     hdr.SubChunk1ID   );  // Contains the letters "fmt "
    printf(" SubChunk1Size: %dn",     hdr.SubChunk1Size );  // 16 for PCM.  This is the size of the rest of the Subchunk which follows this number.
    printf("   AudioFormat: %dn",     hdr.AudioFormat   );  // PCM = 1 (i.e. Linear quantization). Values other than 1 indicate some form of compression.
    printf("   NumChannels: %dn",     hdr.NumChannels   );  // Mono = 1, Stereo = 2, etc.
    printf("    SampleRate: %dn",     hdr.SampleRate    );  // 8000, 44100, etc.
    printf("      ByteRate: %dn",     hdr.ByteRate      );  // == SampleRate * NumChannels * BitsPerSample/8
    printf("    BlockAlign: %dn",     hdr.BlockAlign    );  // == NumChannels * BitsPerSample/8. The number of bytes for one sample including all channels.
    printf(" BitsPerSample: %dn",     hdr.BitsPerSample );  // 8 bits = 8, 16 bits = 16, etc.
    printf("   SubChunk2ID: %sn",     hdr.SubChunk2ID);
    printf(" SubChunk2Size: %dn",     hdr.SubChunk2Size);
    printf("n*********************************n");
    /// ...
    return 0;
}

Answer 1

%s格式说明符使printf打印一个以空结尾的字符串。你的字符串不是以空结束的

当printf尝试打印以空结尾的字符串时，它只是打印字符，直到遇到零字节。在您的示例中，这会导致字符串预期结束后的数据被解释为字符并打印，直到遇到零字节。