c - 在c中读取二进制文件的各个部分

Question

我正在编写一个程序来读取二进制文件的特定部分，但无法让它在文件中的正确位置读取正确的二进制文件长度。一段时间以来我一直在试图找出我做错了什么，但没有成功。

这是我的代码的样子

int project(char * rel, char * atr){  takes in the name of the relation to look in and the name of the attribute to look for

    char tmpstr[MAX_LEN+5], tmpstr2[MAX_LEN+5], attr[MAX_LEN+5], type[MAX_LEN+5], names[MAX_LEN+5];//attr is the attribute read by pass of fscanf()
    int numbytes = 0, numtups = 0, nummove = 0, skip = 0, i = 0;    //the length in bytes of the atr, number of tuples to read
    FILE* f1;
    FILE* f2;
    strcpy(tmpstr, rel);
    strncat(tmpstr,".sch", 4); // appends '.sch' extension to the file name
    if((f1 =(FILE*)fopen(tmpstr, "r")) == NULL){  // opens "InsertFileNameHere.sch"
        return -1;
    }
    if(fscanf(f1, "%d", &numattr) != 1){
        return -1;
    }
    strcpy(tmpstr2, rel);
    strncat(tmpstr2,".dat", 4); // appends '.dat' extension to the file name
    if((f2 =(FILE*)fopen(tmpstr2, "rb")) == NULL){  // opens "InsertFileNameHere.dat"
        return -1;
    }

    fscanf(f1, "%s", attr);  
    fscanf(f1, "%1s", type);
    fscanf(f1, "%d", &numbytes);
    while((strcmp(attr, atr) != 0) || numattr == 0){ //While the attr is = to the atr or you have run out of attributes to scan, keep scanning
        skip = skip + numbytes*2;
        fscanf(f1, "%s", attr);
        fscanf(f1, "%1s", type);
        fscanf(f1, "%d", &numbytes);
        numattr--;
        if(numattr == 0){
            return -2;
        }
        fseek(f2, skip, SEEK_CUR);
    }
    nummove = tuplen(rel) - numbytes*2;
    numtups = count(rel);
    printf("%d\n", nummove);
    while(i < numtups){
        fread(names, 1, numbytes*2, f2);
        fprintf(stdout, "%s\n", names);
        fseek(f2, skip, SEEK_CUR);
        i++;
    }
    puts("\n");
    fclose(f1);
    fclose(f2);
    return 1;

}

我用来测试此代码的文件如下:

Students.sch:该文件的第一行是文件中后续行的数量。每行都有一个字符串，后跟要从二进制文件读取的数据类型，然后是要从二进制文件读取的字节数。该文件用于告诉程序如何读取二进制文件。

5
Name   S  25
Major  S  4
Minor  S  4
Totcr  I  4
Majcr  I  4

Students.dat:该文件包含所有要读取的信息

536d 6974 682c 526f 6265 7274 0000 0000
0000 0000 0000 0000 0050 5359 0043 5349
0000 0000 3900 0000 2757 6f6f 6473 2c4a
616e 6500 0000 0000 0000 0000 0000 0000
0000 4353 4900 4255 5300 0000 0061 0000
0044 5261 6d73 6579 2c45 6c61 696e 6500
0000 0000 0000 0000 0000 0042 5553 0050
5359 0000 0000 6b00 0000 5857 6861 7274
6f6e 2c54 6f6d 0000 0000 0000 0000 0000
0000 0000 4255 5300 5053 5900 0000 0075
0000 0062 4261 6b65 722c 4e6f 726d 6100
0000 0000 0000 0000 0000 0000 0042 494f
0043 5349 0000 0000 2700 0000 19

感谢任何帮助!谢谢!

Answer 1

你这里有问题

fscanf(f1, "%s", attr);
fscanf(f1, "%1s", type);
fscanf(f1, "%d", &numbytes);

您必须确保读取正确的值，因此您需要检查 fscanf() 的返回值，并且所有这些都可以适合像这样的单个格式字符串

if (fscanf(f1, "%s%1s%d", attr, type, &numbyts) != 3)
    /* ... there is a problem with the data, do not continue ... */

此外，为每个 "%s" 说明符提供长度修饰符以防止缓冲区溢出。