理解 C 语言中的 fseek（）

尝试更改为使用而不是fopenw+rb+

此外，如前所述，在创建文件时删除 肯定会在写入数据后推进文件偏移量（前提是确实写入数据）。fseekfwritefwrite

以下是使用下面修改后的代码观察到的输出。

gcc main.c
./a.out
51 a 0 0.000000

// main.c

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

typedef struct tools {
    int recordno;
    char toolname[50];
    int quantity;
    float cost;
} tools;

void recordprinter(tools a) {
    printf("%d %s %d %f\n", a.recordno, a.toolname, a.quantity, a.cost);
}

int main() {
    FILE * fp;
    
    // recommend for this example using w+
    // w because it creates the file if the file doesn't exist
    // r fails if the file doesn't exist (and that doesn't seem useful here)
    // + because you are reading and writing
    // avoiding b and choosing POSIX - linux
    // may be wrong, if libc docs says b is needed then use b
    // my doc "man fopen" says b is ignored
    fp = fopen("file.txt", "w+");
    // check return values, file pointer exist? fail if not
    if (fp==NULL) { printf( "oops file not opened\n" ); return 1; }
    tools * a = (tools * ) malloc(100 * sizeof(tools));
    for (int i = 0; i < 100; i++) {
        a[i].cost = 0;
        a[i].toolname[0] = 'a';
        a[i].toolname[1] = '\0';
        a[i].quantity = 0;
        a[i].recordno = i + 1;
    }
    // alternative way to save 100 objects
    // if ( fwrite(a, sizeof(tools), 100, fp) != 100 )
    // {
    //   printf( "oops 100 objects not written to file\n" );
    //  return 1;
    // }
    for (int i = 0; i < 100; i++) {
        fwrite(a + i, sizeof(tools), 1, fp);
        // remove fseek, not needed, fwrite does what is needed here
        //fseek(fp, sizeof(tools), SEEK_CUR);
        // I used fseek here just because fwrite doesnot move the cursor when
        // it writes something to the file.(and fwrite(a + i, sizeof(tools), 100, fp) gives weird gliches)
    }

    // no review after this line, it seems to do what author intends

    fseek(fp, 0, SEEK_SET); // to bring cursor back to start of the file.

    fread(a, sizeof(tools), 1, fp);

    fseek(fp, sizeof(tools) * 50, SEEK_SET); // now I expect the cursor to be at 51th structure.

    fread(a + 3, sizeof(tools), 1, fp); // I am now writing the 51th structure in a[3]    

    recordprinter(a[3]);
    // this gives output 51 as desired   

    return 0;
}