基于C中的密钥更新文件

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct person 
{
char  name[10];
char size[6];
char timestamp[15];
};

int main ()
{
FILE *outfile;

// open file for writing
outfile = fopen ("ads.txt", "a");
if (outfile == NULL)
{
fprintf(stderr, "nError opend filen");
exit (1);
}

struct person input1 = {"runner", "100", "4376482682"};
//struct person input2 = {"maze", "300", "3232365436"};

// write struct to file
fwrite (&input1, sizeof(struct person), 1, outfile);
//fwrite (&input2, sizeof(struct person), 1, outfile);

if(fwrite != 0) 
printf("contents to file written successfully !n");
else 
printf("error writing file !n");

// close file
fclose (outfile);

FILE *infile;
struct person input;
infile = fopen ("ads.txt", "r");
if (infile == NULL)
{
fprintf(stderr, "nError opening filen");
exit (1);
}

// read file contents till end of file
char name[10] = "maze";
char size[6] = "500";
char timestamp[15] = "437838322";
int remaining_size = 100;
int alreadythere =0;
//unpcoming file size
int incoming_file_size = 200;
int target_file_size_toremove = incoming_file_size - remaining_size;
while(fread(&input, sizeof(struct person), 1, infile)){
if(target_file_size_toremove > 0) {
int x = atoi(input.size);
if(target_file_size_toremove < x) {
strcpy(input.name, name);
strcpy(input.size, size);
strcpy(input.timestamp, timestamp);
}
}else {
if(strcmp(input.name, name) == 0) { // if name is eqUAL
if(strcmp(input.size, size) != 0) {
strcpy(input.size, size);
}
if(strcmp(input.timestamp, timestamp) !=0) {
strcpy(input.timestamp, timestamp);
}
alreadythere = 1;
}
printf ("id = %s name = %s %sn", input.name,
input.size, input.timestamp);
}
}
if(alreadythere == 0) {
struct person incoming = {name, size, timestamp};
fwrite (&incoming, sizeof(struct person), 1, outfile);
}
// close file
fclose (infile);
return 0;
}

代码是用C语言编写的。当name等于"1"时，我想更新CCD_；迷宫"；。如何在读取文件时做到这一点？

首先，写入文件的数据是-
runner 100 4376482682迷宫300 3232365436

更新后-
runner 100 4376482682
迷宫100 3232365436

大小应从300更新到100。

Answer 1

我已经修复了代码中的一些小错误，这些错误与以正确的模式打开文件以及检查fwrite和fread是否成功有关。然后我添加了你真正要求的部分：更新记录"迷宫"；。

要更新记录，我们需要读取它，更改值，然后将它写回我们读取它的位置。因此，在读取记录之前，我们用ftell查询当前文件指针，在写入之前，我们调用fseek将文件指针移回我们读取的位置。需要调用fflush(另一个fseek也可以(，以便在正确的位置发生下一个fread。

为了测试，我在"；迷宫"；这样我们就可以通过查看文件内容来看到，我们不会覆盖数据。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct person
{
char name[10];
char size[6];
char timestamp[15];
};
int main()
{
FILE* outfile;
// open file for writing binary
outfile = fopen("ads.txt", "wb");  
if (outfile == NULL) {
fprintf(stderr, "nError opening filen");
exit(1);
}
struct person input1 = { "runner", "100", "4376482682" };
struct person input2 = { "maze",   "300", "3232365436" };
struct person input3 = { "street", "400", "4232365486" };
// write struct to file, checking for success
if ((fwrite(&input1, sizeof(struct person), 1, outfile) != 1) ||
(fwrite(&input2, sizeof(struct person), 1, outfile) != 1) ||
(fwrite(&input3, sizeof(struct person), 1, outfile) != 1))
printf("error writing file !n");
else
printf("contents to file written successfully !n");
// close file
fclose(outfile);

FILE* infile;
struct person input;
// Open for both reading and writing, binary
infile = fopen("ads.txt", "r+b");
if (infile == NULL) {
fprintf(stderr, "nError opening file for updaten");
exit(1);
}
// read file contents till end of file, update "maze"
while (1) {
long pos = ftell(infile);
if (pos < 0) {
fprintf(stderr, "nError getting file positionn");
break;
}
if (fread(&input, sizeof(struct person), 1, infile) != 1)
break;
// update the value of size here, if name is equal to "maze"
printf("id = %s name = %s %sn", input.name, input.size, input.timestamp);
if (strcmp(input.name, "maze") == 0) {
if (fseek(infile, pos, SEEK_SET) != 0) {
fprintf(stderr, "nError moving file pointern");
break;
}
strcpy(input.size, "100");
if (fwrite(&input, sizeof(struct person), 1, infile) != 1) {
fprintf(stderr, "nError writing filen");
break;
}
// fflush() is required so that fread() take place a the correct position
if (fflush(infile) != 0) {
fprintf(stderr, "nError flushing filen");
break;
}
printf("  Updated id = %s name = %s %sn", input.name, input.size, input.timestamp);
// Since this code update a single record, we could break the loop
}
}
// close file
fclose(infile);
return 0;
}