update file based on a key in C - c

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 file\n");
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 file\n");
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 %s\n", 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;
}
The code is in C language. I want to update the size variable when name is equal to "maze". How I can do it, while reading from file?
first the data that is writing in file is -
runner 100 4376482682
maze 300 3232365436
After updating -
runner 100 4376482682
maze 100 3232365436
size should updated from 300 to 100.

Answer 1

I have fixed a number of small bugs in your code related to opening the file in correct mode and checking if fwrite and fread where successful. Then I added the part you where really asking: update the record "maze".
To update the record, we need to read it, change the value and write it back to the file exactly where we have read it. So before reading a record, we query the current file pointer with ftell and before writing, we call fseek to move the file pointer back to the place we read. A call to fflush (another fseek would do as well) is required so that the next fread take place at the correct position.
For testing, I added a third record after "maze" so that we can see - by looking at the file content - that we don't overwrite data.
#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 file\n");
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 update\n");
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 position\n");
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 %s\n", input.name, input.size, input.timestamp);
if (strcmp(input.name, "maze") == 0) {
if (fseek(infile, pos, SEEK_SET) != 0) {
fprintf(stderr, "\nError moving file pointer\n");
break;
}
strcpy(input.size, "100");
if (fwrite(&input, sizeof(struct person), 1, infile) != 1) {
fprintf(stderr, "\nError writing file\n");
break;
}
// fflush() is required so that fread() take place a the correct position
if (fflush(infile) != 0) {
fprintf(stderr, "\nError flushing file\n");
break;
}
printf(" Updated id = %s name = %s %s\n", input.name, input.size, input.timestamp);
// Since this code update a single record, we could break the loop
}
}
// close file
fclose(infile);
return 0;
}