C program reading and writing to binary files - c

Question

Hello this is the first time I am working with binary file in C.
I am trying to first write the data to the file and then I to read the same data from the file. But somehow I am not reading the data correctly (that's what I think) because I think the file writing part has no issue. And I am doing all of this in unix.
So basically I am trying to write and read data from the binary file in unix and I am not able to read the data properly. Any help would be appreciated. Thanks in advance.
I have attached the code and the output below so it is understandable:
#include <stdio.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/stat.h>
#include <stdlib.h>
int main(int argc, char *argv[]){
char data[100];
if(argc<2){
printf("Usage: gift <textstring>\n");
return -1;
}
int n = 0;
int num = 0;
char data[100];
if(argc<2){
printf("Usage: gift <textstring>\n");
return -1;
}
int n = 0;
int num = 0;
int fd = open("gifts.dat", O_CREAT | O_WRONLY | O_RDONLY);
if((strcmp(argv[1],"new"))==0){
for(int i = 2; i < argc; i = i+2){
int number = atoi(argv[i+1]);
strcpy(data, argv[i]);
int length = strlen(data) + 1;
write(fd,&length, sizeof(int));
write(fd,data, length);
write(fd,&number, sizeof(int));
}
}
else{
printf("Not worked");
}
for(int k =0;k < ((argc-2)/2);k++){
int length;
char*name = NULL;
int money;
read(fd, &length, sizeof(int));
name = malloc(length);
read(fd, name, length);
read(fd, &money, sizeof(int));
printf("%10s: %.2f\n", name, money);
}
return 0;
}
And then here is the output of the binary file
gcc -o gifts board.c
./gifts new patel 200 Ram 500
: 0.00
: 0.00
od -c gifts.dat
0000000 006 \0 \0 \0 p a t e l \0 310 \0 \0 \0 004 \0
0000020 \0 \0 R a m \0 364 001 \0 \0 \0 \0
0000034
cat gifts.dat
patelÈRamô{cslinux1:~/CS3377_hmb180006}
Make sure to use alias gcc='gcc -std=c99'before compiling with gcc

Answer 1

The primary problem is with your open() call. You are attempting to open the file as both read-only and write-only at the same time. From man 2 open:
The argument flags must include one of the following access
modes: O_RDONLY, O_WRONLY, or O_RDWR.
Refactoring your code as follows works fine.
char data[100];
int fd = open ("gifts.dat", O_CREAT | O_WRONLY);
int n = 0;
if ((strcmp(argv[1],"new")) == 0) {
for (int i = 2; i < argc; i = i+2){
int number = atoi (argv[i+1]);
strcpy (data, argv[i]);
int length = strlen(data) + 1;
write (fd, &length, sizeof(int));
write (fd, data, length);
write (fd, &number, sizeof(int));
n++;
}
}
else{
printf("Not worked");
}
close (fd);
fd = open ("gifts.dat", O_CREAT | O_RDONLY);
for (int k = 0; k < n; k++){
int length;
char name[100];
int money;
read (fd, &length, sizeof(int));
read (fd, name, length);
read (fd, &money, sizeof(int));
printf("%10s: %.2d\n", name, money);
}
close (fd);
Example Use/Output
$ ./bin/readwrite_bin new patel 200 Ram 500
patel: 200
Ram: 500
Additionally, you must validate all opens, reads, writes (and close - after write) by checking the return of each call to ensure is succeeds, or handle any error if it fails. You should replace atoi() with at minimum sscanf() or better strtol() as atoi() provides zero error detection and will happily accept atoi ("my cow");
Let me know if you have further questions.