This algorithm doesn't work - c

Question

I've created a binary file with three persons (Code-Name-Sex) when I write the data into the file and then I read them, it work perfectly so..
I want that the next function read all the information of the X person have (if it exist).
Syntax:
#include <stdio.h>
struct alu{
int cod;
char name[30]; //alu[0]="juan" alu[1]="pedro" alu[2]="leo"
int sex;
};
int FSearch(char path[],char X[]) {
char Name[30];
FILE *arc;
arc=fopen(path,"rb");
fseek(arc,sizeof(int),SEEK_SET);
while (fread(Name,sizeof(char[30]),1,arc)) {
/*Here is when the errors happen..
The next sentence tell me that A.name don't have
the name from the second time*/
printf("%s and %s.",X,Name);
if (strcmp(Name,X)==0) return 1;
fseek(arc,2*sizeof(int),SEEK_CUR);
}
fclose(arc);
return 0
}
int main(int argc, char **argv)
{
char path[]="file.bin";
printf("\n%d",FSearch(path,"pedro"));
return 0;
}
The output is the following:
pedro and juan.pedro and .pedro and .
0
That means that is found the first name ('juan') but the second and third isn't (pedro and leo).
What is wrong?

Answer 1

Here's how it was:
fseek(arc,sizeof(int),SEEK_SET);
while (fread(Name,sizeof(char[30]),1,arc)) {
if (strcmp(A.name,X)==0) return 1;
fseek(arc,2*sizeof(int),SEEK_CUR); //--> ERROR
}
Here's how it should be:
fseek(arc,sizeof(int),SEEK_SET);
while (fread(Name,sizeof(char[30]),1,arc)) {
if (strcmp(A.name,X)==0) return 1;
fseek(arc,2*sizeof(int)+sizeof(char[2]),SEEK_CUR); //--> SOLVED
}
The problem was the 2nd argument, the Number of bytes to offset from origin, passed to fseek() call, within the while loop. It was counting two INTs but not the two CHARs (before reaching to the second string). You can see the sizeof(char[30]) after the fread() call, it shows 32 bytes, but 30 bytes are allocated to the string.
Why have to move two more bytes? Because any string have in its end, reserved bytes (for indicating the beginning and end of the string). e.g.:
char a[10]="Example";
If you save this into a binary file, this file will have a size of 12 bytes.

Answer 2

I would print the result of the fread call each time through the loop. I bet after the first object you're hitting EOF on the file. My prediction is that the first fread call returns 1, and the others return 0.

Answer 3

I don't know your binary file format.
Your code tell the binary file have names only with 30 characters.
struct alu{
int cod;
char name[30]; //juan - pedro - leo
int sex;
};
This structure size is not 38 bytes.
Check structure size.
printf("%d", sizeof(struct alu));
structure size depend on your compiler option...
If your binary format has bellow format.
{ // 1st record
cod = 10
names[30] = "juan"
sex = 1
}
{ // 2nd record
cod = 20
names[30] = "pedro"
sex = 1
}
{ // 3rd record
cod = 12
names[30] = "leo"
sex = 2
}
See your code.
#include <stdio.h>
#pragma pack(push, 1) or 4 or 8 depend on your binary format.
struct alu{
int cod;
char name[30]; //juan - pedro - leo
int sex;
};
#pragma pack(pop)
int FSearch(char path[],char X[]) {
char Name[30];
struct alu A;
FILE *arc;
arc=fopen(path,"rb");
//fseek(arc,sizeof(char[30]),SEEK_SET);
fseek(arc,0,SEEK_SET); // first record.
//while (fread(Name,sizeof(char[30]),1,arc)) {
while (fread(A,sizeof(A),1,arc)) {
/*Here is when the errors happen..
The next sentence tell me that A.name don't have
the name from the second time*/
printf("%s and %s.",X,A.name);
if (strcmp(A.name,X)==0) {
printf("Information of %s:\n",X);
//fshow_str(A);
fclose(arc);
return 1; // found
}
}
fclose(arc);
return 0; // not found
}
int main(int argc, char **argv)
{
char path[]="file.bin";
printf("\n%d",FSearch(path,"pedro"));
return 0;
}
cod size depend on compiler OPTION!!!
int FSearch(char path[],char X[]) {
char Name[30];
struct alu A;
FILE *arc;
arc=fopen(path,"rb");
fseek(arc,0,SEEK_SET); // first record.
//while (fread(Name,sizeof(char[30]),1,arc)) {
while (1) {
//if ( -1 == fseek(arc,4,SEEK_CUR)) // Important!!! case structure pack(4) for skip cod or
// break;
if ( -1 == fseek(arc,8,SEEK_CUR)) // Important!!! case structure pack(8) for skip cod
break;
if (!fread(Name,sizeof(Name),1,arc))
break;
A.name = Name;
/*Here is when the errors happen..
The next sentence tell me that A.name don't have
the name from the second time*/
printf("%s and %s.",X,A.name);
if (strcmp(A.name,X)==0) {
printf("Information of %s:\n",X);
//fshow_str(A);
fclose(arc);
return 1; // found
}
}
//if ( -1 == fseek(arc,4,SEEK_CUR)) // Important!!! case structure pack(4) for skip sex or
// break;
if ( -1 == fseek(arc,8,SEEK_CUR)) // Important!!! case structure pack(8) for skip sex
break;
fclose(arc);
return 0; // not found
}
Binary file and structure are alined with 4 bytes.
// Attention! pragma pack(push, 4) needs!!!
// Your now running machine is 32 bits system.
// This source code will invoke addressing fault 64 bits NON Intel processor.
#pragma pack(push, 4) // align 4 bytes.
struct alu{
int cod; // Integer type. 4 bytes on 32bits system. but 8 bytes on 64 bits system.
// I recommend using "long" type. "long" type is 4 bytes on any system.
char name[30]; //juan - pedro - leo
int sex; // also, Integer type.
};
#pragma pack(pop)
int FSearch(char path[],char X[]) {
char Name[30];
struct alu A;
FILE *arc;
arc=fopen(path,"rb");
fseek(arc,0,SEEK_SET); // first record.
//while (fread(Name,sizeof(char[30]),1,arc)) {
while (1) {
if ( -1 == fseek(arc,4,SEEK_CUR)) // Skip "cod" 4 bytes.
break;
if (!fread(Name,sizeof(Name),1,arc)) // Read "name" 30 bytes.
break;
if ( -1 == fseek(arc,2,SEEK_CUR)) // Skip "name"'s aligned 2 bytes.
break;
A.name = Name;
/*Here is when the errors happen..
The next sentence tell me that A.name don't have
the name from the second time*/
printf("%s and %s.",X,A.name);
if (strcmp(A.name,X)==0) {
printf("Information of %s:\n",X);
//fshow_str(A);
fclose(arc);
return 1; // found
}
}
if ( -1 == fseek(arc,4,SEEK_CUR)) // Skip "sex" 4 bytes.
break;
fclose(arc);
return 0; // not found
}

Answer 4

printf in main() printed the return value of FSearch(), if you want to see if the result is found, should at least return a bool value in FSearch()