Searching a string in a binary file in C

Question

In this code snippet I'm trying to add a string in a binary file. If the string already exists, I return error, otherwise I add it at EOF. I creataed two functions, void AddNewTeam(FILE* fp, char* newTeam), and int SearchTeam(FILE* fp, char* newTeam) as shown below, but that didn't work. Everytime I enter a string, it is added at EOF, even if is in the binary file. I think the problem is the fread function, I tried to print the value returned by the fread but it is always 0. Can someone help me trying to understand what's wrong with this code and why it's not working?

void AddNewTeam(FILE* fp, const char* newTeam){
    int found;

    if((fp = fopen("File.dat", "rb")) == NULL){
        fp = fopen("File.dat", "ab");
        fclose(fp);
    }
            
    printf("\tEnter New Team: ");
    scanf("%s", newTeam);
            
    found = SearchTeam(fp, newTeam);
            
    if(found == 0){
        if((fp = fopen("File.dat", "ab"))){
            fwrite(newTeam, sizeof(newTeam), 1, fp);
            printf("\tThe following team has been successfully loaded\n");
            fclose(fp);
        }
    }else if(found == 1){
        printf("\tThis team already exists\n");
        exit(EXIT_FAILURE);
    }
}

int SearchTeam(FILE* fp, const char* newTeam){
    char string[MAX][MAX_LENGTH];
    int counter, result, found = 0;
    if((fp = fopen("File.dat", "rb"))){
        fseek(fp, 0, SEEK_SET);
        for(counter = 0; !feof(fp); counter++){
            if(fread(string[counter], sizeof(string[counter]), 1, fp) == 1){
                result = strcmp(newTeam, string[counter]);
                if(result == 0){
                    found = 1; break;
                }else if(result != 0){
                    found = 0;
                }       
            }
        }
        fclose(fp);
        return found;
    }else if(fp == NULL){
        printf("\tError opening binary file\n");
        exit(EXIT_FAILURE);
    }
}

This is the main function and the function prototypes

int SearchTeam(FILE* fp, const char* newTeam);
void AddNewTeam(FILE* fp, const char* newTeam);

int main(void){
    FILE* fp;
    char newTeam[MAX_LENGTH];
    AddNewTeam(fp, newTeam);
    return 0;
}

Why do you need an array in `searchTeam`? You can use the same string for each `fread()`. — Barmar, Jun 02 '21 at 23:13
`for(counter = 0; !feof(fp); counter++){` Dont use feof(). Never. https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong — wildplasser, Jun 02 '21 at 23:20
Consider opening a single `FILE*` and passing that as a parameter to any function that needs it. That way you can avoid *Hardcoded Filenames* which you use to open a stream 4 -different times within the two functions. — David C. Rankin, Jun 03 '21 at 01:17
`const char* newTeam) .... scanf("%s", newTeam);` is a problem - writing to a `const`. Post `AddNewTeam()` calling code. — chux - Reinstate Monica, Jun 03 '21 at 04:02

score 3 · Answer 1 · answered Jun 02 '21 at 23:18

3

You're not appending to the file correctly. The search code expects each team name to be in a block of MAX_LENGTH bytes. But when you write, you do:

fwrite(newTeam, sizeof(newTeam), 1, fp);

newTeam is a pointer, not an array, so sizeof will be the size of a pointer, typically 4 or 8 bytes. You need to write MAX_LENGTH bytes so this will match the way you read from the file.

fwrite(newTeam, MAX_SIZE, 1, fp);

answered Jun 02 '21 at 23:18

Barmar

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Thanks! That worked! I've been struggling for hours on that code. Before using functions, I did the AddNewTeam and SearchTeam functions inside the main function and the code worked, why is that? – Tyler Lambert Jun 03 '21 at 06:26
Because in the main function `newTeam` was an array, not a pointer. Just like in `searchTeam`. – Barmar Jun 03 '21 at 13:46

Searching a string in a binary file in C

1 Answers1