Thread: Mysteriously losing contents of a string between file I/O operations

Haven't worked with file I/O very much, and it feels like there's something happening behind the scenes here that I don't understand.

Here's the code I'm writing:

Code:

#include <stdio.h>
#include <stdlib.h>


char* getTeamCode(char* team);
int checkTeamInGame(char* teamCode, char* game);
char* getCell(char* line, int index);
int findNextCell(char* line, int index);
void addLineCSV(char* line, char* csv);



int main()
{
    char* OSUteamCode = malloc(sizeof(char)*5);
    char* OSU = "Oregon State";
    char* OSUgameLog = "OSUgamelog.csv";
    char* fileLine[1024];
    FILE* fp;

    OSUteamCode = getTeamCode(OSU);

    fp = fopen("2012/game.csv","r");



    while(fgets(fileLine,1024,fp))
   {
       if(checkTeamInGame(OSUteamCode,fileLine)){
            printf("Writing game %s\n",fileLine);
            addLineCSV(fileLine,OSUgameLog);
        }
    }

    fclose(fp);

    return 0;
}




/* Takes a team name in the form of a string
Returns the team's ID number as a string */
char* getTeamCode(char* team)
{
    FILE* stream = fopen("2012/team.csv","r");

    char line[1024];
    char teamCode[10];

    while (fgets(line, 1024, stream))
    {
        char temp[100];
        int i = 0;
        int j = 0;

        i = findNextCell(line, i);        // Look for first comma

        i++;                          // Hop past the comma and quote

        while(line[i] != '"')           // Copy current team name into temp
        {
            temp[j] = line[i];
            i++;
            j++;
        }
        temp[j] = '\0';                 // Append temp with null

        if(!strcmp(team, temp))         // Check to see if we have the right team
        {
            i = 0;
            j = 0;
            while(line[i] != ',')
            {
                teamCode[j] = line[i];
                i++;
                j++;
            }
            teamCode[j] = '\0';
            break;
        }

        free(temp);
    }

    fclose(stream);
    return teamCode;
}



/*Checks to see if a specific team is part of a game
Takes a team's code and a specific game
Returns 1 if a team is in the game, 0 if not*/
int checkTeamInGame(char* teamCode, char* game)
{
    char* temp;
    int i = 0;

    i = findNextCell(game, i);  //Skip game ID cell

    i = findNextCell(game, i);  //Skip date Cell

    temp = getCell(game, i);    //Store first team ID
    if(!strcmp(temp,teamCode))  //Check it
        return 1;
    else
    {
        i = findNextCell(game, i);      //Go to cell with Second team ID
        temp = getCell(game,i);         // Store second team ID
        if(!strcmp(temp,teamCode))      // Check it
            return 1;
        else
            return 0;
    }
    puts(teamCode);

}


/*Gets the content of a cell and returns it
Takes a line of a CSV and an index for that line
Returns a string containing the contents of one cell*/
char* getCell(char* line, int index)
{
    char* temp = malloc(sizeof(char)*100);
    int i = index;
    int j = 0;

    while(line[i] != ',' && line[i] != '\n')
    {
        temp[j] = line[i];
        i++;
        j++;
    }
    temp[j] = '\0';

    return temp;
}


/*Moves an index to the next cell in a line of a CSV
    THIS WILL NOT WORK PROPERLY IF THE CELL CONTAINS A RELEVANT COMMA
Takes a line of a csv and an index
Returns an index*/
int findNextCell(char* line, int index)
{
    int i = index;

    if(line[i] == ',') // Check to see if we're starting on a comma by mistake
        i++;
    while(line[i] != ',') // Travel down string to the next comma
        i++;
    i++; // Hop past comma

    return i;
}


/*Appends a CSV with the given line of text */
void addLineCSV(char* line, char* csv)
{
    FILE* fp = fopen(csv, "a+");
    fprintf(fp,"%s",line);
    fclose(fp);
}

What I can't figure out is why the placement of the code on line 21 above matters.

It grabs the correct string just fine. If I write a printf just below it, it prints the string it should correctly. However, if I do a printf of the string OSUteamCode below the fopen call on line 23, it prints blank.

So the first thing I did was move it below the fopen line. It worked, finding and outputting the first game in "game.csv" just fine, but not the other 11. Debugging with printf shows that the contents of OSUteamCode again disappear after the fopen call in the addLineCSV function.

I'm not understanding why that happens. The only thing I can figure is there's something going on with the file I/O commands that I just don't understand, but I can't find anything online that explains what that might be.

getTeamCode() is returning a pointer to the (first element of) an array that is local to that function. That array is not guaranteed to exist after the function returns. Any usage of that return value (in main()) therefore gives undefined behaviour.