CODEWITHSUNDEEP
clinuxfile-access
Eoin O Connell
Eoin O Connell

Reputation: 101

Unexplained file NULL pointer for file that exists

I have a file that exists in the same directory as my .c and my executable under Linux. 99 times out of 100 it will open the file correctly and not return a null pointer. But this 1 time out of 100 it will return a null pointer, the code is shown below. Why would I only sometimes get a null pointer for this file? And why when I do get the null pointer does it fail to create a file with the same name if Linux thinks it doesn't exist?

int ID_Lookup(int serialNumber, int NodeID)
{
    FILE *nodeidfile = NULL;
    char temp[8];
    char *filebuff;
    int commapos, endpos;
    int fileserialnum = 0, id = 9, match = 0, x = 0, linechar = 0, y;
    size_t len = 0;

    nodeidfile = fopen("NodeIDs","r");
    if (nodeidfile == NULL)
    {
        printf("file not found, creating it\r\n");
        nodeidfile = fopen("NodeIDs","w");
        fclose(nodeidfile);
    }

    nodeidfile = fopen("NodeIDs","r");

    if (nodeidfile == NULL)
        return -1;

    printf("file found\r\n");
    fseek(nodeidfile,0,SEEK_END);
    len = ftell(nodeidfile);
    fclose(nodeidfile);

    return 0;
}

Upvotes: 0

Views: 376

Answers (1)

L. Scott Johnson
L. Scott Johnson

Reputation: 4382

If fopen succeeds the first time, you open the file again (abandoning the first FILE* -- you're no longer able to close that first file). This system eventually runs out of file handles.

You should handle the two cases independently.

int ID_Lookup(int serialNumber, int NodeID)
{
    FILE *nodeidfile = NULL;
    char temp[8];
    char *filebuff;
    int commapos, endpos;
    int fileserialnum = 0, id = 9, match = 0, x = 0, linechar = 0, y;
    size_t len = 0;

    nodeidfile = fopen("NodeIDs","r");
    if (nodeidfile == NULL)
    {
        printf("file not found, creating it\r\n");
        nodeidfile = fopen("NodeIDs","w");
        fclose(nodeidfile);
        return 0;
    }

    printf("file found\r\n");
    fseek(nodeidfile,0,SEEK_END);
    len = ftell(nodeidfile);
    fclose(nodeidfile);

    return 0; /* should return len? */
}

Upvotes: 3

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