Thread: Low level file I/O showing 'Segmentation fault (core dumped) error'

Hello friends,

Please have a look in the below given C code programmed in Ubuntu Linux:

Code:

void
Log(char *message)
{
    mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH;
    int fd;
    size_t length = strlen(message);
    
    if (file_exists(logfile))
        fd = open(logfile, O_WRONLY | O_APPEND | O_CREAT, 0666);
    else
        fd = open(logfile, O_WRONLY | O_EXCL | O_CREAT, 0666);

    if(fd == -1)
    {
        printf("%s\n", "Log file open/create error.");
        return;
    }
    else
        fd = write(fd, message, length);
}

Even this code is showing me the same error:

Code:

void
Log(char *message)
{
    mode_t mode = 0666;
    FILE *fd;
    
    if (file_exists(logfile))
        fd = fopen(logfile, "a");
    else
        fd = fopen(logfile, "w");

    if(fd == NULL)
    {
        fprintf(stderr, "Log file open/create error.");
    }
    else
        fprintf(fd, "%s", message);
    
    fclose(fd);
}

The above code is giving me error:

Code:

Segmentation fault (core dumped)

Please help.

There is nothing wrong with that function, per-se, except open() is specific to your operating system (i.e. your usage is not standard C) and that the function does not close the file (which means, each time the function is called, it will leave an open file, and eventually exceed process thresholds for number of open files. However, that code will report an error if it can't open the logfile, rather than just giving a segmentation fault.

The problem is therefore almost certainly in the caller. The caller must ensure that the message passed is valid: specifically, that it is non-NULL, it is initialised to a valid address, and it points at a buffer with a zero character before its end.

For example, each of the following uses of Log() can potentially result in a segmentation fault.

Code:

    Log((char *)0);    /*  passing a NULL pointer */

    char *x;    /*  note that x is not initialised, so accessing its value gives undefined behaviour */
    Log(x);

    char buffer[] = {'A', 'B'};    /*   note no terminating zero char */
    Log(buffer);

    char *p = malloc(10);
    strcpy(p, "Hello");
    free(p);
    Log(p);     /*  passing a buffer that has been released */