在尝试将字符串传递给函数时获取错误分段错误（核心转储）

Question

When I try to pass the current time as a string to the function logger in my code I get an error saying that "Segmentation fault (core dumped)". but when I put static char str[30] the error doesn't come. but with no errors the resulting file makes impossible to open it.

void logger(char * logType, int loggingLevel, char * massage)
{
    FILE *fp = fopen("log.txt", "a");
    fprintf(fp, "%s|%d|%s|%s",logType,loggingLevel,massage,currentTime());
    fclose(fp);   
}

char * currentTime(void)
{
    time_t rawtime;
    char str[30];
    char *string;
    struct tm * timeInfo;
    time(&rawtime);
    timeInfo = localtime(&rawtime);
    strftime(str, 26, "%Y:%m:%d %H:%M:%S", timeInfo);
    return str;

}

so previously I made the currentTime function like this

char * currentTime(void) {
    time_t rawtime;
    struct tm * timeinfo;
    time(&rawtime);
    timeinfo = localtime(&rawtime);
    return asctime(timeinfo);
}

it works fine but that is not the way I need to display the time.

Answer 1

You are returning a local variable from function currentTime() which is undefined behaviour .

Change function signature to: char * currentTime(char *inputBuffer, size_t bufLen)

Answer 2

You are returning a pointer to a local variable (here the str buffer) from the currentTime function.

You cannot do this, because as soon as local variables goes out of scope (that is when you leave the currentTime function), their content is undefined, they will most of the time contain garbage.

Therefore you must declare str as static:

static char str[30];

In that case str exists all the time during the execution of your program and not only during the execution of the currentTime function.

But this can cause other problems.

Example:

char *time1;
char *time2;

time1 = currentTime();
...
/* somewhat later */
time2 = currentTime();

/* now time1 and time2 point to the same memory location     */
/* which contains the current time at the second call        */

Or problems when you use threads.

Answer 3

For the reasons exposed in other answers your function is not reentrant meaning that each time you call it the result is overwritten. To create a dedicated instance for each call you can also simply use an strdup() to create a duplicate of your string that can be deleted using free() after use:

void logger(char * logType, int loggingLevel, char * massage)
{
    FILE *fp = fopen("log.txt", "a");
    char *sTime = currentTime();    //Get the value
    fprintf(fp, "%s|%d|%s|%s",logType,loggingLevel,massage, sTime);
    free(sTime);    //Release the string if no more needed.
    fclose(fp);   
}

char * currentTime(void)
{
    time_t rawtime;
    char str[30];
    struct tm * timeInfo;
    time(&rawtime);
    timeInfo = localtime(&rawtime);
    strftime(str, 26, "%Y:%m:%d %H:%M:%S", timeInfo);
    return strdup(str);
}