If I have an integer, how can I put it in a time_t* structure?

[tom66]

This part is giving problems:



raw_time = (time_t *)(cctvTxt.start_time + (short)(frame / cctvTxt.fps));
time_info = localtime(raw_time);

sprintf(time_format, "%s\n%s", cctvTxt.date_row1, cctvTxt.date_row2);
strftime(time_buffer, 80, time_format, time_info);


raw_time is a time_t value.
time_info is a struct tm *time_info
time_format and time_buffer both have 1,000 bytes allocated to them.

But, when running it, I get a segmentation fault. What have I done wrong?

Thanks,
Tom

[nvteighen]

Why is raw_time being cast to time_t *? Shouldn't it be just time_t? Or, if raw_time is a pointer, then you should do *raw_time = ....

[tom66]

Casting to just time_t causes a compilation error (invalid conversion or something like that). I'll try the other method you suggested.

Thanks

[nvteighen]

Casting to just time_t causes a compilation error (invalid conversion or something like that). I'll try the other method you suggested.

Thanks
Then raw_time is a pointer... if you want to change the contents of the address that pointer points to, you have to use the * operator. Otherwise, you'll be just changing the address the pointer points to... And who knows what the hell is there...

[tom66]

Sorry, I forgot to mention it was. In several C++ guides I've read, I've seen that it must be a pointer for some reason. Using a normal value doesn't work.

However, this code still throws a segmentation fault. I have narrowed it down to the topmost line, as a printf statement after there doesn't get printed. So, I'm confused.



*raw_time = (cctvTxt.start_time + (short)(frame / cctvTxt.fps));
time_info = localtime(raw_time);

sprintf(time_format, "%s\n%s", cctvTxt.date_row1, cctvTxt.date_row2);
strftime(time_buffer, 80, time_format, time_info);

[nvteighen]

Hmm... Compile using the -g flag and run the code using gdb.



gdb myprog


Use the "run" command at gdb to make it start... when it segfaults, it will throw the exact line where it occurred.

[Tony Flury]

in my experience - a segfault is often not caused by the line that actually fails.

It could be a memory allocation problem many lines before.

[nvteighen]

in my experience - a segfault is often not caused by the line that actually fails.

It could be a memory allocation problem many lines before.
No and yes: the SEGFAULT hits exactly at the illegal access line. Of course, then you have to backtrack where the error comes from.

[tom66]

My compiler line is, which includes GDB, libmath, and libgd:



g++ timestamp.c -g -lm -lgd


which isn't giving much useful information:



[New Thread 0xb7a538d0 (LWP 3878)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7a538d0 (LWP 3878)]
0xb7fad754 in ?? () from /lib/ld-linux.so.2

[nvteighen]

My compiler line is, which includes GDB, libmath, and libgd:



g++ timestamp.c -g -lm -lgd


which isn't giving much useful information:



[New Thread 0xb7a538d0 (LWP 3878)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7a538d0 (LWP 3878)]
0xb7fad754 in ?? () from /lib/ld-linux.so.2


I love the ?? () warnings from gdb... :p

Do the same and then, use the "bt" command to look at the backtrace and see from where that function has been called.

[tom66]

I ended up fixing my problem, but my going down a route I didn't want to take... it's a very esoteric route to take.

I used PHP, wrote a small script and piped it's output into a file, then read the file, using that in my program. I expected it to be a performance hog, but it's actually OK.

Thanks for your help.

[dwhitney67]

Sorry, I forgot to mention it was. In several C++ guides I've read, I've seen that it must be a pointer for some reason. Using a normal value doesn't work.

However, this code still throws a segmentation fault. I have narrowed it down to the topmost line, as a printf statement after there doesn't get printed. So, I'm confused.



*raw_time = (cctvTxt.start_time + (short)(frame / cctvTxt.fps));
time_info = localtime(raw_time);

sprintf(time_format, "%s\n%s", cctvTxt.date_row1, cctvTxt.date_row2);
strftime(time_buffer, 80, time_format, time_info);

This problem could have been solved in a couple of minutes if you had provided more code that demonstrated how raw_time was declared and initialized. Instead you found a questionable workaround.

time_t is nothing more than a value stored in a 4-byte space (I believe it is defined to be a 'long' integer).

Thus the proper usage would be something along the lines of:


#include <time.h>
...

time_t t = (cctvTxt.start_time + (short)(frame / cctvTxt.fps));

struct tm* tm = localtime(&t);

...

where struct tm has the following definition:


struct tm {
int tm_sec; /* seconds */
int tm_min; /* minutes */
int tm_hour; /* hours */
int tm_mday; /* day of the month */
int tm_mon; /* month */
int tm_year; /* year */
int tm_wday; /* day of the week */
int tm_yday; /* day in the year */
int tm_isdst; /* daylight saving time */
};