Std Library Functions

* Library Calls And System Calls
- Library calls are parts of the language or application while system calls are part of OS. A system call would be triggered by using trap or interrupt. The C library is the same on every ANSI C implementation. They are calls to routines in a library and linked with user program. They executes in the user address space with lower calling overhead and counts as part of user time. On the other hand, the system calls are different in each OS. They are calls to the kernal for a service. They are entry points to the OS. They executes in the kernal address space with high calling overhead and counts as part of system time.

In practice, many C library functions do their jobs by making system calls.

* getchar
- Its return value is integer instead of char.

* strcpy, strcat, strncpy and strncat
- strcpy and strcat do not check the buffer size and the ending condition is '\0'. So it might cause buffer boundary problem. Try to use strncpy and strncat.

* strlen
- Its return value does not include the character '\0'

* scanf
- It expects a pointer to an integer instead of char to read an integer. The formats of float and double are different. %f for the float and %lf for the double. In printf, %f for both.

* memcpy and memmove
- memcpy could not copy overlapped memory blocks of src and dst and memmove could at a cost of performance.

* setbuf
- When the main function returns, the library would FREE and clean up the memory setbuf used. Therefore, this buffer could not be one in the stack and it is supposed to be memory in the heap or static/global array.

* fread, fseek and fwrite
- fseek needs to be called between the callings of fread and fwrite. One file can not be read and write in sequence without the state change.

* errno
- This global variable would always be the last error number. The general usage should be:
if (error return value)
{
     check errno
}