The difference between getc and fgetc is that getc can be
implemented as a macro, whereas fgetc cannot be implemented as a macro.
This means three things:

The argument to getc should not be an expression with side effects.

Since fgetc is guaranteed to be a function, we can take its address.
This allows us to pass the address of fgetc as an argument to another
function.

Calls to fgetc probably take longer than calls to getc , as it
usually takes more time to call a function.

…

2.

Seems like the differences are, in 99.9% of the cases, meaningless.

One point which may make a difference - The man page says getc() may be implemented as a macro which evaluates stream more than once .

It could lead to strange behavior in some (not very useful) cases, e.g.:

FILE *my_files[10] = {...}, *f=&my_files[0];
for (i=0; i<10; i++) {
    int c = getc(f++);    // Parameter to getc has side effects!
}

If getc evaluates f++ more than once, it will advance f more than
once per iteration. In comparison, fgetc is safe in such situations.

http://stackoverflow.com/questions/18480982/getc-vs-fgetc-what-are-the-
major-differences

2014-04-16

C/C++/Linux

Clock time, User CPU time and System CPU time in UNIX?

The clock time, sometimes called wall clock time, is the amount of time the
process takes

to run, and its value depends on the number of other processes being run on
the system.

The user CPU time is the CPU time attributed to user instructions.

The system CPU time is the CPU time attributed to the kernel when it executes
on behalf of the process.

Wall-clock time is the time that a clock on the wall (or a stopwatch in hand)
would measure as having elapsed between the start of the process and ‘now’.

从程序起点到现在的时间

The user-cpu time and system-cpu time are pretty much as you said - the amount
of time spent in user code and the amount of time spent in kernel code.

程序在系统code和用户code上运行的CPU时间

The units are seconds (and subseconds, which might be microseconds or
nanoseconds).

The wall-clock time is not the number of seconds that the process has spent on
the CPU; it is the elapsed time, including time spent waiting for its turn on
the CPU (while other processes get to run).

refs:

APUE

http://stackoverflow.com/questions/7335920/what-specifically-are-wall-clock-
time-user-cpu-time-and-system-cpu-time-in-uni

2014-04-16

C/C++/Linux

Linux下的snprintf

snprintf是将format之后的字符传保存到s所指向的地方，但是最大的长度是由n指定的。

见例子：

#include <stdio.h>
#include <string.h>

int main()
{
  char tmp[2];
  char tmp2[25];
  int cx2 = snprintf(tmp, 7, "%s", "ddd22d");
  snprintf(tmp2, 25, "%d", cx2);
  puts(tmp2);
  puts(tmp);
  puts("\n");

  return 0;
}

上面的例子输出：

_6
ddd22d _

如果将snprintf中的7改成6，输出：

_6
ddd22 _

如果将snprintf中的7改成5，输出：

_6
ddd2 _

注意我的tmp的长度只是2，但是在三次的输出时每次的长度都不一样，这时因为snprintf会将format之后的字符串保存到n-1个长度的char
数组中去，

然后将第n个位置为\0. puts方法输出字符串是寻找\0结尾，所以每次的输出不一样，这个会不会也可以做成溢出来利用呢？

http://www.cplusplus.com/reference/cstdio/snprintf/

2014-04-10

杂项