The First C Program

History

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Program

main Function

• A program exits successfully if and only if its main function returns 0 .
  • If the return type is compatible with int and control reaches the terminating } , the value returned to the environment is the same as if executing return 0;

printf & scanf

• #include <stdio.h>
• printf
  • Will %d convert? More below at
• scanf
  • %?: type of variable
  • NOTE: %dwill skip any leading whitespaces
    • "whitespace": space, newline || return(\n / \r), tab(\t), etc
      • Standard:
        whitespace characters: any single whitespace character in the format string consumes all available consecutive whitespace characters from the input (determined as if by calling isspace in a loop). Note that there is no difference between "\n" , "" , "\t\t" , or other whitespace in the format string.
      • More below at [[#Note about format string in scanf]]

Note about format string in scanf

TLDR

• %d: Reads nums when non-whitespace char occurred
• \n: Reads newline and discard any whitespace char after it (or until non-whitespace char occurred)

---

According to the ref:

• The format of the number is the same as expected by strtol with the value 10 for the base argument.
  • strol is a function to convert string to long int
    • ref: Interprets an integer value in a byte string pointed to by str
• How does strol parse a string?
  • Discards any whitespace characters (as identified by calling isspace ) until the first non-whitespace character is found, then...

Case I: "%d\n%d\n"

// Input #0
1 // Enter
2 // Enter
// Enter (Did not stop, still waiting)
3 // Enter
// Stopped, received 1, 2

// Input #1
1 2// Enter
// Enter (Did not stop, still waiting)
3 //Enter
// Stopped, received 1, 2

// Input #2
1 2 3//Enter
// Stopped, received 1, 2

Summary: It will wait you untill you've entered a non-whitespace character

Case II: %d\n%d

// Input #0
1 2// Enter
// Stopped, received 1, 2

Case III: %d%d

// Input #0
1 2// Enter
// Stopped, received 1, 2

WHY?

• %d: Reads nums when non-whitespace char occurred
• \n: Reads newline and discard any whitespace char after it (or until non-whitespace char occurred)

So

• Case I
  • Read 1\n
    • \n marks the end of %d, so scanf accpeted 1 and gave \n back to input buffer
  • Read \n (or many other whitespace charaters)
    • \n meets the need of \n, and scanf accepted it and wait until another whitespace char occurred(2)
  • Read 2\n
    • 2 is a non-whitespace charater, marks the end of \n
    • 2\n meets the need of the 2^nd %d, and give \n back to buffer again
  • Read \n*n
    • \n meets the need of \n, wait non-whitespace charater
  • Read 3
    • Here 3 is a non-whitespace charater, it marks the end of \n, stop.

Note about printf

• print %ld with %d?

// Case I:
double pi = 3.14;
printf("%d\n", pi);
// Output: 1374389535

// Case II:
double pi = 3.14;
int pi_int = pi; // pi_int 值为 3
printf("%d\n", pi_int);
// Output: 3

这里的 double 为什么这么奇怪

conversion specifier 与对应的变量类型不匹配是 undefined behavior，不是每个平台、环境都会得到这个输出

但是我们来理解一下：

在 x86-64 clang 的编译环境下，3.14 将会按照IEEE 754标准进行存储

.quad   0x40091eb851eb851f     # double 3.1400000000000001

二进制表示为：

0100000000001001000111101011100001010001111010111000010100011111

如果输出为 long long ，即可看到对应的数字：4614253070214989087