Implicit type conversion in C language, C Language

Implicit type conversion in C language, C Language

I read a blog post, which is described as follows:

1 #include <stdio.h>
  2   
  3 int main ()
  4 {
  5 unsigned short a = 1;
  6 unsigned short b = 0;
  7
  8 if (a <(b-1)) // The result -1 of a and b-1 is converted to int type for comparison, 1 <-1, obviously false
  9     {  
10 printf ("in if \ n");
11}
12
13 return 0;
14
15}

"In if" is printed in the expected results, but nothing is output during actual running. Change the following code to output "in if", which is consistent with the expectation.

1 #include <stdio.h>
  2   
  3 int main ()
  4 {
  5 unsigned short a = 1;
  6 unsigned int b = 0;
  7
  8 if (a <(b-1)) // The result of b-1 is converted to 0xffffffff, 1 <0xffffffff, obviously true.
  9     {  
10 printf ("in if \ n");
11}
12
13 return 0;
14
15}

Use assembler to directly compare the two results (the company is not secure from sending Internet images ..), the main differences are found in the following five rows. The short is extended in the first four rows, from 2 bytes to 4 bytes, and the short is increased to 0. The reason for the non-conformity between the running result and expectation lies in row 5th: jge 8048461

1 08048425 <main>:
 2  8048425:      8d 4c 24 04              lea    0x4(%esp),%ecx
 3  8048429:      83 e4 f0                 and    $0xfffffff0,%esp
 4  804842c:      ff 71 fc                 pushl  -0x4(%ecx)
 5  804842f:      55                       push   %ebp
 6  8048430:      89 e5                    mov    %esp,%ebp
 7  8048432:      51                       push   %ecx
 8  8048433:      83 ec 14                 sub    $0x14,%esp
 9  8048436:      66 c7 45 f6 01 00        movw   $0x1,-0xa(%ebp)
10  804843c:      66 c7 45 f4 00 00        movw   $0x0,-0xc(%ebp)
11  8048442:      0f b7 45 f6              movzwl -0xa(%ebp),%eax
12  8048446:      0f b7 55 f4              movzwl -0xc(%ebp),%edx
13  804844a:      83 ea 01                 sub    $0x1,%edx
14  804844d:      39 d0                    cmp    %edx,%eax
15  804844f:      7d 10                    jge    8048461 <main+0x3c>
16  8048451:      83 ec 0c                 sub    $0xc,%esp
17  8048454:      68 00 85 04 08           push   $0x8048500
18  8048459:      e8 a2 fe ff ff           call   8048300 <puts@plt>
19  804845e:      83 c4 10                 add    $0x10,%esp
20  8048461:      b8 00 00 00 00           mov    $0x0,%eax
21  8048466:      8b 4d fc                 mov    -0x4(%ebp),%ecx
22  8048469:      c9                       leave  
23  804846a:      8d 61 fc                 lea    -0x4(%ecx),%esp
24  804846d:      c3                       ret    
25  804846e:      66 90                    xchg   %ax,%ax

 

There are no symbols for the values in the Assembly, but the commands are signed separately. The jge used above is a symbolic comparison method, resulting in the comparison result: if (1 <-1), false, no string is printed

Command	Description	Operator number
Jbe	Unsigned below or equal (lower or same)	<=
Jae	Unsigned above or equal (higher or same)	> =
Jb	Unsigned below (lower)	<
Ja	Unsigned above (higher)	>
Jle	Signed less or equal	<=
Jge	Signed greater or equal	> =
Jl	Signed less	<
Jg	Signed greater	>