GCC argument descriptor

Problem 1. Basic knowledge Problem 2. Polynomial evaluation Input format Output format Example Notes Problem 3. GCC argument descriptor Problem background Problem description

1. -Wall
2. -Wpedantic
3. -Wextra
4. -Werror
5. -o xxx
6. -I xxx
7. -std=xxx
8. Other Notes Example Problem 4. 6174 Hint Input format Output format Example Note Problem 5: CPU emulator Description Input format Output format Example Note Problem 1. Basic knowledge
9. Suppose c is a variable of type char. If c is a decimal digit character, how can we obtain its numeric value? int(c) c - 48 c - '0' c - '\0' *(int *)&c
10. Regarding undefined behaviors, which of the following statements is/are true? An undefined behavior results in one of a set of valid results. For example, the following code (assuming int is 32-bit) will assign the variable x with some value, but the value is not valid. int ival = 10000000; int x = ival * ival; An undefined behavior means that there are no restrictions on the behavior of the program. The standard does not require such programs to do anything meaningful. An undefined behavior means that two or more behaviors are permitted by the standard and that the compiler will choose a meaningful one. The behavior remains unchanged when the program is run again, although we don't know what the behavior is. Correct C programs shall not have undefined behaviors.
11. Suppose int is 32-bit and long long is 64-bit. Select the code snippet(s) that involve(s) integer overflow. int x = 42 / 0; int ival = 1000000; long long llval = ival * ival; unsigned uval = -42; unsigned u1 = 10000000; unsigned u2 = u1 * u1;
12. Select the code snippet(s) that involve(s) undefined behavior. int random(void) { int x; return x; } int main(void) { printf("%d\n", random()); } unsigned uval = -5; printf("%u\n", uval); int table[100]; int exists_in_table(int value) { for (int i = 0; i <= 100; ++i) if (table[i] == value) return 1; return 0; } int foo(int value) { printf("%d\n", value); } int main(void) { int x = 42; int y = foo(x); printf("%d\n", x + y); } int i = 42; printf("%d%d\n", i, i++); double pi = 3.14; printf("%d\n", pi);
13. Read the following code. void swap(int *pa, int *pb) { int tmp = *pa; *pa = *pb; pb = tmp; } int main(void) { int x = 10, y = 15; swap(&x, &y); printf("%d %d\n", x, y); } (1) Which of the following statements is/are true? The asterisk () in the function parameter declaration int *pa is a part of int , meaning that pa is a pointer to int. The asterisk () in the statement int tmp = pa is the dereference operator, which is used to access the object that pa points to. The asterisk () in the statement pb = tmp is the pointer symbol, meaning that pb is a pointer. The asterisk () in the statement *pb = tmp is the dereference operator, which is used to access the object that pb points to. (2) Which of the following statements is/are true? If we rewrite the function swap as follows void swap(int a, int b) { int tmp = a; a = b; b = tmp; } and replace swap(&x, &y) with swap(x, y) in the main function, the values of x and y are still exchanged successfully. The parameter declarations of swap can be rewritten as int *pa, pb, because pa and pb are of the same type. The parameter declarations of swap can be rewritten as int *pa, *pb, because pa and pb are of the same type. The function swap will output the values of *pa and *pb. None of the above. (3) Write down the type of each expression. For pointer types, place exactly one space between the pointee type and the first asterisk, and no spaces between consecutive asterisks (e.g. int *, int **, or int **, but not int or int * *). Expression pa *pa &pa x &x Type
14. Read the following code. int min_element(int *array, int l, int r) { int pos = l; while (l < r) { if (array[l] < array[pos]) pos = l; ++l; } return pos; } int a[] = {1, 4, 2, 8, 5, 7}; (1) Which of the following statements is/are true? min_element(array, l, r) returns array[i] . min_element(array, l, r) returns array[i] . min_element(array, l, r) returns the index of the first occurrence of array[i] . min_element(array, l, r) prints the index of the first occurrence of array[i] to the standard output. To make min_element(array, l, r) return the index of the last occurrence of array[i] , replace array[l] < array[pos] (on line 4) with array[l] <= array[pos]. (2) Suppose we have the following main function. int main(void) { printf("%d\n", min_element(a, 0, 6)); } min{ ∣ i ∈ [l,r)} min{ ∣ i ∈ [l,r]} min{ ∣ i ∈ [l,r)} min{ ∣ i ∈ [l,r)} min{ ∣ i ∈ [l,r)} Which of the following statements is/are true? The function main contains undefined behavior. The type of a is int [6]. The type of a is int *. The type of the parameter array is int [6]. The type of the parameter array is int [], where the length is determined at run-time and may change. The type of the parameter array is int *. Since we want to find the minimum element in the entire array a, we can simply write min_element(a), and the parameters l and r will be assigned with 0 and 6 automatically.
15. Given the functions swap and min_element as defined in the previous two questions. Read the following code. void sort(int *array, int n) { for (int i = 0; i < n - 1; ++i) { int min_pos = min_element(array, i, n); swap(&array[i], &array[min_pos]); // Note: If you have difficulty understanding this function, uncomment the following lines, run the code and look at the output. // for (int i = 0; i < n; ++i) // printf("%d ", array[i]); // printf("\n"); } } int main(void) { int a[] = {8, 5, 7, 1, 4, 2}; sort(a, 6); for (int i = 0; i < 6; ++i) printf("%d ", a[i]); printf("\n"); } (1) Regarding the sort function, which of the following statements is/are true? swap(&array[i], &array[min_pos]) is equivalent to swap(array + i, array + min_pos). At each iteration, the smallest element in {array[i], ..., array[n - 1]} is found and then placed at index i by swapping with array[i]. The effect of sort on the array is not changed if we rewrite the loop condition as i < n. sort(array, n) sorts the elements {array[0], ..., array[n - 1]} in non-decreasing order. (2) Which of the following statements is/are true? The program prints 1 2 4 5 7 8. If we replace sort(a, 6) as sort(a, 4), the output becomes 1 2 4 5 8 7. If we replace sort(a, 6) as sort(a, 3), the output becomes 5 7 8 1 4 2. If we replace sort(a + 2, 4), the output becomes 8 5 1 2 4 7, i.e. only the last four elements are sorted. Problem 2. Polynomial evaluation Given a polynomial P(x) of degree n n P(x) = a0 + a1x + ⋯ + anx n = ∑ aix i , i=0 we want to evaluate the polynomial at several points x0, x1, ⋯ , xm−1. Input format On the first line, a nonnegative integer n, which is the degree of the polynomial. On the second line, n + 1 numbers a0 , a1 , ⋯ , an separated by space, which are the coefficients of the polynomial. On the third line, a nonnegative integer m. Then m lines follow, the i-th of which is a number xi(i = 0, 1, ⋯ , m − 1). Output format m lines, the i-th of which (i = 0, 1, ⋯ , m − 1) is a number P (xi), rounded to three decimal places. Example Input 2 -0.5 1 2.5 5 0 -6.6 1000 -1 32 Output -0.500 101.800 2500999.500 1.000 2591.500 Notes It is guaranteed that n ⩽ 30. An array is enough to store the coefficients. Do not use heap memory. Your program will not be compiled and linked against the math library, so do not use the functions in <math.h>. The evaluation of P (xi) at a given point xi should be done using only one loop without call to standard library functions. Think about how to do this efficiently. Problem 3. GCC argument descriptor Problem background To compile C/C++ code, we need to execute a command that calls the compiler with some arguments. For example: gcc hello.c -o hello.exe -std=c17 -Wall -Wpedantic -Wextra This command compiles the C source code file hello.c with gcc, setting the output file to be hello.exe, setting the language standard to be ISO C17, and enabling some kinds of warnings and errors by -Wall -Wpedantic -Wextra. Taking a look at GCC's manual, you will find an overwhelmingly long list of supported arguments. 代写GCC argument descriptor Is there a tool that can generate some explanations of the arguments, or at least for some of the most common ones? For example, suppose the name of that tool is gcc_descriptor.exe (without .exe on Mac OS X and Linux). It would be helpful if the following command (with .\ replaced by ./ on Mac OS X and Linux) .\gcc_descriptor hello.c -o hello.exe -std=c17 -Wall -Wpedantic -Wextra could print hello.c: C source code as input file. -o hello.exe: Place the primary output in file hello.exe. -std=c17: Set the language standard to ISO C17. -Wall: Enable all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning). -Wpedantic: Issue all the warnings demanded by strict ISO C and ISO C++ and reject all programs that use forbidden extensions. -Wextra: Enable some extra warning flags that are not enabled by -Wall. Your task is to implement such a tool that generates explanations for some very common GCC arguments. Problem description Write a program that accepts command-line arguments, which are the arguments that we want to pass to GCC, and print the descriptions for them. The supported kinds of arguments are as follows.
16. -Wall Description: -Wall: Enable all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning).
17. -Wpedantic Description: -Wpedantic: Issue all the warnings demanded by strict ISO C and ISO C++ and reject all programs that use forbidden extensions.
18. -Wextra Description: -Wextra: Enable some extra warning flags that are not enabled by -Wall.
19. -Werror Description: -Werror: Make all warnings into errors.
20. -o xxx Description: -o xxx: Place the primary output in file xxx. where xxx is replaced by the actual file name (see the example below). Note that this option consists of two arguments, where the first one is -o and the second one is the file name.
21. -I xxx Description: -I xxx: Add the directory xxx to the list of directories to be searched for header files during preprocessing. where xxx is replaced by the actual directory name (see the example below). Note that this option consists of two arguments, where the first one is -I and the second one is the directory name.
22. -std=xxx Description: -std=xxx: Set the language standard to yyy. where xxx is replaced by the actual value (see the example below) and yyy is replaced by the name of the standard according to the following table. xxx yyy example example output cN ISO CN c17 ISO C17 c++N ISO C++N c++20 ISO C++20 gnuN GNU dialect of CN gnu11 GNU dialect of C11 gnu++N GNU dialect of C++N gnu++14 GNU dialect of C++14 N consists of two digits. For example, the 2003 ISO C++ standard is named C++03.
23. Other Anything that does not match the patterns above is treated as an input file. Description: xxx: yyy as input file. where xxx is replaced by the file name and yyy is replaced by the type of the file according to the following table. xxx yyy example *.c C source code hello.c *.h C/C++ header file stdio.h *.cpp, *.C, *.cc, *.cxx C++ source code checker.cc, game.cpp *.hpp, *.hxx C++ header file utils.hpp Notes There is no input for this problem. The arguments to be explained are given as the command line arguments of your program. It is guaranteed that the arguments are valid: Anything that does not match the patterns in 1 ~ 7 must be a valid C/C++ source or header file name as described in the table above. For the language standards (pattern 7), you don't need to check whether the standard that it refers to actually exists. Things like - std=c++100000 won't appear. The names of the files and directories involved in the arguments do not contain whitespaces or quotes. The name following -o or -I is not a C/C++ source or header file. Print the explanations in order of the arguments. Please be extremely careful about the output contents, especially the spaces, newlines and punctuations. Example Suppose the name of your program (executable) is my_program.exe (without .exe on Mac OS X and Linux). If the following command (with .\ replaced by ./ on Mac OS X and Linux) is executed, .\my_program -std=c++20 -Wall -Wpedantic -Wextra a.c b.hxx c.cpp -Werror -o output_file -I /usr/local/boost_1_80_0/ the output is -std=c++20: Set the language standard to ISO C++20. -Wall: Enable all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning). -Wpedantic: Issue all the warnings demanded by strict ISO C and ISO C++ and reject all programs that use forbidden extensions. -Wextra: Enable some extra warning flags that are not enabled by -Wall. a.c: C source code as input file. b.hxx: C++ header file as input file. c.cpp: C++ source code as input file. -Werror: Make all warnings into errors. -o output_file: Place the primary output in file output_file. -I /usr/local/boost_1_80_0/: Add the directory /usr/local/boost_1_80_0/ to the list of directories to be searched for header files during preprocessing. Problem 4. 6174 The number 6174 is known as Kaprekar's constant after the Indian mathematician D. R. Kaprekar. This number is renowned for the following rule:
24. Take any four-digit number, using at least two different digits (leading zeros are allowed).
25. Arrange the digits in descending and then in ascending order to get two four-digit numbers, adding leading zeros if necessary.
26. Subtract the smaller number from the bigger number.
27. Go back to step 2 and repeat. The above process, known as Kaprekar's routine, will always reach its fixed point, 6174, in at most 7 iterations. Once 6174 is reached, the process will continue yielding 7641 − 1467 = 6174. For example, choose 1459: Write a program that simulates this process. Note that The input number should not contain more than 4 digits and should contain at least two different digits (i.e. not a repdigit like , , ...). These numbers are said to be invalid. The input number may contain less than 4 digits. For example, start with : Hint The functions given in [Problem 1] Basic knowledge may be helpful for this problem. Input format On the first line, a nonnegative integer . Then lines follow, the -th of which contains a nonnegative integer . It is guaranteed that is representable by int. Output format For each of the input integers, either report that it is invalid or simulate the Kaprekar's routine and print the steps (see below). If for some the integer contains more than 4 digits, print xxx contains more than 4 digits. where xxx is replaced with . If contains no more than 4 9541 8820 8532 − 1459 − 288 − 2358 = 8082 = 8532 = 6174 1111 2222 9 9000 9981 8820 8532 − 9 − 1899 − 288 − 2358 = 8991 = 8082 = 8532 = 6174 n n i xi xi n i xi xi xi digits but is a repdigit, print xxx is a repdigit. where xxx is replaced with xi . A step in the Kaprekar's routine should be printed in the form xxx - yyy = zzz, where xxx, yyy and zzz are replaced with the corresponding numbers. Note that leading zeros are not printed (see the example below). The process stops when zzz reaches 6174. If the input is already 6174, you should print nothing and start processing next input. You don't have to start printing after all inputs are consumed! Do not waste efforts saving the things to be printed. Example Input 5 123456 0 22 4444 1459 Output 123456 contains more than 4 digits. 0 is a repdigit. 2200 - 22 = 2178 8721 - 1278 = 7443 7443 - 3447 = 3996 9963 - 3699 = 6264 6642 - 2466 = 4176 7641 - 1467 = 6174 4444 is a repdigit. 9541 - 1459 = 8082 8820 - 288 = 8532 8532 - 2358 = 6174 Note Your program will not be compiled and linked against the math library, so do not use the functions in <math.h>. Problem 5: CPU emulator Description To understand how the computer executes your program, let me give a short introduction to CPU. The CPU basically contains 2 parts: registers and ALU. You can consider registers as an array. Each cell of this array has a name (for example, x0, x1, x2, …, x31 in RISC-V), and can store a single number. Arithmetic calculations (addition, subscription, multiplication, division, ...) are done by ALU. When executing your program, the assembly codes (written in binary) are sent to CPU. Each line of assembly code specifies some operation of registers, and CPU will use a special register named PC (program counter) to identify the current execution line number. For example, if the code is add x3 x1 x2, then CPU will take the values in registers x1 and x2, add them up and store the result in the register x3. This is shown in the figure below (only 5 registers shown here). Then, the CPU increments PC by 1 to execute the next instruction. In this problem, you are required to implement a toy emulator which is able to execute a very simple assembly language (similar to RISC-V). You only need to consider 6 registers, named x0, x1, x2, x3, x4 and x5, all in lowercase, and each register stores a 16-bit integer. The x0 register is a special one, whose value is always zero. Also, there are only has 6 instructions in this assembly language: add, sub, mul, div, let and print, which are introduced below. In this problem, we use a 16-bit machine code. An example of a 16-bit instruction is as follows. The instructions add, sub, mul and div are similar. The syntax is shown in the following table. Note that in these operations, the imm part is discarded. Instruction Syntax Explanation OpCode add add r1 r2 value in r1 += value in r2 000 sub sub r1 r2 value in r1 -= value in r2 001 mul mul r1 r2 value in r1 *= value in r2 010 div div r1 r2 value in r1 /= value in r2 011 The let instruction is used to assign value to a register. The syntax is: Instruction Syntax Explanation OpCode let let r1 imm value in r1 = imm 100 where r1 is the name of some register (x0 to x5), and imm is an integer within the range . In this instruction, the r2 part is discarded and the imm part is used. For example, the execution of instruction let x1 80 is shown below. Also, we need a print instruction to print the value in some register so that we can test whether the execution result is correct. For example, if the value in register x1 is 5, your emulator should print x1 = 5 on the screen when the instruction print x1 is executed. The r2 part is also discarded in this instruction. Instruction Syntax Explanation OpCode print print r1 print value in r1 101 [0, 2 7 − 1] Notes:
28. In the div instruction, the division result is truncated towards zero, and the denominator will always be non-zero.
29. Value in all registers should be initialized to 0 before execution.
30. Remember that the value in x0 is always zero. Any instruction that attempts to modify it should have no effect.
31. We only consider unsigned immediate in this problem. Input format The first line of the input is a number n, indicating the total lines of code. The following n lines are the instructions written in hexadecimal. Output format Your emulator should execute the input program, and only print the result when a print instruction is executed. It is guaranteed that the input and output are both non-empty. Example Input: 2 0x8401 0xA400 Output: x1 = 1 Input: 10 0x8401 0x8801 0x8C01 0x9001 0x9401 0xA400 0xA800 0xAC00 0xB000 0xB400 Output: x1 = 1 x2 = 1 x3 = 1 x4 = 1 x5 = 1 Explanation: 0x8401 is 1000010000000001 in binary, whose opcode is 100, r1 is 001 (x1), r2 is 000 (x0) and imm is 0000001 (1). From the opcode we know that this is a let instruction, so the instruction is let x1 1. 0xA400 is 1010010000000000 in binary, whose opcode is 101 and r1 is 001 (x1). This is a print instruction, so the instruction is print x1. Note Your program will not be compiled and linked against the math library, so do not use the functions in <math.h>. WX：codinghelp