man ascii

image.png
image.png

ASCII and ctype

  1. int isxdigit(int c)
  2. {
  3.     return isdigit(c) || ((unsigned)c|32)-'a' < 6;
  4. }

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|32保证了可以检测到A等大写字母，因为ASCII码为65，aASCII码为97，如果或上32，就相当于补上了他们的差值32。

strcpy and strncpy

  1. char *strcpy(char *dst, const char *src)
  2. {
  3.     char *result = dst;
  4.     while ((*dst++ = *src++)) ;
  5.     return result;
  6. }
  7. // taken from the strncpy man page
  8. char *strncpy(char *dest, const char *src, size_t n)
  9. {
  10.     size_t i;
  11.     for (i = 0; i < n && src[i] != '\0'; i++)
  12.     dest[i] = src[i];
  13.     for ( ; i < n; i++)
  14.     dest[i] = '\0';
  15.     return dest;
  16. }

Aside: In the days of yore, pointer arithmetic had a performance advantage over array indexing, but smart modern compilers make this largely moot. A lot of old code and old programmers haven’t yet gotten that memo. Pointer arithmetic is especially rife in code that wrangles C-strings where it is perhaps tolerable, but for arrays of type other than char, array indexing is almost always going to be more readable.

gdb x command

(gdb) x/8bc buf
image.pngbuf是main中的一个变量。

Using Valgrind

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backtrace
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myprintenv

echo $USER $HOST $HOME $SHELL
image.png
printenv, 打印环境变量。env，修改环境变量
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c编程获取并打印环境变量

  1. /* 
  2.  * File: myprintenv.c
  3.  * ------------------
  4.  * Read lab2 writeup for more information.
  5.  */
  7. #include <stdio.h>
  8. #include <string.h>
  10. const char *get_env_value(const char *envp[], const char *varname)
  11. {
  12.   for (int i = 0; envp[i] != NULL; i++){
  13.     //printf("%s\n", envp[i]);
  14.     char *result = strstr(envp[i], varname);
  15.     if (result == envp[i]){
  16.     //if result is null, don't do anything
  17.     //envp[i] can be null but will be blocked by for
  18.       result += strlen(varname);
  19.       if (*result == '=')
  20.         return ++result;
  21.     }
  22.   }
  23.   return NULL;    // TODO
  24. }
  27. /* Function: main
  28.  * --------------
  29.  * Ordinarily main() has just two arguments (argc/argv); this alternate
  30.  * prototype includes an optional third argument which is an array of
  31.  * environment variables (like argc/argv, the values passed to this third
  32.  * arg are set automatically by the operating system when the program is 
  33.  * launched). Programs can use either form for main. The 2-argument 
  34.  * version is more common, but this program will use the main with
  35.  * 3rd argument to get access to the environment variables.
  36.  * (In fact, many programmers don't even know about this optional 3rd
  37.  * argument, so now you're already elite! :-) )
  38.  *
  39.  * Each array entry in this third argument is a string (pointer char*), 
  40.  * where the string pointed to is of the form "NAME=value", for different
  41.  * environment variable NAMEs and configuration values. The array's last 
  42.  * entry is a NULL pointer, which acts as a sentinel to identify the
  43.  * end of the array (which is why we can get away with not including an 
  44.  * accompanying array size argument, as we would normally do when passing 
  45.  * arrays in C). 
  46.  */
  47. int main(int argc, char *argv[], const char *envp[])
  48. {
  49.     if (argc == 1) {
  50.         for (int i = 0; envp[i] != NULL; i++)
  51.             printf("%s\n", envp[i]);
  52.     } else {
  53.         for (int i = 1; i < argc; i++) {
  54.             const char *value = get_env_value(envp, argv[i]);
  55.             if (value != NULL)  // if not found, don't print anything
  56.                 printf("%s\n", value);
  57.         }
  58.     }
  59.     return 0;
  60. }

代码逻辑非常棒。