这次作业很有用a2.pdf

atoi

  1. int atoi(const char *s)
  2. {
  3.     int n=0, neg=0;
  4.     while (isspace(*s)) s++;
  5.     switch (*s) {
  6.         case '-': neg=1;
  7.         case '+': s++;
  8.     }
  9.     /* Compute n as a negative number to avoid overflow on INT_MIN */
  10.     while (isdigit(*s))
  11.         n = 10*n - (*s++ - '0');
  12.     return neg ? n : -n;
  13. }

int型的最大值、最小值C/C++中int类型是32位的，范围是-2147483648到2147483647 。

strtok

我觉得作业里的这个函数有错

  1. char *my_strtok(char * s, const char * sep)
  2. {
  3.     static char *p = NULL;
  5.     if (s == NULL && ((s = p) == NULL))
  6.         return NULL;
  7.     s += strspn(s, sep);
  8.     if (!*s) 
  9.         return p = NULL;
  10.     p = s + strcspn(s, sep);
  11.     if (*p) 
  12.         *p++ = '\0';
  13.     else 
  14.         p = NULL;
  15.     return s;
  16. }

静态变量的值不能在编译时确定，应该是在执行时确定。C语言初始化一个全局变量或static变量时，只能用常量赋值，不能用变量赋值
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opendir/readdir

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  1. #include <dirent.h>
  2. #include <stdio.h>
  3. void list_filenames(const char *dirpath)
  4. {
  5.     DIR *dp = opendir(dirpath);
  6.     if (dp == NULL) return;
  7.     // loop through directory entries
  8.     struct dirent *entry;
  9.     while ((entry = readdir(dp)) != NULL) {
  10.         printf("%s\n",entry->d_name);
  11.     }
  12.     closedir(dp);
  13. }

PATH_MAX

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Implement the mywhich program

  1. //#include "samples/prototypes.h"
  2. #include <limits.h>
  3. #include <stdio.h>
  4. #include <string.h>
  5. #include <dirent.h>
  6. #include <stdbool.h>
  8. // You wrote this as an exercise in lab2! Grab the code from your lab checkoff sheet
  9. // at https://cs107.stanford.edu/cgi-bin/lab2
  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. }
  26. bool my_scan_token(const char **p_input, const char *delimiters, char buf[], size_t buflen)
  27. {
  28.   int deletion = strspn(*p_input, delimiters);
  29.   *p_input += deletion;
  30.   int num_tokens = strcspn(*p_input, delimiters);
  32.   if (num_tokens > 0) {
  33.     if (num_tokens > buflen - 1) {
  34.       strncpy(buf, *p_input, buflen - 1);
  35.       *p_input += buflen - 1;
  36.       buf[buflen - 1] = '\0';
  37.     } else {
  38.       strncpy(buf, *p_input, num_tokens);
  39.       *p_input += num_tokens;
  40.       buf[num_tokens] = '\0';
  41.     }
  42.     return true;
  43.   } else {
  44.     return false;
  45.   }
  46. }
  48. // This main is incomplete. It sketches the expected behavior for the case when
  49. // mywhich is invoked with no arguments. You are to first read and understand 
  50. // this code, work out how to change/extend to suit your needs, and finally add
  51. // comments to document your strategy.
  53. int main(int argc, char *argv[], const char *envp[])
  54. {
  55.   const char *searchpaths = get_env_value(envp, "MYPATH");
  56.   if (searchpaths == NULL)
  57.     searchpaths = get_env_value(envp, "PATH");
  59.   if (argc == 1){
  60.     char dir[PATH_MAX+1];
  61.     const char *remaining = searchpaths;
  63.     printf("Directories in search path:\n");
  64.     while (my_scan_token(&remaining, ":", dir, sizeof(dir))){
  65.       printf("%s\n", dir);
  66.     }
  67.   }else{
  68.     argc--;
  69.     argv++;
  70.     char dir[PATH_MAX+1];
  72.     //loop through arguments
  73.     for (int i = 0; i < argc; i++){
  74.       //check argument
  75.       if (!strlen(argv[i]))
  76.         continue;
  77.       unsigned char ifwild = 0;
  78.       if (argv[i][0] == '+'){
  79.         argv[i]++;
  80.         if (!strlen(argv[i]))
  81.           continue;
  82.         ifwild = 1;
  83.       }
  84.       const char *remaining = searchpaths;
  85.       //loop through dirs in path
  86.       while (my_scan_token(&remaining, ":", dir, sizeof(dir))){
  87.         DIR *dp = opendir(dir);
  88.         if (dp == NULL)
  89.           continue;
  91.         struct dirent *entry;
  92.         unsigned char ifbreak = 0;
  93.         //loop through files in dir
  94.         while ((entry = readdir(dp)) != NULL){
  95.           if (!ifwild){
  96.             if (!strcmp(argv[i], entry->d_name)){
  97.               printf("%s/%s\n", dir, entry->d_name);
  98.               ifbreak = 1;
  99.               break;
  100.             }
  101.           }else{
  102.             char *find = strstr(entry->d_name, argv[i]);
  103.             if (find)
  104.               printf("%s/%s\n", dir, entry->d_name);
  105.           }
  106.         }
  107.         closedir(dp);
  108.         if (ifbreak)
  109.           break;
  110.       }
  111.     }
  112.   }
  113.   return 0;
  114. }

一个比较综合的练习。