K&R c book a stripped-down version of Unix sort

When I do this examples on section 5.6, which is a sort program to sort a set of textlines into alphabetic order: I just write the code and start to run it, and I found something not quite right:

#include <stdio.h>  #include <string.h>  #include "alloc.c"  #define MAXLENGH 100 /* max length per line  */  #define MAXLINES 100 /* max lines to be sorted  */  char *lineptr[MAXLINES];/* pointers to the text lines  */  int readlines(char *lineptr[], int nlines);  void writelines(char *lineptr[], int nlines);  void strcopy(char *s, char *t);  void qsort(char *linestr[], int left, int right);  int stringcmp(char *, char *);  /* sort input lines  */  int main(){     int nlines; /* the number of input lines read  */     if ((nlines = readlines(lineptr, MAXLINES)) > 0){         qsort(lineptr, 0, nlines -1);         writelines(lineptr, nlines);         return 0;     }else{         return 1;     }  }  int my_getline(char *, int);  /* read input lines  */  int readlines(char *lineptr[], int maxlines){      int len, nlines;      char *p, line[MAXLENGH];      nlines = 0;      while((len = my_getline(line, MAXLENGH)) > 0){          //if (nlines >= MAXLENGH || (p = alloc(len)) == NULL)            //  return -1;          //else {              line[len -1] = '\0';              strcpy(p, line);              lineptr[nlines++] = p;      }      return nlines;  }    void writelines(char *lineptr[], int lines){      while(lines-- > 0)          printf("%s\n", *lineptr++);  }  int my_getline(char *s, int limit){      int i, c;      for (i = 0; i < limit -1 && (c = getchar())!= '\n' && c != EOF;i++)           *s++ = c;      if (c == '\n'){           *s++ = '\n';           i++;      }      *s = '\0';      return i;  }  /* qsort:sort v[left]...v[right] into increasing order  */  void swap(char *v[],int i, int j);  void qsort(char *v[], int left, int right){      int i, last;      if (left >= right)/* do nothing if the array contains less than 2 elements*/         return;      swap(v, left, (left + right) / 2);      last = left;      for (i = left + 1; i <= right; i++){          if (stringcmp(v[i], v[left]) > 0){              swap(v, ++last, i);          }      }      swap(v, last, left);      qsort(v, left, last -1);      qsort(v, last + 1, right);  }  * qsort:sort v[left]...v[right] into increasing order  */  void swap(char *v[],int i, int j);  void qsort(char *v[], int left, int right){      int i, last;      if (left >= right)/* do nothing if the array contains less than 2 elements*/         return;      swap(v, left, (left + right) / 2);      last = left;      for (i = left + 1; i <= right; i++){          if (stringcmp(v[i], v[left]) > 0){              swap(v, ++last, i);          }      }      swap(v, last, left);      qsort(v, left, last -1);      qsort(v, last + 1, right);  }  int stringcmp(char *s, char *t){      while (*s++ == *t++)              ;      if (*t == '\0')         return 0;      else         return 1;  }  void swap(char *v[], int i, int j){      char *temp;      temp = v[i];      v[i] = v[j];      v[j] = temp;  }  void strcopy(char *s, char *t){      while(*s++ = *t++)              ;  }  

For most of part, there is no need to worry about, but for this line:

line[len -1] = '\0';  strcpy(p, line);  lineptr[nlines++] = p;  

when it comes to strcpy, which is to copy a string, it will cause segmentation fault;

A string can be initialized in different ways  Segmentation fault  

then I write another program to verify my thought:

#include <stdio.h>  #include <string.h>  int main(){     char *p;     char line[] = {'h','e','l','l','o','\0'};     strcpy(p, line);     return 0;  }  

It will give me same error, I assume the code in this book is correct, but It does run into crash, (by the way I only currently care about the segmentation error part, for other parts, ignore them please) am I missing some code part? any advice is thankful!

https://stackoverflow.com/questions/66881430/kr-c-book-a-stripped-down-version-of-unix-sort March 31, 2021 at 11:56AM