字符串突然在函数中丢失其值

Question

I have code which should read the filenames from a file, open those, and do some calculations. Those files contain columns of data. I have declared arrays to read in the data. The strings containing file names are behaving in an erratic manner when I initialise the arrays before reading in. If the initialisation is removed, the problem is solved.

The erratic behaviour is: the string declared first isn't remaining in the memory. In the code, if I declare d[90] first, then a print statement at a later time does not print the previously saved value. What could be the cause?

Here is the code:

#include<stdio.h>
#include<math.h>

main()
{

  FILE *data,*model,*filenames;

  int i,imax,j,k,l;
  float fd[1000],fm[1000],td[1000],tm[1000];
  float a;
  char d[90],m[90];


  k=1;
  l=0;

  filenames=fopen("filenames.dat","r");


  while(fscanf(filenames,"%s %s",d,m)!=EOF)
   {
printf("model: %s\ndata: %s\n",m,d);

    for(i=1;i<=1000;i++)
      {
        fd[i]=0;
        fm[i]=0;
        td[i]=0;
        tm[i]=0;
      }


printf("datafile name: %s\n",d);
printf("modelfile name: %s\n",m);

    data=fopen(d,"r");

    for(i=1;i<=1000;i++)
      if(fscanf(data,"%f %f %f %f",&td[i],&fd[i],&a,&a)!=EOF)
        k++;
      else
        break;

    fclose(data);

printf("modelfile name: %s\n",m);
    model=fopen(m,"r");

    for(i=1;i<=1000;i++)
      if(fscanf(model,"%f %f %f %f %f %f",&tm[i],&fm[i],&a,&a,&a,&a)!=EOF)
       l++;
      else
        break;

    fclose(model);


   }

}

Answer 1

In your code, with a definition like float fd[1000] and so on, using the for loop like

for(i=1;i<=1000;i++)
      {
        fd[i]=0;
        fm[i]=0;
        td[i]=0;
        tm[i]=0;
      }

creates off-by-one error and hence cause undefined behaviour , as essentially, you're trying to access past the allocated memory.

As C uses 0 -based indexing, you should be using index from 0 to (size-1) for an array with size elements.

Also, always check the return value of fopen() against NULL to ensure it's success. In case fopen() failed, using the returned pointer will again invoke UB.