Status access overflow while accessing array of chars

    FILE *input;
     FILE *output;
 
     input=fopen("in.txt", "r");
     output=fopen("out.txt", "w");
 
 
     char buffer[1000];
     char bytebuffer=0;
     char tempchr=0;
     char huffmancode[100]={0};
     int bufferindex=7;
 
     fgets(&buffer[0],255,input);
 
     int length=0;
     while (buffer[length]!=0) length++;
 
 
 
     fputc(length,output);
 
     int j;
     int k;
 
 
 
     for (j=0;j<length;j++){
         tempchr=buffer[j];
         strcpy(&huffmancode[0],code[tempchr-97]);
         k=0;
 
         while(huffmancode[k]!=0){
             if (huffmancode[k]!='0'){
                 setBit(&bytebuffer,bufferindex);
             }
             bufferindex-=1;
             if (bufferindex==-1){
                 fputc(bytebuffer,output);
                 bytebuffer=0;
                 bufferindex=7;
             }
             k++;
         }
 
     }
 

this code is failing due

tempchr=buffer[j];
 

It works perfectly fine for j<10 but for j>=10 the program throws a status access overflow exception. buffer variable is allocated for 1000 bytes so I don't have a clue why the program is failing to access an index in bounds. Why is this failing?

There are some array definitions which I didn't include. I don't think they are the problem but here are they anyways.

    int agac[1000]={21,12,9,7,5,
                    5,4,0,0,0,
                    0,3,2,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0,
                    0,0,0,0,0
     };
     char sembol[1000]={0,0,0,'a','b',
                       0,'f',0,0,0,
                       0,'u','k',0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0,
                       0,0,0,0,0
     };
 
     char *code[27]={ "00","01",0,0,0,
                      "11",0,0,0,0,    
                      "101",0,0,0,0,    
                      0,0,0,0,"100",            
                      0,0,0,0,0,            
                      0,0
     };  
 

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