Question

I’m writing a code for a MIPS pipeline simulator in C++. One of my functions is fetch. After some debugging, I narrowed down to my fetch function, where the segmentation fault occurs. Can someone help me figure out why it happens? The code is here below:

void Simulator::fetch(){
int flag =0;
string buf, rd;
int i;
for(i = 0;i<4;i++){
 if(pre_issue_buffer[i]==";"){
     flag = 1;
     break;
 }
 }
 if(flag ==1){
 if(i<3){
string instr = memory.read_memory(PC);
 stringstream ss(instr);
 vector<string> tokens;
 while (ss >> buf)
    tokens.push_back(buf);
 string instruction = tokens.at(0);
 if(instruction == "BREAK"){
        brk =1;
        instr_string=instruction;
 }
 else if(instruction=="NOP"){

    instr_string=instruction;
 }
 else if(instruction=="J"){
    int address=toInt(tokens.at(1));
    if(address>this->break_addr){
        cerr<<"Invalid Jump Address at: "<<PC<<endl;
    }
    PC = address;
    exec_instr=instruction+"\t#"+tokens.at(1);
 }
 else if(instruction=="JR"){
    rd = tokens.at(1);
    if(regInUse[rd]==0){
    p=regFile.find(tokens.at(1));
    PC = p->second;
    exec_instr=instruction+"\t"+tokens.at(1);
 }
    else
        waiting_instr= instruction+"\t"+tokens.at(1);
 }
 else if(instruction=="BEQ"){
    int rs,rt;
    rd = tokens.at(1);
    if(regInUse[rd]==0){
    p=regFile.find(tokens.at(1));
    rs = p->second;
    p=regFile.find(tokens.at(2));
    rt = p->second;
    if(rs==rt){
        int offset=toInt(tokens.at(3));
        PC = PC+offset+4;
    }
    else
        PC=PC+4;
    exec_instr=instruction+"\t"+tokens.at(1)+", "+tokens.at(2)+", #"+tokens.at(3);
 }
    else
    waiting_instr=instruction+"\t"+tokens.at(1)+", "+tokens.at(2)+", #"+tokens.at(3);
 }
 else if(instruction=="BLTZ"){
    rd = tokens.at(1);
    if(regInUse[rd]==0){
    p = regFile.find(tokens.at(1));
    int rs = p->second;
    if(rs<0){
        int offset=toInt(tokens.at(2));
        PC = PC + offset+4;
    }
    else
        PC=PC+4;
    exec_instr=instruction+"\t"+tokens.at(1)+", #"+tokens.at(2);
 }
    else
        waiting_instr=instruction+"\t"+tokens.at(1)+", #"+tokens.at(2);
 }
 else if(instruction=="BGTZ"){
    rd = tokens.at(1);
    if(regInUse[rd]==0){
    p = regFile.find(tokens.at(1));
    int rs = p->second;
    if(rs>0){
        int offset=toInt(tokens.at(2));
        PC = PC + offset+4;
    }
    else
        PC=PC+4;
    exec_instr=instruction+"\t"+tokens.at(1)+", #"+tokens.at(2);
 }
    else
    waiting_instr=instruction+"\t"+tokens.at(1)+", #"+tokens.at(2);
 }
 else{
 rd = tokens.at(1);
 pre_issue_buffer[i]=instr;
 cout<<i<<endl;
 PC=PC+4;
 }

}

Answer 1

Check, for every array, the position you are trying to access and verify it’s inside the array bounds.

We don’t have enough informations to answer. For what I can tell it may be at the beginning as well as at the end of your function.