Thread: Segmentation fault stepping into function in debug mode

Hi,
I have to parse a string and fill a structure with data. But when i try to step into function parse_mp_info while debugging i receive a segmentation fault. The weired thing is that if i don`t initialize pch pointer with NULL no error occures. What can it be and how to fix/bypass it?
(I use Qt Creator 7.0.1 on Ubuntu 20.04. Plain C project)
=====================================

Code:

#include <stdio.h>
typedef struct MPRInfo{
    long long cnt;
    char   smb[8];
    size_t r1;
    size_t r2;
    size_t r3;


    double l1;
    double l2;
    double l3;
} stMPR;




int parse_mp_info(stMPR* stData,const char* pData){
    char *pch=NULL;
    //char *pch; //Here no problem
    
    //Main code....
    
    return 0;


}


int main()
{
    const char *pData     = "Some data......";
    stMPR stParsed       = {0};
    int res               = parse_mp_info(&stParsed,pData);


    if(res!=0)
        printf("Error parsing data!\n");
    return 0;
}

===========================================

All local variables should ALWAYS be initialized, especially for pointers! It is illegal to access a NULL pointer resulting in a segmentation fault. You need to assign pch with the address of some valid nul terminated char string first.

In most cases, if not initialized, pch is getting some garbage data on the stack, and is being interpreted as an address, most always an address that it should not be accessing, resulting in a segmentation fault. Why it does not create a fault in your case, is unclear.

Your code as presented does compile and run correctly. You need to provide the missing code if you continue to have problems.

Originally Posted by Salem

Lemme guess, you're trying to do strtok() in your parser on a const char* pData string.

Yes, you are right! In fact the code looks like this:

Code:

#include <stdio.h>
#include <string.h>
typedef struct MPRInfo{
    long long cnt;
    char   smb[8];
    size_t r1;
    size_t r2;
    size_t r3;


    double l1;
    double l2;
    double l3;
} stMPR;




int parse_mp_info(stMPR* stData,const char* pData, const char* pName){
    if(pName!=NULL){
        strcpy(stData->smb,pName);


    }
    char buffer[128] = {0};
    char *p_tmp      = NULL;
    char *p_tag      = strstr(pData,"ticks");


    if(p_tag){
        p_tmp = strchr(p_tag,':');
        if(p_tmp){
            char *p_start   = ++p_tmp;
            char *p_end     = strchr(p_tag,',');
            unsigned len    = p_end-p_start;
            for(unsigned i=0;i<len;i++){
               buffer[i]=p_start[i];
            }
            stData->cnt = strtoll(buffer,NULL,0);
        }
    }


    p_tag = strstr(pData,"rates");


    if(p_tag){
        char *pch  = strtok(p_tag,":,");
        unsigned proc_val = 0;
        while(pch!=NULL){
           if(strcmp(pch,"\"r1\"")==0){
              proc_val=1;
           }


           if(proc_val==1){ //should store value in struct field
               char *pdiv = strchr(pch,':');
               memset(buffer,0,128*sizeof(char));
               for(int i=0;i<strlen(pch);i++){
                  buffer[i]=pch[i];
               }
               stData->r1 = strtol(buffer,NULL,0);
           }




           pch  = strtok(NULL,":,");
        }








    }


    return 0;


}


int main()
{
    const char *pData     = "{\"ticks\":1657195557242,\"rates\":[{\"r1\":145,\"r2\":716,\"r3\":2145}],\"lows\":[{\"l1\":0.05,\"l2\":0.0135,\"l\":1.02}]}";
    stMPR stParsed        = {0};
    int res               = parse_mp_info(&stParsed,pData,"Test1");


    if(res!=0)
        printf("Error parsing data!\n");
    return 0;
}

Under Windows i get to the bold line and then receicve the error. Under linux i receicve the error the moment i try to step into the function..

Shouldn't the compiler be able to warn when passing a const char* to a function that accepts a char* ?

I see. So it's more an API problem.

Originally Posted by thmm

I see. So it's more an API problem.

Yep, this is one of the shortcomings/limitations of C.

C++ has two overloaded versions of strstr, one that takes and returns const char * and the other that takes and returns a non-const char *. C doesn't have overloaded functions, but it still could have had two differently-named strstr functions (one for const and another for non-const), but what's done is done at this point.

Thanks a lot for commentaries! Now it`s cleared up more or less! Frankly i didn`t expect it to turn out that tricky..))
I guess i should try to use some other way

Your data looks like JSON, so using a JSON library might be a good idea.