C program pointers crashing system calls - c

Question

// Struct for Country Data
typedef struct
{
char name[50]; // Country name
char code[3]; // Country code
int population; // Country Population
double lifeExp; // Country Life expectancy
} CountryData;
// Struct for Dir File
typedef struct
{
char code[3];
int offSet;
} DirData;
// Function Declarations
void fillCountryStructs(CountryData ** dataPtr, int nLines, int fd);
void fillDirectoryStructs(CountryData **dataPtr, DirData **director, int nLines,int fd2);
void sortStructs(DirData **director, int nLines);
int verifyString(char *s1, char *s2);
// Main Function
// - This function starts the program, get the number of lines as a
// parameter, fills the structs and writes the data to the Country
// File and the Directory file.
int main(int argc, char *argv[]) // Always remember to pass an argument while executing
{
// Some variables
int nLines; // The number of lines
char *pEnd; // For String functions
FILE *Fin,*Fout; // File pointers
int fd;
int fd2;
nLines = strtod(argv[1], &pEnd);
CountryData **countryDataPtr; // Array of structs
CountryData **tempStruct;
DirData **director;
// Allocate memory for the struct pointers
countryDataPtr = calloc(nLines, sizeof(CountryData*));
director = calloc(nLines, sizeof(DirData*));
// File Stream for "AllCountries.dat"
if((fd = open("AllCountries.dat", O_RDWR)) ==-1)
err_sys("File not found...\n");
// File Stream for "RandomStruct.bin"
if ((fd2 = open("RandomStruct.bin", O_RDWR)) == -1)
err_sys("Failed to open binary\n");
// Filling the Country stucts
fillCountryStructs(countryDataPtr, nLines, fd);
close (fd);
//fclose(Fin); // Closing the file "AllCountries.dat"
// Writing Binary File
write(fd2, (countryDataPtr[0]->name[0]), sizeof(CountryData));
close (fd2);
//fclose(Fout);
printf("RandomStruct.bin written Sucessfully\n");
// Filling the Directory File
// File Stream for "RandomStructDir.dir"
if ((fd2 = open("RandomStructDir.dir",O_RDWR|O_TRUNC)) != -1)
err_sys("Failed to open binary\n");
fillDirectoryStructs(countryDataPtr, director, nLines, fd2);
sortStructs(director, nLines); // Sorting the structs
// Write the number of lines in the FIRST LINE
// of the Directory File
write(fd2, nLines, sizeof(nLines));
// Writing Directory File after the number of lines was written
write(fd2,(director[0]->code[0]), sizeof(DirData));
close (fd2);
//fclose(Fout);
printf("RandomStructDir.dir written Sucessfully\n\n");
exit(0);
}
// Filling the Country structs
// - This function extracts the data from the file using strtok
// and fills all the structs with their corresponding values.
void fillCountryStructs(CountryData **dataPtr, int nLines, int fd)
{
int curLine = 0; // Current line
int index = 0; // The index
char buf[BUFSIZE]; // The Buffer with the size of BUFSIZE
char *tok; // Token
char *pEnd; // For the String functions
char ch = 'a'; // The temp character
int temPop;
double temLifeExp;
int num=0;
for(curLine = 0; curLine < nLines; curLine++)
{
// Reading each line
dataPtr[curLine] = (CountryData *)calloc(1, sizeof(CountryData));
index = 0;
do
{
read(fd, &ch, 1);
buf[index++] = ch;
}
while(ch != '\n');
// Strtoking...
tok = strtok(buf, ",\n");
index = 1;
while(tok != NULL)
{
tok = strtok(NULL, ",\n");
// Get the Country Code
if(index == 1)
{
strcpy(dataPtr[curLine]->code, tok); // Copying code to the struct
}
// Get the Country Name
if(index == 2)
{
strcpy(dataPtr[curLine]->name, tok); // Copying name to the struct
}
// Get the Country Population
if(index == 7)
{
temPop = (int)strtol(tok, &pEnd, 10);
dataPtr[curLine]->population = temPop; // Copying population to the struct
}
// Get the Country Life expectancy
if(index == 8)
{
num=countchar(tok);
printf ("The number of characters entered is %d\n", num);
printf ("The character entered is %s\n",tok);
temLifeExp = strtod(tok, &pEnd);
dataPtr[curLine]->lifeExp = temLifeExp; // Copying life expectancy to the struct
}
index++;
}
}
}
int countchar (char list[])
{
int i, count = 0;
for (i = 0; list[i] != '\0'; i++)
count++;
return (count);
}
// Filling the Directory Structs
// - This function fills the directory with the offset
void fillDirectoryStructs(CountryData **dataPtr, DirData **director, int nLines, int fd2)
{
int i = 0;
for(i = 0; i < nLines; i++)
{
strcpy(director[i]->code, dataPtr[i]->code); //It crashes in this Line
director[i]->offSet = sizeof(CountryData) * (i);
}
}
// Sorting the Dir Structs
// - This function sorts the Directory Structs.
void sortStructs(DirData **director, int nLines)
{
int maxNumber;
int i;
DirData **temp;
temp = calloc(1, sizeof(DirData));
// Sorting the array of pointers!
for(maxNumber = nLines - 1; maxNumber > 0; maxNumber--)
{
for(i = 0; i < maxNumber; i++)
{
if((verifyString(director[i]->code, director[i+1]->code)) == 1)
{
temp[0] = director[i];
director[i] = director[i+1];
director[i+1] = temp[0];
}
}
}
}
// Veryfying the strings
// - This function compares two strings and return a specific value
// accordingly.
int verifyString(char *s1, char *s2)
{
int i;
if(strcmp(s1,s2) == 0)
return(0); // They are equal
for(i = 0; s1[i] != 0; i++)
{
if(s1[i] > s2[i])
return(1); // s1 is greater
else if(s1[i] < s2[i])
return(2); // s2 is greater
}
return (2); // s2 is greater
}
So I get segmentation fault and I have no Idea why? maybe is something about the pointers. I specified where it crashes (void fillDirectoryStructs) that method the first line.
When I compile I get :
Countries.c: In function 'main':
Countries.c:68: warning: passing argument 2 of 'write' makes pointer from integer without a cast
Countries.c:84: warning: passing argument 2 of 'write' makes pointer from integer without a cast
Countries.c:86: warning: passing argument 2 of 'write' makes pointer from integer without a cast
Countries.c:232:2: warning: no newline at end of file
I don't know a lot about pointers but I have to use system calls, so I can't use any of the FILE * functions (fwrite(), etc) that is why I'm using plain write() and read().
When I run it I get segmentation fault when It gets to that point I just specified.
for test purposes I'm trying to print this
printf("test: %s\n", countryDataPtr[0]->code[0]);
instead of writing and it crashes there, why? what am I doing wrong? shouldn't that get the code of that first country in my struct? thanks

Answer 1

Well, you need to listen to your compiler and take its warnings seriously.
This:
write(fd2, nLines, sizeof(nLines));
is wrong, and would explain the warning. The variable nLines has type int, but if you look at the [documentation for write()] you can see that the 2nd argument has type void *.
So it will interpret your integer value as a pointer, and start reading memory which you have no right to be reading.
You need:
write(fd2, &nLines, sizeof nLines);
Note that sizeof is not a function, it only needs parenthesis when the argument is a type name (since it then needs a cast expression to the type in question, and casts are writen as a type name enclosed in parenthesis).
Also, you need to be prepared for the reality that I/O can fail. The write() function has a return value which you should be checking.

Answer 2

There are a number of other problems with your code, in addition to the serious one unwind pointed out.
This:
CountryData **countryDataPtr; // Array of structs
is not an Array of structs. Once allocated, it could be an array of pointers to structs.
This:
write(fd2, (countryDataPtr[0]->name[0]), sizeof(CountryData));
does not write one CountryData instance (much less a whole array of them). It takes the integer value of the first character of the first element's name, and treats it as a pointer just like you do with nLines.
If you want to write the first element, it would look like this:
write(fd2, countryDataPtr[0], sizeof(CountryData));
and if you wanted to write all the elements, you'd either need a loop, or a contiguous array of structs you can write in one go.