CSV Parsing - Second Field Returning Null Values - c

Question

I use the following code to parse this CSV
me;val1;val2;val3;val4;val5;
me;val1;val2;val3;val4;val5;
void readcsv()
{
FILE* stream = fopen("input.csv", "r");
char line[1024];
while (fgets(line, 1024, stream))
{
char* tmp = strdup(line);
// printf("Field 1 would be %s\n", getcsvfield(tmp, 1));
printf("Field 1 would be %s\n", getcsvfield(tmp, 1));
printf("Field 2 would be %s\n", getcsvfield(tmp, 2));
// NOTE strtok clobbers tmp
free(tmp);
}
}
//Used for parsing CSV
const char* getcsvfield(char* line, int num)
{
const char* tok;
for (tok = strtok(line, ";");
tok && *tok;
tok = strtok(NULL, ";\n"))
{
if (!--num)
return tok;
}
return NULL;
}
But i keep getting NULL value in the second Field
Output:
Field 1 would be me
Field 2 would be (null)
Field 1 would be me
Field 2 would be (null)
What im i doing wrong?

Answer 1

strtok(line, ";");
strtok modifies the string (in this case line) being passed to it. So you should not use the same line (returned from first call to getcsvfield) also during the second time you call getcsvfield, because after first call to getcsvfield, line now has different content.
Notice this is not an issue within the function getcsvfield, because when you pass NULL second time to strtok inside that function, it knows how to proceed with the modified input string in a correct way.
From manual about strtok parameters:
Notice that this string is modified by being broken into smaller
strings (tokens).
Something like this should do the trick. This is the most "basic" approach, you can try other ones too. Leave the getcsvfield function as you had in your code initially, just on the caller side do:
char line[1024];
char buffer[1024];
while (fgets(line, 1024, stream))
{
// char* tmp = strdup(line); not necessary in this case
strcpy(buffer, line);
printf("Field 1 would be %s\n", getcsvfield(buffer, 1));
strcpy(buffer, line);
printf("Field 2 would be %s\n", getcsvfield(buffer, 2));
// free(tmp);
}
As it currently stands above, each call to getcsvfield, returns poitner to same memory address - buffer. For printing it works ok (because at the time of printing it shows what is there in buffer), but if you want to store result of each call to getcsvfield for later use, you may want to copy result of each invocation of getcsvfield to some different memory location each time.