Pointers and Char
Author
Hi,
I have problems with pointers and char's and can't just get my head around what's wrong!
I have a function in C++ which reads in data from a file, I then want to compare the data read in, with a char* which is passed into the function.
//The function loooks like this
getCat(char *filename, char* inName);
I declare a variable
char *name;
Then I read in the data using fgets(buffer, 255, fp);
and read the data into the variable by doing
sscanf(buffer, "%s", &name);
Now I can print name to prove that the data is in there using
printf("name = %s\n\n", &name);
now I want to compare name with inName so I try
if(strcmp(inName, name) == 0){
//do stuff
}
The problem is it crashes with a segmentation error. Any ideas what I'm doing wrong??
Quote:Original post by siliconsurferprintf("name = %s\n\n", &name);
One problem is that ampersand before name in the printf. It should be:
printf("name = %s\n\n", name);otherwise it's trying to print the pointer value itself as a string...
There may be other problems in your function, but that is impossible to tell without the full code...
Tom
Well your main problem is that you've misspelt "string".:
Enigma
void someNameBetterThanGetCatUnlessYouAreActuallyReturningAFeline(std::string filename, std::string expectedName){ std::ifstream reader(filename.c_str()); std::string name; std::getline(reader, name); if (name == expectedName) { // do stuff }}Enigma
Author
Here is the complete code. The problem is when I try to do the strcmp on line 34. If I create a variable such as
char *newvar = "anystring";
And compare it with inUrl the code works fine. But comparing inUrl with domain causes the program to crash with a segmentation error. I presume this is to do with how sscanf works, but I can't figure out why, or how to fix it.
char *newvar = "anystring";
And compare it with inUrl the code works fine. But comparing inUrl with domain causes the program to crash with a segmentation error. I presume this is to do with how sscanf works, but I can't figure out why, or how to fix it.
int getCat(char *filename, const char *inUrl) 18 { 19 FILE *fp; 20 char buffer[255]; 21 22 char *domain; 23 //char *cat; 24 25 if((fp=fopen(filename, "r")) == NULL) 26 { 27 printf("Error opening file %s\n", filename); 28 return -1; 29 } 30 while(!feof(fp)) 31 { 32 fgets(buffer, 255, fp); 33 sscanf(buffer, "%s", &domain); 34 if(strcmp(domain, inUrl) == 0){ 35 printf("match found\n"); 36 break; 37 } 38 39 } 40 fclose(fp); 41 42 return 0; 43 }
you declare domain as a pointer to char. You need an array:
Also, if you're doing this in C++, it's preferred to use std::string.
char domain[ MAX_SIZE ];sscanf(buffer, "%s", domain);Also, if you're doing this in C++, it's preferred to use std::string.
if you have ANY trouble AT ALL using char* as a string type you should not be using it. The only time that you should use char* is when you are already quite experienced with std::string and have found one of those rare cases where using char* is better than using the real string type.
#include <string> //note, not string.h#include <iostream>#include <fstream>using namespace std;void GetFeline(string filename, string expected_data){ fstream fin(filename.c_str()); string actual_data; fin>>actual_data; if(actual_data==expected_data) { //blah blah blah }}
Author
I'm using char * because I'm working on code, which already uses char * rather than the string data structure.
It's also because I have to return a char * to a calling rpc.
It's also because I have to return a char * to a calling rpc.
need a std::string from a char*
no problem:
std::string str = char_ptr;
need a char* from a string
no problem:
do_something_with_a_char_pointer( str.c_str() );
no problem:
std::string str = char_ptr;
need a char* from a string
no problem:
do_something_with_a_char_pointer( str.c_str() );
Here's how I would do it.
[Edited by - the_dannobot on September 7, 2005 10:34:27 AM]
#define STRING_LENGTH 255int getCat(const char *filename, const char *inUrl) 18 { 19 FILE *fp; 20 char buffer[STRING_LENGTH]; if (filename == 0 || inUrl == 0) { return -1; } memset(buffer, 0, (sizeof(char) * STRING_LENGTH)); 23 //char *cat; 24 25 if((fp=fopen(filename, "r")) == NULL) 26 { 27 printf("Error opening file %s\n", filename); 28 return -1; 29 } 30 while(!feof(fp)) 31 { 32 fgets(buffer, STRING_LENGTH, fp); 34 if(strcmp(buffer, inUrl) == 0) { 35 printf("match found\n"); 36 break; 37 } 39 } 40 fclose(fp); 42 return 0; 43 }[Edited by - the_dannobot on September 7, 2005 10:34:27 AM]
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