Segmentation fault executing file writing using FILE pointer - c++

Question

I get a "Segmentation fault" for the following C++ code:
#include <cstdio>
int main(int, char**) {
FILE *fp;
fp = fopen("~/work/dog.txt", "w");
fprintf(fp, "timer, timer3, timer5, timer6, timer7");
fclose(fp);
}

Answer 1

Your path is invalid and will never work, thus fopen sets fp to NULL and you get a segfault. Hint: the ~ character is expanded by the shell, you can't use it in an argument to fopen.
A correct, safe implementation of what you're trying to do might look as follows. This is tested. It's also the reason why sane people don't write in C unless they have no other way of doing it :)
// main.cpp
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <unistd.h>
int main(int, char**)
{
const char * home = getenv("HOME");
if (home == NULL || strlen(home) == 0 || access(home, F_OK) == -1) abort();
const char * subPath = "/work/dog.txt";
char * path = (char*)malloc(strlen(home) + strlen(subPath) + 1);
if (path == NULL) abort();
strcpy(path, home);
strcat(path, subPath);
FILE *fp = fopen(path, "w");
if (fp != NULL) {
fprintf(fp, "timer, timer3, timer5, timer6, timer7");
fclose(fp);
}
free(path);
}

Answer 2

A few things:
you need to check fp for NULL before using it, else you'll get a segfault whenever the file isn't found.
you need to resolve the full path before passing it to fopen (fopen doesn't know what to do with "~")
example:
FILE *fp = NULL;
char path[MAX];
char *home = getenv ("HOME");
if ( home )
{
snprintf(path, sizeof(path), "%s/work/dog.txt", home);
// now use path in fopen
fp = fopen(path, "w");
if ( fp )
{
fprintf(fp, "timer, timer3, timer5, timer6, timer7");
fclose(fp);
}
else
{
std::cout << "your dog is missing" << std::endl;
}
else
{
std::cout << "You are homeless" << std::endl;
}

Answer 3

Segfault happens is the file you're trying to open does not exist. This has nothing to do with Qt.
Test for the nullity of 'fp' and handle the error correctly. Something like
FILE *fp = fopen("/path/to/work/dog.txt", "w");
if (fp == NULL)
{
printf("File does not exist.\n");
// throw exception or whatever.
}