C Segmentation fault - saving struct in .dat file - c

Question

Can someone please tell me, what causes the segementation fault in my C - code? I am trying to save a struct into a file, and calling it afterwards, following this guide. I don't see anything assigned wrongly, so I'm happy to learn what's my mistake by someone more experienced.
Here is my code, simplified:
int main(int argc, char **argv)
{
char *key = (argc > 4) ? argv[4]: 0;
if(0==strcmp(key, "write")){
struct MyStruct s;
FILE *myoutfile;
myoutfile = fopen("file.dat","w")
if (myoutfile == NULL)
{
fprintf(stderr, "\nError opend file\n");
exit (1);
}
s = get_struct(argv[2]);
fwrite(&s, sizeof(struct MyStruct), 1, myoutfile);
fclose(myoutfile);
}else{
struct MyStruct t;
FILE *myinfile;
myinfile = fopen("file.dat", "r")
if (myinfile == NULL)
{
fprintf(stderr, "\nError opend file\n");
exit (1);
}
while (fread(&t, sizeof(struct MyStruct), 1, myinfile))
printf("Done reading");
}
work_with_struct(t);
fclose(myinfile);
}
Also, as I read in another stack overflow post, doing this:
fread(&t.one, sizeof(t.one), 1, myinfile);
fread(&t.two, sizeof(t.two), 1, myinfile);
fread(&t.three, sizeof(t.three), 1, myinfile);
did not work either.
EDIT: I now think i have located the problem a bit more. The first part of the function (if) works fine. What I thought was not necessary to mention first, was that in the end of the "else" I have a function that works with t. This is the one throwing the error, i believe.
It works fine, when I leave out the .dat-file-part, so by just saying
t = get_struct(argv[2]);
work_with_struct(t);
which I actually want to avoid, because "get_struct" is huge. Doing it once, and working with the data in the .dat file was my solution, that I calculate it only once.
My assumption now is, that putting the struct into the fstream and getting it back will somehow destroy it, or makes it somehow not-readable for work_with_struct. What I think is also worth mentioning is, that inside the struct there are three members: two char **, and one custom data type.
I didn't find any other solution suggesting other ways of reading in the file.
Maybe with this explanation someone gets alerted, where the segmentation fault might come from. Thanks a lot!

Answer 1

The issue is that you are passing a NULL pointer to strcmp which expects a const char*. When no arguments are passed to the program key = 0 and the program segfaults because strcmp attempts to dereference this NULL pointer.
Edit:
I attempted to correct the example program to make it compile and run, here it is:
Edit 2: print content of saved struct
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
struct MyStruct {
int a;
};
int main(int argc, char **argv) {
const char *key = (argc > 4) ? argv[4] : "write";
if (0 == strcmp(key, "write")) {
struct MyStruct s;
FILE *myoutfile;
myoutfile = fopen("file.dat", "w");
if (myoutfile == NULL) {
fprintf(stderr, "\nError opend file\n");
exit(1);
}
s = (struct MyStruct){ 'a' };
fwrite(&s, sizeof(struct MyStruct), 1, myoutfile);
fclose(myoutfile);
} else {
struct MyStruct t;
FILE *myinfile;
myinfile = fopen("file.dat", "r");
if (myinfile == NULL) {
fprintf(stderr, "\nError opend file\n");
exit(1);
}
while (fread(&t, sizeof(struct MyStruct), 1, myinfile))
printf("Done reading\n");
printf("%c\n", t.a);
fclose(myinfile);
}
}
Edit 3:
After updating the description of the struct MyStruct the problem is obvious. You need to find a different way to save the struct to the file then by using fwrite since you have pointer types in it. The way you would go about that is a topic for a different question.