What is causing the segmentation fault in my program when reading a txt file?

[nenyan]

I wrote a program to read a txt file. When its running, system appear "Segmentation fault <core dumped>"

the code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define BUF 1024
#define pi 3.14159

typedef struct skymapdata
{
double ra;
double dec;
double p;
double pn;
} skydata;

/* This function is for readding skymap*/
int
readskymap( FILE *fp, skydata **sky)
{
skydata *sky1;
int i=0, n=BUF; /* n is the buffer. */

if ( !(*sky=(skydata *)malloc(n*sizeof(skydata))))
printf("memory error 1\n");

while (!feof(fp))
{ fscanf(fp, "%lf %lf %lf %lf", (*sky+i)->ra, (*sky+i)->dec, (*sky+i)->p, (*sky+i)->pn);
i++;
if (i==(n-1)) /*the number of lines is going to be longer than buffer. */
{
if(!(sky1=realloc(*sky, 2*n*sizeof(skydata))) ) {
printf("memory error 2\n"); }
*sky=sky1;
n*=2; } /*double the buffer*/
}

return i; /* Returning i tell us the accurate number.*/
}


int
main()
{
int i, n1;
skydata *sky;

FILE *fp;

if ( !( fp=fopen("skymap.txt", "r"))) {
printf ("cannot open the skymap.txt file\n");
return 1;}


n1=readskymap(fp, &sky);
for (i=0; i<n1; i++)
printf("%e %e %e %e\n", sky.ra, sky.dec, sky.p, sky.pn);



}



skymap.txt:
3.222 4.323 0.23 0.32
5.434 43.34545 0.3434 0.22

 

[chiro]

I wrote a program to read a txt file. When its running, system appear "Segmentation fault <core dumped>"

the code:

Hello nenyan.

For future posts you should embed your code in code tags. Basically use CODE and /CODE with [ and ] around the tags. I will use this so that I can see a cleaned up version of your code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define BUF 1024
#define pi 3.14159

typedef struct skymapdata
{
	double ra;
	double dec;
	double p;
	double pn;
} skydata;

/* This function is for readding skymap*/
int
readskymap( FILE *fp, skydata **sky)
{
	skydata *sky1;
	int i=0, n=BUF;       /* n is the buffer. */

	if ( !(*sky=(skydata *)malloc(n*sizeof(skydata))))  
		printf("memory error 1\n");

	while (!feof(fp))
	{ fscanf(fp, "%lf %lf %lf %lf", (*sky+i)->ra, (*sky+i)->dec, (*sky+i)->p, (*sky+i)->pn);
	i++;
	if (i==(n-1))                              /*the number of lines is going to be longer than buffer. */
	{
		if(!(sky1=realloc(*sky, 2*n*sizeof(skydata))) ) {
				printf("memory error 2\n"); }
		*sky=sky1;
		n*=2; }    /*double the buffer*/
	}  

	return i;    /* Returning i tell us the accurate number.*/
}int
main()
{
	int i, n1;     
    skydata *sky;

	FILE *fp;

	if ( !( fp=fopen("skymap.txt", "r"))) {
		printf ("cannot open the skymap.txt file\n");
		return 1;}	n1=readskymap(fp, &sky);
	for (i=0; i<n1; i++)
		printf("%e %e %e %e\n", sky[i].ra, sky[i].dec, sky[i].p, sky[i].pn);

   

}

skymap.txt:
3.222 4.323 0.23 0.32
5.434 43.34545 0.3434 0.22

Have you run this through a debugger? What line does it crash on?

I'm thinking that maybe you are passing by reference when you should be passing by value, or passing by value when you should be passing by reference.

To check this you could create some temporary pointer variables for your *(sky+i) and in the debugger make sure that all of these pointers have the right contents.

Also in terms of allocating your memory for your sky data structure, zero the memory and then check while you are debugging that the memory is indeed zeroed and also the right size.

 

[nenyan]

I found the reason.
fscanf(fp, "%lf %lf %lf %lf", (*sky+i)->ra, (*sky+i)->dec, (*sky+i)->p, (*sky+i)->pn);
should be
fscanf(fp, "%lf %lf %lf %lf", &((*sky+i)->ra), &((*sky+i)->dec), &((*sky+i)->p), &((*sky+i)->pn));