Dynamic memory allocation issue

Question

I've been trying to make this C program to work for some time, but I'm still having some great trouble with the memory allocation. The main point of this piece of work is that it's supposed to read a string from the first line of a file, then 2 numbers from the 2nd line and then 3 column vectors each having thrice as many members as the difference between the 2 read numbers.

#include <stdlib.h> #include <stdio.h> #include <string.h> void read(char **banda, int *s, int *sf, int **vs, char **vcs, char **vd) { FILE *fin; int i; fin = fopen("date.in", "r"); *banda = (char*)malloc(50*sizeof(char)); fgets(*banda, 50, fin); fscanf(fin, "%d", s); fscanf(fin, "%d", sf); fseek( fin, 1, SEEK_CUR ); int l = 3* ( (*s)-(*sf) ); *vcs = (char*) malloc( l * sizeof(char) ); *vd = (char*) malloc( l *sizeof(char) ); *vs = (int*) malloc( l * sizeof(int) ); for( i = 0; i< l ; i++ ) { fscanf(fin, "%d", vs[i]); fscanf(fin, " %c", vcs[i]); fscanf(fin, " %c", vd[i]); fseek( fin, 1 , SEEK_CUR ); } fclose(fin); } int main() { char *banda; int i; int s, sf; char *vcs; char *vd; int *vs; read(&banda, &s, &sf, &vs, &vcs, &vd); for( i = 0; i < strlen(banda) ; i++ ) { printf("%c", banda[i]); } printf("%d %d\n", s, sf); for( i = 0; i < 3*(s-sf) ; i++ ) { printf("%d %c %c\n", vs[i], vcs[i], vd[i]); } } Example input: Helen 2 1 1 H A 2 B C 5 K I

However, I can't seem to allocate the memory for it. If I go past i=0(aka the first position in the arrays) the program shows me a neat "Segmentation fault". However, if I only go and write in the array ONE value for each, it doesn't crash. There's something I'm seriously missing and I just can't figure it out.

You allocate vcs as char *vcs; (a single char pointer) but try to use it as a char* array in read (an array of char*) — Colonel Thirty Two
– Colonel Thirty Two, Commented Oct 24, 2015 at 18:15

dbush · Accepted Answer · 2015-10-24 18:16:00Z

1

The problem is here:

fscanf(fin, "%d", vs[i]); fscanf(fin, " %c", vcs[i]); fscanf(fin, " %c", vd[i]);

The variables vs, vcs, and vd are pointers to arrays. Here, you're treating them as arrays of pointers. You need to first dereference the pointer, then get the array element you want to write to, then take its address:

fscanf(fin, "%d", &(*vs)[i]); fscanf(fin, " %c", &(*vcs)[i]); fscanf(fin, " %c", &(*vd)[i]);

answered Oct 24, 2015 at 18:16

dbush

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5 Comments

campescassiano Over a year ago

There is no need to do that @dbush, just dereference once and all will works fine. fscanf(fin, "%d", *vs); BTW, what you did is the same, but a little more complex to understand/read.

Snappy Over a year ago

Hi dbush. Tried your option, now it does indeed add to the 2nd position in the arrays, but it fails at the 3rd position(for my example) and seg faults :(

Snappy Over a year ago

Never mind. I was braindead, sorry, your answer was perfect. Thanks a lot :D

dbush Over a year ago

@Ccampes No, that won't work. Your example will only write to the first element of the array.

dbush Over a year ago

@RjYslf Glad I could help. Feel free to accept this answer if you found it useful.

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