sprintf with struct array field - gets segmentation fault - c

Question

This idea is to format text info messages bellowing to a structure within a module.
It works like a charm when trying to define the message with (cf module.c):
/*this works*/
module_text3.info_text[0] = "toto[0]";
module_text3.info_text[1] = "toto[1]";
But when using sprintf, I got segmentation fault (cf module.c):
/*this gives segmentation fault*/
for(cpt=0; cpt < 2; cpt++)
{
sprintf(module_text3.info_text[cpt], "info[%u]", cpt);
}
3 different files: main.c, module.h and module.c
/*main.c*/
/*gcc -o test main.c module.c*/
#include <stdio.h>
#include "module.h"
int main(int argc, char **argv)
{
int i;
struct message3 *ptext3 = moduleFcn3();
for (i= 0; i < ptext3->info_nb; i++)
{
printf("ptext3->info_text[%u]: %s\n", i, ptext3->info_text[i]);
}
printf("ptext3->error_text: %s\n", ptext3->error_text);
printf("ptext3->id: %u\n", ptext3->id);
printf("ptext3->info_nb: %u\n", ptext3->info_nb);
printf("ptext3->info_nb_max: %u\n", ptext3->info_nb_max);
return 0;
}
/*------------------------------------------------------*/
/*module.h*/
#define NB_LINE_MAX 10
struct message3
{
char *info_text[NB_LINE_MAX]; /*a few info lines.*/
char *error_text; /*only one line for error.*/
int id;
int info_nb_max;
int info_nb;
};
extern struct message3* moduleFcn3(void);
/*------------------------------------------------------*/
/*module.c*/
#include <stdio.h>
#include "module.h"
/*static is in "Stack".*/
static struct message3 module_text3;
struct message3* moduleFcn3(void)
{
int cpt = 0;
struct message3 *ptext;
/*this gives segmentation fault*/
for(cpt=0; cpt < 2; cpt++)
{
sprintf(module_text3.info_text[cpt], "info[%u]", cpt);
}
/*this works*/
// module_text3.info_text[0] = "toto[0]";
// module_text3.info_text[1] = "toto[1]";
// cpt = 2;
module_text3.error_text = "This is error";
module_text3.id = 4;
module_text3.info_nb_max = NB_LINE_MAX;
module_text3.info_nb = cpt;
ptext = &module_text3;
return ptext;
}
I would appreciate any advises on how to format my information messages (with our without using sprintf).
Thank you,

Answer 1

You have not allocated space for the strings in the info_text field. The simplest thing to do would be to change the struct:
/*module.h*/
#define NB_LINE_MAX 10
#define INFO_MAX 25
struct message3
{
char info_text[NB_LINE_MAX][INFO_MAX]; /*a few info lines.*/
char *error_text; /*only one line for error.*/
int id;
int info_nb_max;
int info_nb;
};
extern struct message3* moduleFcn3(void);

Answer 2

You did not allocate any memory for the info_text strings. You either have to use malloc() first, or if your C library supports it (the GNU one does), use asprintf() instead of sprintf() to have it allocate enough memory to hold the whole output string for you:
for(cpt = 0; cpt < 2; cpt++)
asprintf(&module_text3.info[cpt], "info[%u]", cpt);
Don't forget that you also have to free the memory again at some point.
The reason that the following line works:
module_text3.info_text[0] = "toto[0]";
Is that the compiler ensures the string "toto[0]" is stored in memory somewhere, and you just make the pointer module_text3.info_text[0] point to that string.