Send formatted output to character array elements in C

Question

I have an array of strings. I need to initialize each array element with some formatted data, i.e. each element should hold a constant string "data" plus a generated number. The final array should hold elements that look something like this, "data1", "data2", "data3", ... "dataN".

I used sprintf() but whenever the compile sees my sprintf() function, the program crashes. What am I doing wrong ?

//import necessary headers
#include <stdio.h>
#include <stdlib.h>

//variable to hold server data 
char *serverData[255];

//loop variable
int loop_index = 0;

//program entry
int main() {
    //initialize the serverData array elements with string "data" and
    //append number x to each element. i.e. x <= 255
    //use a for loop for the initialization
    for (; loop_index < sizeof(serverData) / sizeof(serverData[0]); loop_index++) {
        sprintf((char*)serverData[loop_index], "%s%d", "data", loop_index); //program crashes when this line is reached
    }
    return 0;
}

(char*)serverData[loop_index] - Only cast if you 1) fully understand why the cast is required 2) Know all implications and 3) accept all effects. — too honest for this site
– too honest for this site, Commented Feb 6, 2016 at 14:47

Sourav Ghosh · Accepted Answer · 2016-02-06 14:09:45Z

3

The problem here is, you're using serverData[n] while it is not assigned any valid memory. char *serverData[255]; being a global array of pointer type, each member is implicitly initialized to a null pointer.

Refer C11, chapter §6.7.9,

If an object that has static or thread storage duration is not initialized explicitly, then:

— if it has pointer type, it is initialized to a null pointer;
— if it has arithmetic type, it is initialized to (positive or unsigned) zero;
— if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;
- [.....]

A null pointer is not a valid one to access.

You need to allocate memory to serverData[n] before you can write to it. You can consider using malloc() to allocate memory to serverData[n] before you actually use it.

edited Feb 6, 2016 at 14:09

answered Feb 6, 2016 at 13:55

Sourav Ghosh

135k17 gold badges192 silver badges271 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

lost_in_the_source · Accepted Answer · 2016-02-06 23:50:30Z

0

Each pointer in your array is unitialized. Try this

#define NSERVERS    255
#define MAXDATA     32
char serverdata[MAXDATA][NSERVERS];

for (i = 0; i < NSERVERS; ++i)
    if (snprintf(serverdata[i], MAXDATA, "%s%d", "data", i) < 0)
        fprintf(stderr, "snprintf returns <0\n");

snprintf is recommended over sprintf, because it doesn't allow buffer overflows to occur. In this code, each element is initialized--you can't just write onto a pointer. To remove the limits of this array, we could use malloc, and realloc.

edited Feb 6, 2016 at 23:50

answered Feb 6, 2016 at 14:18

lost_in_the_source

11.3k10 gold badges50 silver badges79 bronze badges

4 Comments

chux Over a year ago

Using 1) snprintf() and not checking its results vs . 2) sprintf() trades one class of errors for another. Neither is recommended for robust code.

lost_in_the_source Over a year ago

What's wrong with sprintf and snprintf? How does it trade one class of errors for another? What would be a better solutuon, then? @chux

chux Over a year ago

What does snprintf() prevent that sprintf() may cause: buffer overflows. What does snprintf() without error checking do: Creates un-flagged invalid data in a string. char s[22]; snprintf(s, sizeof s, "Release prisoner %d", 123456789); --> "Release prisoner 1234", (Prisoner 1234 hates programmers) Simple check if the result of snprintf(s, n, format,...) is negative or >= n and then handle the error.

lost_in_the_source Over a year ago

fixed [15 chars] @chux

Collectives™ on Stack Overflow

Send formatted output to character array elements in C

2 Answers 2

Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related