snprintf and array of strings syntax

Question

I have a struct that contains an array of strings but I'm lost regarding how to use it sometimes
For instance :

struct A
{
  char** b;
};

size_t two = 2;
struct A a = malloc(sizeof(struct A));
b = (char**) malloc(sizeof(char*) * 2);
a->b[0] = "1";
snprintf(a->b[1], 4, "%d", two); //this line makes a mess

It works perfectly fine if I use snprintf for a variable that I define as char* type, but isn't a->b[1] a char* type itself ?
Why doesn't it work (if I use a printf on a->b[1], it displays the value but the program crashes when I do any malloc afterward) and what should I do to make it work ?

Sergey Kalinichenko · Accepted Answer · 2016-04-09 21:55:13Z

2

This line

snprintf(a->b[1], 4, "%d", two);

dereferences a->b[1], which has not been set. You cannot write into it. If you would like to write into a->b[1], do it like that:

a->b[1] = malloc(4);
snprintf(a->b[1], 4, "%3d", two);

Now a->b[1] has a writeable block of memory of 4 bytes. %3d limits the output to three digits, in case variable two is set to a larger number.

answered Apr 9, 2016 at 21:55

Sergey Kalinichenko

729k85 gold badges1.2k silver badges1.6k bronze badges

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

4 Comments

user3548298 Over a year ago

Seems to work perfectly fine But I'm kinda disapoint that I have to manually do a malloc and not just point to a new value returned by the function :( Thank you

Sergey Kalinichenko Over a year ago

@user3548298 Apart from non-standard %sa, there's no good option here :-(

cremno Over a year ago

The second argument to snprintf() limits the output. The 3 in %3d sets a different limit: the minimum field width.

Chris Dodd Over a year ago

Use asprintf if you want a function that allocates the memory for you. Non-standard, but available on Windows, OSX, Linux, *BSD and many others.

Collectives™ on Stack Overflow

snprintf and array of strings syntax

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related