Sprintf crashing my program

Question

I am just tryng to use sprintf to concatenate some strings but I had this problem, I don't understand why my program is crashing using sprintf in C. Why this code runs?

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char* dateTime = malloc(16*sizeof(char));

    printf("Date: %s\n", __DATE__);
    printf("Time: %s\n", __TIME__);

    sprintf (dateTime, "%s, %s\0", __DATE__, __TIME__);

    printf("%s", dateTime);
    free(dateTime);

    return 0;
}

And this one not?

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char* dateTime = malloc(16*sizeof(char));

    //printf("Date: %s\n", __DATE__);
    //printf("Time: %s\n", __TIME__);

    sprintf (dateTime, "%s, %s\0", __DATE__, __TIME__);

    printf("%s", dateTime);
    free(dateTime);

    return 0;
}

Curious, Why 16 in char* dateTime = malloc(16*sizeof(char));? — chux
– chux, Commented Dec 24, 2016 at 5:10
Note that the \0 at the ends of the strings is redundant; the C compiler adds a null byte at the end of a string automatically. — Jonathan Leffler
– Jonathan Leffler, Commented Dec 24, 2016 at 5:34

Jean-François Fabre · Accepted Answer · 2016-12-23 22:33:22Z

On my compiler, the string you're creating is 21 characters long (Dec 23 2016, 23:29:57), so you're basically allocating too few bytes for your string.

You get undefined behaviour. So it crashes without the printf statements, and kind of works with them because the computer is not doing the exact same thing, but it's still wrong.

BTW you could achieve what you want safely by just doing this:

const char *dateTime= __DATE__ " " __TIME__;

since __DATE__ and __TIME__ are already string macros. Preprocessor can do the concatenation at compile time.

If you want to compute the numbers of characters needed you could use snprintf with a NULL buffer (c99 only):

Calling snprintf with zero bufsz and null pointer for buffer is useful to determine the necessary buffer size to contain the output:

const char *fmt = "sqrt(2) = %f";
int sz = snprintf(NULL, 0, fmt, sqrt(2));
char buf[sz + 1]; // note +1 for terminating null byte
snprintf(buf, sizeof buf, fmt, sqrt(2));

http://en.cppreference.com/w/c/io/fprintf

Jonathan Leffler · Accepted Answer · 2016-12-24 05:37:33Z

4

It appears that your buffer is too small for sprintf:

char* dateTime = malloc(16*sizeof(char));

The __DATE__ and __TIME__ always takes more than 15 bytes (not including NUL character). Writing past the array boundary causes undefined behaviour.

edited Dec 24, 2016 at 5:37

Jonathan Leffler

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answered Dec 23, 2016 at 22:34

tdao

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