C Concat String Int Sprintf

Question

why is the the second sprintf not working?

char* jc;
    char* tn;
    char* result = malloc((256)*sizeof(char));
    int thread=99;
    int jobcounter=88;
    sprintf(jc, "%d", jobcounter);
    sprintf(tn, "%d", thread);
    strcpy(result,"file_");
    strcat(result,jc);
    strcat(result,"_");
    strcat(result, tn);
    strcat(result,".html");
    printf("%s",result);

Output:

file_88_Þ*m.html

Because `char* tn;` has no memory allocated. It is an uninitialised pointer variable. — Weather Vane, Apr 28 '17 at 16:51

score 1 · Accepted Answer · answered Apr 28 '17 at 16:51

In your case

sprintf(jc, "%d", jobcounter);
sprintf(tn, "%d", thread);

causes undefined behavior as none of those pointers (first arguments) point to any valid memory.

You need to make sure that the pointer(s) you're using to access a(ny) memory location points to a valid memory. You can either

make them point to statically/automatic allocated variable or
use a memory allocator function like malloc() or family.

C Concat String Int Sprintf

1 Answers1