String format works in fprintf but doesn't work in sprintf, gives segmentation fault

fprintf(fp,"IP: %d:  %.*s\n",
        ip, 
        strstr(strstr(p->data, "User-Agent:"),"\n") - strstr(p->data, "User-Agent:"),
        strstr(p->data, "User-Agent: ") );
    fclose(fp);

Hi All, as you can see, in the above statement, I am trying to write off just the U开发者_StackOverflowser Agent header from a char pointer which contains the entire http packet data. The thing is, After fiddling with the string format, I came up with this %.*s format which lets me, dynamically select the number of characters to be printed to the file, and then prints them. What the code is basically doing is, first, it's printing an int, then the number of chars from the occurrence of "User-Agent:" to the very next occurrence new line character is passed, and that amount of chars are then passes starting at where the "User-Agent:" starts, from the entire packet data string. I know it's all pretty messy, but it's working fine. Except that it's not working in sprintf.

Please save all my hard word! Any help is appreciated!

    char *stat;
    sprintf(stat,"%.*s\0",
        strstr(strstr(p->data, "User-Agent:"),"\n") - strstr(p->data, "User-Agent:"),
        strstr(p->data, "User-Agent: ")) ;

You are not allocating memory for stat. Try

char *stat = malloc(MAXLEN);
snprintf(stat, MAXLEN, ...);
 ^              ^

When you use sprintf, you need an array of characters to write into. You're writing to an uninitialized pointer.

Try this instead:

char stat[200];
sprintf(stat, etc...

Well, you are trying to write the data into uninitialized unallocated random memory location. Now that can't possibly work.

Either do:

char stat[SUFFICIENTLY_LARGE_NUMBER];
snprintf(stat, SUFFICIENTLY_LARGE_NUMBER, ...);

or:

char *stat = malloc(SUFFICIENTLY_LARGE_NUMBER);
snprintf(stat, SUFFICIENTLY_LARGE_NUMBER, ...);

and make sure "SUFFICIENTLY_LARGE_NUMBER" is enough bytes that the string fits in and not unnecessarily huge.

PS: snprintf, because your format does not include length limits. If it does, sprintf is OK, but never ever use sprintf with unlimited %s. Your %.*s, while formally limited, is not enough, because the expression will happily return more than the size of the allocated buffer, so you need another check to avoid overruning it.