c - Multiple fscanf -

i have written following program intended read string file variable "title":

#include <stdio.h> #include <stdlib.h>  int main() {     int m, b;     char *title;     file *fp;      fp = fopen("input2.txt", "r");     if (fp == null)     {          printf ("error: file cannot found\n");          return 1;     }      fscanf(fp, "<%d>\n<%d>", &m, &b);     printf("%d\n%d", m, b);     fscanf(fp, "<%s>", title);      fclose(fp);     return 0; } 

the above program crashes @ second call fscanf. why happen?

your main problem you've not allocated space string read into. can in multiple ways:

char title[256]; 

or:

char *title = malloc(256); if (title == null) {     fprintf(stderr, "out of memory\n");     exit(1); } 

either of should used with:

if (fscanf(fp, " <%255[^>]>", title) != 1) {     fprintf(stderr, "oops: format error\n");     exit(1); } 

or, if have system implementation of fscanf() that's compliant posix 2008, can use m modifier %s (or %c, or, in case, scanset %[...] — more on below):

char *title = 0;  if (fscanf(fp, " <%m[^>]>", &title) != 1)  // note crucial & {     fprintf(stderr, "oops: format error\n");     exit(1); } 

this way, if fscanf() succeeds in entirety, function allocate memory title. if fails, memory have been released (or never assigned).

note changed %s %m[^>]. necessary because original conversions never match >. if there > in input, incorporated result string because reads white space, , > not white space. further, won't able tell whether trailing context ever matched — that's > in original format, , it's still problem (or not) in revised code i'm suggesting.

i added space @ start of string match optional white space. without that, < @ start of string must on same line > after second number, assuming > present @ all. should check return first fscanf():

if (fscanf(fp, "<%d>\n<%d>", &m, &b) != 2) {     fprintf(stderr, "oops: format error\n");     exit(1); } 

note embedded newline looks white space between > , < — that's 0 or more blanks, tabs or newlines. note you'll never know whether second > matched or not.

you use exit(exit_failure); in place of exit(1); — or, since code in main(), use either return 1; or return(exit_failure); parentheses optional in either case presence evokes unwarranted ire in people.

you improve error messages. , should consider using fgets() or posix's getline() followed sscanf() because makes easier (by far) error reporting, plus can rescan data if first attempt @ converting fails.