Thread: fclose() causes segment fault on i686

hello

working on problem days.
program reads lines form text file , writes these lines successively buffer pointed pb_start.
program can compiled , run on 2 of x86_64 suse 9.3 machines, failed both on i686 suse9.3 , ubuntu 9.04 machines.
following program.

code:

#include <stdio.h> #include <stdlib.h> #include <string.h> #include <errno.h> #include <unistd.h> #define max_length 64  //assumming fixed line size of 64 characters  int main(int argc, char **argv) {          file *f_in;          int n_line;                     //number of lines read file         int index = 0;          struct string {                 char data[max_length];         } *pb_start, *pb_current;       //pb_start- pointer start of counter, pb_current- pointer currnet position of counter          if (argc < 3) {                 puts("usage: command filename number_of_lines\n");                 exit(0);         }          //open input file         if (( f_in = fopen (argv[1], "r")) == null) {                 fprintf(stderr, "open input file failed: %s\n", strerror(errno));                 exit(0);         }          //derive number of lines read file          n_line = atoi(argv[2]);          if ( (pb_start = pb_current = (struct string *) calloc(n_line, sizeof(struct string)) ) == null ) {                 fprintf (stderr, "calloc() failed: %s\n", strerror (errno));                 exit(0);         }           while (fgets(pb_current->data, max_length, f_in) != null && index < n_line) {                 ++pb_current;                 ++index;         }          ( index = 0, pb_current = pb_start ; index < n_line; index++) {                 printf("%s", pb_current->data);                 ++pb_current;         }   fclose (f_in); free (pb_start); return (0); }

i run program ascii text file on i686 ubuntu 9.4 machine (linux shuttle 2.6.28-15-generic #52-ubuntu smp wed sep 9 10:49:34 utc 2009 i686 gnu/linux.)

different n_line value chosen each test , each test repeated 1000 times. test results consistent. pending on value of n_line,
smaller n_line values generate segment fault while larger n_line values passed. if whole text read (when n_line larger or equal
lines of test file) program complete successfully.

sure calloc() , free() work fine, , believe problem comes form fopen() , fclose() part. program/library seems having difficult release memory allocated input file when file partially read.

following show gdb trace of program when segment fault take place.
[code]
program received signal sigsegv, segmentation fault.
0xb7dfcfc8 in ?? () /lib/tls/i686/cmov/libc.so.6
(gdb) bt full
#0 0xb7dfcfc8 in ?? () /lib/tls/i686/cmov/libc.so.6
no symbol table info available.
#1 0xb7dfd5b6 in free () /lib/tls/i686/cmov/libc.so.6
no symbol table info available.
#2 0xb7deafe4 in fclose () /lib/tls/i686/cmov/libc.so.6
no symbol table info available.
#3 0x080487b0 in main (argc=3, argv=0xbfd1aab4) @ t_ring_1.c:52
f_in = (file *) 0x9c9f008
n_line = 2
index = 2
pb_start = (struct string *) 0x9c9f170
pb_current = (struct string *) 0x9c9f1f0
(gdb)

planning install development version of library , trace on library see went wrong (which out of league.) so, please let me know if have experience same problem , possible solutions. comments , suggestion grateful. thanks.

putting blame on standard library wrong.
problem in code.
line problem:

code:

while (fgets(pb_current->data, max_length, f_in) != null && index < n_line) {

it executes fgets on non existing buffer because boundary condition checked after fgets
evaluation of && left right (opposed evaluation of = used earlier [and bad style])
should be:

code:

while (index < n_line && fgets(pb_current->data, max_length, f_in) != null) {

now boundary condition enforced before writing

missing boundary check when n_line greater lines in file while printing
add a:

code:

n_line = index;

after reading while loop

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