I just found out when I comment out the line 27, which calls free() to free the line got from getline(), the program doesn't segmentation fault. But the manual of getline() tells us to free the "*lineptr" when it is NULL before call getline(). It's the situation in my program. With this src.txt: (x = 1) 1 it reports "double free or corruption" when doesn't comment line 27: *** glibc detected *** ./compiler: double free or corruption (!prev): 0x08c0d2d8 ======= Backtrace: ========= /lib/libc.so.6[0x617ac1] /lib/libc.so.6(cfree+0x90)[0x61b0f0] ./compiler[0x8048660] /lib/libc.so.6(__libc_start_main+0xe0)[0x5c4390] ./compiler[0x8048451] ======= Memory map: ======== 00110000-00111000 r-xp 00110000 00:00 0 [vdso] 0058f000-005aa000 r-xp 00000000 fd:00 51555 /lib/ld-2.7.so 005aa000-005ab000 r-xp 0001a000 fd:00 51555 /lib/ld-2.7.so 005ab000-005ac000 rwxp 0001b000 fd:00 51555 /lib/ld-2.7.so 005ae000-00701000 r-xp 00000000 fd:00 51556 /lib/libc-2.7.so 00701000-00703000 r-xp 00153000 fd:00 51556 /lib/libc-2.7.so 00703000-00704000 rwxp 00155000 fd:00 51556 /lib/libc-2.7.so 00704000-00707000 rwxp 00704000 00:00 0 00dee000-00df9000 r-xp 00000000 fd:00 51568 /lib/libgcc_s-4.1.2-20070925.so.1 00df9000-00dfa000 rwxp 0000a000 fd:00 51568 /lib/libgcc_s-4.1.2-20070925.so.1 08048000-08049000 r-xp 00000000 fd:01 4259848 /home/hpt/temp/algorithms_in_c/4.17/compiler 08049000-0804a000 rw-p 00000000 fd:01 4259848 /home/hpt/temp/algorithms_in_c/4.17/compiler 08c0d000-08c2e000 rw-p 08c0d000 00:00 0 b7e00000-b7e21000 rw-p b7e00000 00:00 0 b7e21000-b7f00000 ---p b7e21000 00:00 0 b7f2a000-b7f2c000 rw-p b7f2a000 00:00 0 b7f3c000-b7f3e000 rw-p b7f3c000 00:00 0 bf943000-bf958000 rw-p bffea000 00:00 0 [stack] Aborted
