My resize program works per cs50 check, but it has a segmentation fault per valgrind. It appears to be with the atoi function, but I'm lost as to why

    ==750== Memcheck, a memory error detector
==750== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==750== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==750== Command: ./resize
==750== Invalid read of size 1
==750==    at 0x517A517: ____strtol_l_internal (strtol_l.c:298)
==750==    by 0x5176F5F: atoi (atoi.c:27)
==750==    by 0x4008C5: main (resize.c:18)
==750==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==750== Process terminating with default action of signal 11 (SIGSEGV)
==750==  Access not within mapped region at address 0x0
==750==    at 0x517A517: ____strtol_l_internal (strtol_l.c:298)
==750==    by 0x5176F5F: atoi (atoi.c:27)
==750==    by 0x4008C5: main (resize.c:18)
==750==  If you believe this happened as a result of a stack
==750==  overflow in your program's main thread (unlikely but
==750==  possible), you can try to increase the size of the
==750==  main thread stack using the --main-stacksize= flag.
==750==  The main thread stack size used in this run was 8388608.
==750==     in use at exit: 0 bytes in 0 blocks
==750==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==750== All heap blocks were freed -- no leaks are possible
==750== For counts of detected and suppressed errors, rerun with: -v
==750== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Segmentation fault

The error is in a .c file I don't even recognize, so I'm not sure how to start finding it. I'm assuming it's somehow in my use of atoi? Here are lines 17-27 of my code (can post more as needed) since it says the error is "by" these lines.

    // ensure proper usage and calculator sizefactor
int sizefactor = atoi(argv[1]);
if (argc != 4 || sizefactor < 1 || sizefactor > 100)
    printf("Usage: ./resize n infile outfile where n is 1 to 100\n");
    return 1;

// remember filenames
char* infile = argv[2];
char* outfile = argv[3];

1 Answer 1


What happens when you execute the program with no parameters? Based on what little code is listed above, it looks like you are using argv[1] to assign a value to another variable before you have verified that there are the right number of arguments.

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  • That's exactly what I did. Thanks! I guess I didn't realize that I couldn't assign it to a variable before checking that it exists, but it makes sense. Jul 15, 2016 at 19:20

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