The following code causes a segmentation fault. The executable is named ‘./struct’ #include <stdio.h>

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Asked: June 4, 20262026-06-04T23:01:14+00:00 2026-06-04T23:01:14+00:00

The following code causes a segmentation fault. The executable is named ‘./struct’ #include <stdio.h>

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The following code causes a segmentation fault. The executable is named ‘./struct’

#include <stdio.h>
#define VERSION_NUMBER_LEN 32
#define MAX_DESCRIPTION_COUNT 32
#define DESCRIPTION_LEN 128
int main(void)
{
    struct foo {
        char number[VERSION_NUMBER_LEN + 1];
        char description[MAX_DESCRIPTION_COUNT][DESCRIPTION_LEN];
    };

    struct foo asdf = {
        "1.1", { "clap", "clap", "stomp", NULL }
    };

    struct foo hjkl = {
        "1.2", { "clop", "clop", "stamp", NULL }
    };

    int i;
    printf( "%s\n", asdf.number );
    for( i = 0; (asdf.description)[i] != NULL; i++ ){
        printf( "\t%s\n", (asdf.description)[i]);
    }
    printf("\n");
    printf( "%s\n", hjkl.number );
    for( i = 0; (hjkl.description)[i] != NULL; i++ ){
        printf( "\t%s\n", (hjkl.description)[i]);
    }
}

The output looks like this:

1.1
    clap
    clap
    stomp






��
    N���~�����������ջ�����e���t�����������A���P���b���������������̽��㽊����,���V���g���y���������������̾��    ���k�����������Ͽ��迊�
    迊�





    ome/tiger
    56
    y
    vZxy/ssh
    ptop:/tmp/.ICE-unix/2710
    usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:~/bin:~/vitetris-0.3.6:/var/lib/gems/1.8/bin/
    baz
    GNOME_KEYRING_PID=2692
    t \w\n\$
    XDG_DATA_DIRS=/usr/share/gnome:/usr/local/share/:/usr/share/
    9a6bf0ef61ded7872065094fca55d1
    se
Segmentation fault

I ran valgrind:

$ valgrind -v --leak-check=full --track-origins=yes ./struct  

<snip>

==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x402605B: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x4026067: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228== Invalid read of size 1
==15228==    at 0x4026058: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0xbec1007c is not stack'd, malloc'd or (recently) free'd
==15228==
==15228==
==15228== Process terminating with default action of signal 11 (SIGSEGV)
==15228==  Access not within mapped region at address 0xBEC1007C
==15228==    at 0x4026058: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  If you believe this happened as a result of a stack
==15228==  overflow in your program's main thread (unlikely but
==15228==  possible), you can try to increase the size of the
==15228==  main thread stack using the --main-stacksize= flag.
==15228==  The main thread stack size used in this run was 8388608.
==15228== Syscall param write(buf) points to uninitialised byte(s)
==15228==    at 0x4107DC3: __write_nocancel (syscall-template.S:82)
==15228==    by 0x40B0A1E: new_do_write (fileops.c:530)
==15228==    by 0x40B0D35: _IO_do_write@@GLIBC_2.1 (fileops.c:503)
==15228==    by 0x40B181C: _IO_file_overflow@@GLIBC_2.1 (fileops.c:881)
==15228==    by 0x40B2DED: _IO_flush_all_lockp (genops.c:849)
==15228==    by 0x40B3A4F: _IO_cleanup (genops.c:1010)
==15228==    by 0x41670F0: ??? (in /lib/tls/i686/cmov/libc-2.11.1.so)
==15228==    by 0x401F4F3: _vgnU_freeres (vg_preloaded.c:62)
==15228==    by 0xBEC0D5F7: ???
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0x402a054 is not stack'd, malloc'd or (recently) free'd
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)

<snip>

==15228== HEAP SUMMARY:
==15228==     in use at exit: 0 bytes in 0 blocks
==15228==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==15228==
==15228== All heap blocks were freed -- no leaks are possible
==15228==
==15228== ERROR SUMMARY: 7 errors from 4 contexts (suppressed: 12 from 7)
==15228==
==15228== 1 errors in context 1 of 4:
==15228== Syscall param write(buf) points to uninitialised byte(s)
==15228==    at 0x4107DC3: __write_nocancel (syscall-template.S:82)
==15228==    by 0x40B0A1E: new_do_write (fileops.c:530)
==15228==    by 0x40B0D35: _IO_do_write@@GLIBC_2.1 (fileops.c:503)
==15228==    by 0x40B181C: _IO_file_overflow@@GLIBC_2.1 (fileops.c:881)
==15228==    by 0x40B2DED: _IO_flush_all_lockp (genops.c:849)
==15228==    by 0x40B3A4F: _IO_cleanup (genops.c:1010)
==15228==    by 0x41670F0: ??? (in /lib/tls/i686/cmov/libc-2.11.1.so)
==15228==    by 0x401F4F3: _vgnU_freeres (vg_preloaded.c:62)
==15228==    by 0xBEC0D5F7: ???
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0x402a054 is not stack'd, malloc'd or (recently) free'd
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228==

==15228== 1 errors in context 2 of 4:
==15228== Invalid read of size 1
==15228==    at 0x4026058: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Address 0xbec1007c is not stack'd, malloc'd or (recently) free'd
==15228==
==15228==
==15228== 1 errors in context 3 of 4:
==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x402605B: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
==15228==
==15228== 4 errors in context 4 of 4:
==15228== Conditional jump or move depends on uninitialised value(s)
==15228==    at 0x4026067: __GI_strlen (mc_replace_strmem.c:284)
==15228==    by 0x408A79E: vfprintf (vfprintf.c:1617)
==15228==    by 0x40912BF: printf (printf.c:35)
==15228==    by 0x8048910: main (in /home/tiger/dev/development/c/play/struct)
==15228==  Uninitialised value was created by a stack allocation
==15228==    at 0x4060B01: (below main) (libc-start.c:96)
==15228==
--15228--
--15228-- used_suppression:     12 dl-hack3-cond-1
==15228==
==15228== ERROR SUMMARY: 7 errors from 4 contexts (suppressed: 12 from 7)

Ok… so I have a couple of memory locations which valgrind shows as uninitialized, but I don’t see how… the structure and the strings inside are defined statically, and each instance of the structure is explicitly declared.

I think that the segmentation fault is occuring when one of the elements in the second structure (hjkl) is accessed.

Ran gdb…

(gdb) p asdf
$1 = {number = "1.1", '\000' <repeats 29 times>, description = {"clap", '\000' <repeats 123 times>, "clap", '\000' <repeats 123 times>, 
"stomp", '\000' <repeats 122 times>, '\000' <repeats 127 times> <repeats 29 times>}}

(gdb) p hjkl
$2 = {number = "1.2", '\000' <repeats 29 times>, description = {"clop", '\000' <repeats   123 times>, "clop", '\000' <repeats 123 times>, 
"stamp", '\000' <repeats 122 times>, '\000' <repeats 127 times> <repeats 29 times>}}

I’m just not seeing what’s causing the segmentation fault…

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1 Answer

Editorial Team · Answer 1 · 2026-06-04T23:01:17+00:00

While the macro NULL is intended (at least, was by those who first defined it) to be used exclusively as a pointer value, it’s often #defined with a simple:

#define NULL 0

This is probably the case for your implementation (not that it matters here, except for illustration; it could be defined as (void *)0 without changing the results below—but this would result in a compile-time complaint about your initializers). Let’s expand that first for loop with the above in mind:

for (i = 0; (asdf.description)[i] != 0; i++) {
    printf( "\t%s\n", (asdf.description)[i]);
}

(Side note: The parentheses here are not needed as the binding of the . and subscript operators is already the one forced by the parentheses.) Each asdf.description[i] names one entire array (of size DESCRIPTION_LEN) of char. You are therefore comparing:

<some array of char> != 0

The “value” of an array object is a pointer to the array’s first element, so this has the same meaning as:

&asdf.description[i][0] != 0

Comparison of a pointer value (&asdf.description[i][0]) to the integer constant zero tests whether the pointer is NULL (not “the macro NULL but rather “the system’s internal representation of a NULL pointer”). The address of a valid pointer never compares equal to 0, so the loop runs (in effect) “forever” (certainly until i >= 32).

Eventually, the call to printf passes a pointer value that results in the segmentation fault you see.

Presumably what you really meant to do was to initialize the array following the last valid one with all-zero-byte chars (or at least an initial zero byte). In that case, the loop test should read:

asdf.description[i][0] != '\0'

You might also consider the possibility that the 32-element array (of arrays of DESCRIPTION_LEN of char) is completely filled with valid arrays-of-char. In this case, you should check the value of i before looking at asdf.description[i][anything]:

i < MAX_DESCRIPTION_COUNT && asdf.description[i][0] != '\0'

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