printf access violation in subfunction - Ansi C90

Question

A call to printf inside a subfunction results in an Access Violation.

The program is quite huge in size. but i was able to isolate the problem to the point where only the subfunction call is made.

i am able to crash the system by calling printf with a literal constant like so: printf("test"). Some other people had also access violations by giving a weird object to printf - this is not the case here.

Here is some pseudocode:

subfunction()
{
    printf("all works great"); //Access Violation

    //some other calls here
}

void main()
{
    otherfunctions(); //
    printf("all works great");
    subfunction();        
    //some more calls here
}

From my stachtrace:

msvcr100d.dll!_chkstk()
msvcr100d.dll!_write(int fh, const void * buf, unsigned int cnt)
msvcr100d.dll!_flush(_iobuf * str)
msvcr100d.dll!_ftbuf(int flag, _iobuf * str)
msvcr100d.dll!printf(const char * format, ...)

The Code is in C90 code and compiles with VS2010. It should be treated as C90. It happened after a refactoring where all _(v)snprintf were replaced with their _(v)snprintf_s counterparts. I am not sure if this had an influence.

I think the buffer is getting flushed before anything got written into it.

How can i investigate this further? What systemsettings could my other code touch to crash printf like that?

You probably have stack corruption somewhere else. This code seems all right to me. Try using some dynamic code analysis tool to track access violations. Also try to run the code using "Debug" in VS — Ivaylo Strandjev, Nov 16 '12 at 14:32

score 0 · Accepted Answer · answered May 13 '13 at 09:19

The mistake was that i expected printf to print a string but in reality passed a non string to it.

For Ansi-C I usually write a structure to encapsule strings.

typedef struct TString{
    char buffer[2000];
}TString;

I tend to write:

void mistake( void ){
    TString str;
    TStrnig_Construct(&str);
    prtinf( "%s", str );
    TString_Destuct(&str);
}

This is hard to spot for me since it really looks like str is a string. In reality str is not a string but a structure. This error can manifest anywhere, especially if the content of the tructure is extended with other information (e.g. size_t size).

I should have written:

void corrected( void ){
    TString str;
    TStrnig_Construct(&str);
    prtinf( "%s", str.buffer );
    TString_Destuct(&str);
}

printf access violation in subfunction - Ansi C90

1 Answers1