Passing a string to a function with pointer / C

Question

I want to pass this in my function, but something is going wrong

FILE *f = fopen("out.bmp", "rb");
    int countBit = 0;
    int size = 0;
    char* type;

    for (int i = 0; i < 54; i++)
        fgetc(f);
    printf("/* count bit to pixel */\n");
    scanf("%d", &countBit);
    size=DecodeInformationSize(f, countBit);
    type=DecodeInformationType(f, countBit);
    DecodeMessage(f,countBit,size,type);

before entering the function type is txt

but after :

void DecodeMessage(FILE *f, int countBit, int size, char* type)
{
    char messageSize[8];
    char nameOut[15] = "outMessage.";
    strcat(nameOut, type);
    char* message = (char*)malloc(size * 8);

please explain problem

Paul Ogilvie · Accepted Answer · 2016-03-13 13:03:44Z

1

To be absolutely sure we need to know what DecodeInformationType(f, countBit); does.

However, it seems it uses some data on the stack. Once it returns, this information may only be available for a few instructions. So your debugger shows that the call to DecodeMessage, type points to a valid string, but once you enter DecodeMessage, the stack is overwritten with the variables of DecodeMessage, in particular with char nameOut[15] = "outMessage.";

To solve this, make sure that DecodeInformationType returns a pointer to memory that is not on the stack (not an automatic variable) of DecodeInformationType. That could be memory allocated with malloc or a constant string.

answered Mar 13, 2016 at 13:03

Paul Ogilvie

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