Question

I am not sure why a garbage value is being thrown in the if loop when i try to free dataToWrite. It works fine in else loop.
Can anyone help me with this?

FILE *readFilePointer = NULL;
Message_buf outputMsgBuffer;
char fileData[PIPE_BUF];
char *dataToWrite=NULL;

readFilePointer = fopen(fileToReadFrom, "r");

if (readFilePointer == NULL)
{
    outputMsgBuffer.messageType=1;
    dataToWrite=(char *)malloc(strlen(strcat(fileToReadFrom," does not exists!!"))+1);
    memset(dataToWrite,0,strlen(dataToWrite));
    dataToWrite=strcat(fileToReadFrom," does not exists!!");
    strcat(outputMsgBuffer.messageText,dataToWrite);
    memset(dataToWrite,0,strlen(dataToWrite)+1);
    free(dataToWrite);
}
else
{   
    //outputMsgBuffer.messageText[0] = '\0';
    while (fgets(fileData, sizeof(fileData), readFilePointer) != NULL)
    {
        dataToWrite=(char *)malloc(sizeof(fileData));
        memset(dataToWrite,0,strlen(dataToWrite));
        strcpy(dataToWrite,fileData);
        strcat(outputMsgBuffer.messageText,dataToWrite);    
        free(dataToWrite);
    }

    fclose(readFilePointer);
}
outputMsgBuffer.messageType=1;
outputMsgBuffer.messageLength=strlen(outputMsgBuffer.messageText);
mesg_send(fd,&outputMsgBuffer);
}

Answer 1

dataToWrite=(char *)malloc(strlen(strcat(fileToReadFrom," does not exists!!"))+1);
memset(dataToWrite,0,strlen(dataToWrite));

You are not initialising the allocated memory, so calling strlen on dataToWrite invokes undefined behaviour.

Also, you seem to not be aware of the fact that strcat modifies the memory pointed to by its first argument, and returns a pointer to it,

dataToWrite=strcat(fileToReadFrom," does not exists!!");

throws away the just-allocated memory.