... and not a block to link!!
A junior developer came to me in the morning, seeking help on calloc. I briefly described calloc and malloc to her, but I was somewhat doubtful of her requirements, so I asked what was the specific problem she was facing.
She had allocated a string using the malloc call, and was appending to the string using strcat. The result she got was junk characters.
str = (char *)malloc(N * sizeof(char));
for( i = 0; i <= m; i++)
strcat(str, arr_of_str[i]);
The answer lies in the behavior of malloc and strcat. The memory allocated my malloc is not initialized. So, when 'str' is allocated, it is filled with junk characters. The function strcat appends a new string to an existing string, and to identify the end of the existing string it searches for the null character ['\0']. In this case, strcat appended the new string [arr_of_str[i]] wherever it found a null character in the string 'str' - the initial characters remained as junk, and this is what she saw. In fact she was lucky to get away with a garbled string. Had there been no null character in 'str', strcat function would have written into the memory of some other variable [wherever it found a null character in the memory space adjoining that of 'str'], and caused a crash.
The fix was simply to initialize the newly allocated 'str':
str = (char *)malloc(N * sizeof(char));
strcpy(str,""); /* or alternatively, str[0] = `\0` */
Now, this sounds very obvious. But how often the obvious is overlooked, will perhaps be borne out by the fact that I had come across the very same problem not long back.
A junior developer came to me in the morning, seeking help on calloc. I briefly described calloc and malloc to her, but I was somewhat doubtful of her requirements, so I asked what was the specific problem she was facing.
She had allocated a string using the malloc call, and was appending to the string using strcat. The result she got was junk characters.
str = (char *)malloc(N * sizeof(char));
for( i = 0; i <= m; i++)
strcat(str, arr_of_str[i]);
The answer lies in the behavior of malloc and strcat. The memory allocated my malloc is not initialized. So, when 'str' is allocated, it is filled with junk characters. The function strcat appends a new string to an existing string, and to identify the end of the existing string it searches for the null character ['\0']. In this case, strcat appended the new string [arr_of_str[i]] wherever it found a null character in the string 'str' - the initial characters remained as junk, and this is what she saw. In fact she was lucky to get away with a garbled string. Had there been no null character in 'str', strcat function would have written into the memory of some other variable [wherever it found a null character in the memory space adjoining that of 'str'], and caused a crash.
The fix was simply to initialize the newly allocated 'str':
str = (char *)malloc(N * sizeof(char));
strcpy(str,""); /* or alternatively, str[0] = `\0` */
Now, this sounds very obvious. But how often the obvious is overlooked, will perhaps be borne out by the fact that I had come across the very same problem not long back.
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