Question

I have a c function that I want to return a string.

If I print the string before it is returned then I see croc_data_0186.idx

If I try and print the string that is returned then I see croc_data_á☼

Can anyone see what I am doing wrong?

Problem function:

char* getSegmentFileName(FILE *file, int lineLength, int lineNumber)
{
    char* fileNameString;

    fseek(file, lineNumber * lineLength, SEEK_SET);

    char line[lineLength];
    fgets(line, lineLength, file);

    char *lineElements[3];
    lineElements[0] = strtok(line, ":");
    lineElements[1] = strtok(NULL, ":");
    lineElements[2] = strtok(NULL, ":");

    fileNameString = lineElements[2];

    printf ("getSegmentFileName fileNameString is: %s \r\n", fileNameString);

    return fileNameString;
}

Calling code:

int indexSearch(FILE *file, char* value, int low, int high, char* segmentFileName)
{
    ...

    segmentFileName = getSegmentFileName(file, lineLength, mid);
    printf ("indexSearch: segmentFilename3 is: %s \r\n", segmentFileName);

    ...
}

Answer 1

You are returning a pointer to local data, which is not valid after the function returns. You have to allocate the string properly.

This can be done in either the calling function, by supplying a buffer to the called function, and it copies the string over to the supplied buffer. Like this:

char segmentFileName[SOME_SIZE];
getSegmentFileName(file, lineLength, mid, segmentFileName);

and the getSegmentFileName function:

void getSegmentFileName(FILE *file, int lineLength, int lineNumber, char *segmentFileName)
{
    /* ... */

    strcpy(segmentFileName, fileNameString);
}

The other solution is to allocate the memory for the string in getSegmentFileName:

return strdup(fileNameString);

but then you have to remember to free the string later.