Segmentation fault when trying to allocate memory for a string via a pointer-to-pointer function call

Question

I Initiate my string and call my function like this:

int main() {
...
char *fileBuffer;
readFileToBuffer("/tmp/file.txt", &fileBuffer);
...
}

The purpose of this function is to get the contents of file.txt and put it into the fileBuffer variable. Because the content of file.txt is dynamic, I allocate the memory for fileBuffer dynamically in the readFileToBuffer() function like so:

void readFileToBuffer(char *filePath, char **fileBuffer) {
...
FILE *reqFile = fopen(filePath, "r");

fseek(reqFile, 0, SEEK_END);
long fileSize = ftell(reqFile);

fseek(reqFile, 0, SEEK_SET);

*fileBuffer = malloc(fileSize + 1);

fread(fileBuffer, fileSize, 1, reqFile);
fclose(reqFile);
...
}

This is causing a segmentation fault. I've googled around and this seems to be the correct way when allocating memory inside of a function.

Any idea why this is happening?

use valgrind and gdb, and post your debug information

amdixon
– amdixon

2015-05-21 10:52:07 +00:00
Commented May 21, 2015 at 10:52 — amdixon
– amdixon, Commented May 21, 2015 at 10:52
Take a look at that fread call again.

Some programmer dude
– Some programmer dude

2015-05-21 10:52:17 +00:00
Commented May 21, 2015 at 10:52 — Some programmer dude
– Some programmer dude, Commented May 21, 2015 at 10:52

Adam Nygate · Accepted Answer · 2015-05-22 01:02:55Z

7

In your readFileToBuffer() code, fileBuffer is of type char ** and your function is called as readFileToBuffer("/tmp/file.txt", &fileBuffer);

Then you have rightly allocated memory to *fileBuffer in readFileToBuffer() [so that is gets reflected to the fileBuffer in main()]. So, you need to pass *fileBuffer to fread() to read the contents of the files into the memory pointed by *fileBuffer.

You need to change.

fread(fileBuffer, fileSize, 1, reqFile);

to

fread(*fileBuffer, fileSize, 1, reqFile);  // notice the *

That said,

Always check the return value of malloc() for success.
The recommended signature for main() is int main(void).

edited May 22, 2015 at 1:02

Adam Nygate

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answered May 21, 2015 at 10:52

Sourav Ghosh

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2 Comments

alk Over a year ago

Nitpicking: The need to pass *fileBuffer (to fread()) does not directly arises from "fileBuffer is of type char **" but from the fact that (the memory to be read into) had been allocated to *fileBuffer (by this call *fileBuffer = malloc(fileSize + 1);).

Sourav Ghosh Over a year ago

@alk sir, updated the answer, can you please review whether it is proper now?

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